TECHNICAL INSTRUCTION S.4
Audio-frequency Test Apparatus
BRITISH BROADCASTING , CORPORATION ENGINEERING DlVlSlON - ', : . iv- TECHNICAL INSTRUCTION S.4
Third Issue 1966 instruction S.4 Page reissued May. 1966
CONTENTS
Page Section I . Amplifier Detector AD14 ...... 1.1 Section 2 . Variable Attenuator AT119 ...... Section 3 . Wheatstone Bridge BG/I ...... Section 4 . Calibration IJnit CALI1 ...... Section 5. Harmonic Routine Tester FHP/3 ...... Section 6 . 0.B. Testing Unit 0BT/2 ...... Section . 7 . Fixed-frequency Oscillators OS/9. OS/ 10. OS/ IOA ...... Section 8 . Variable-frequency Oscillators TS/5 ...... TS/7 ...... 1' . . TS/8 ...... TS/9 ...... TS/ 10. TS/ 1OP ...... Section 9 . Portable Oscillators PTS/9 ...... PTS/IO ...... PTS/12 ...... PTS/13 ...... PTS/l5 ...... PTS/l6 ...... Appendix . The Zero Phase-shift Oscillator with Wien-bridge Control Section 10. Transmission Measuring Set TM/I ...... Section 1 I . Peak Programme Meter Amplifiers PPM/2 ...... PPM/6 ...... TPM/3 ...... Section 12 Valve Test Panels VT/4. VT/5 "d . . . . . Section 13. Microphone Cable Tester MCT/I . . Section 14 . Aural Sensitivity Networks ASN/3. ASN/4 Section 15 . Portable Amplifier Detector PAD19 . . Section 16. Portable Intermodulation Tester PIT11 Section 17. A.C. Test Meters ATM/I. ATM/IP . . Section 18 . Routine Line Testers RLT/I. RLT/IP . . Section 19. Standard Level Panel SLP/3 . . . . Section 20 . A.C. Test Bay AC/55 ...... Section 21 . Fixed-frequency Oscillators: OS2 Series Standard Level Meter ME1611 . . INSTRUCTION S.4 Page reissued May 1966 ... CIRCUIT DIAGRAMS AT END
Fig. 1. Amplifier Detector AD14 Fig. 2. Wheatstone Bridge BG/1 Fig. 3. Harmonic Routine Tester FHP/3 Fig. 4. Oscillator OS/9 Fig. 5. Oscillator OS/10 Fig. 6. Oscillator OS/lOA Fig. 7.' Peak Programme Meter Amplifier PPM/2 Fig. 8. Peak Programme Meter Amplifier PPM/6 Fig. 9. Oscillator PTS/9 Fig. 10. Transmission Measuring Set TM/1 Fig. 1I. Test Programme Meter Amplifiers TPM/3 and TPM/3A Fig. 12. Tone Source TS/5 : Amplifier Fig. 12~.Tone Source TS/5: Oscillator and Detector Fig. 13. Tone Source TS/7 Fig. 14. Tone Source TS/8 Fig. 15. Valve Test Panel VT/4: Circuit Fig. 16. Valve Test Panel VT/4: Valve-holder Panel Fig. 16~.Valve Test Panel VT/4 : Adaptor Panel Fig. 17. Valve Test Panel VT/5 Fig. 18. Portable Tone Source PTS/I 3 Fig. 19. Tone Source TS/9 Fig. 20. Portable Tone Source PTS/ 10 Fig. 21. Portable Tone Source PTS/I2 Fig. 22. Portable Amplifier Detector PAD19 Fig. 23. Portable Amplifier Detector PAD/9: Modified for use with P.P.M. Fig. 24. Portable Intermodulation Tester PIT11 . Fig. 25. A.C. Test Meters ATM/l and ATM/lP Fig. 36. Routine Line Tester RLT/1 Fig. 27. Routine Line Tester RLT/lP Fig. 28. Tone Source TS/10 and TS/lOP Fig. 29. Standard Level Panel SLP/3 Fig. 30. AC.. Test Bay AC/55 Fig. 31. Intermodulation Test Oscillator IT011 Fig. 32. Amplifier Test Panel ATPI1 Fig. 33. Telephone Panel TP/12 Fig. 34. Relay and Repeating-coil Panel RRC/1 Fig. 35. Oscillator OS2/4 SECTION 1
AMPLIFIER DETECTOR AD;4
The Amplifier Detector AL)/I' is essentially a anode voltage, a stabiliser, coiisisting of a Metrosil \.alve voltmeter designed to read levels of -+ 10 db disk working in conjunction with R19, is includrtl to - 55 db. Its input impedance is 30 kilohms in the h.t, supply circuit of V.3. which is sufficiently high to permit its connection across most circuits without altering the voltagc Negative Feedback or operating conditions of the sending circuit. 'fhr high gain available from \'1 and 1'2 permits
Fig. I. I. Face Panel AD:4 with Simpson Meter
Circuit Description (Fig. I) tlw use of a consideraldc ;mount oi feedback. The input to the unit is taken via a11 input \Yitli \'I, serirs feedback of 1"l) is ilcwloprtl transformer, TI, to a 10-kilohm potentiomrtcr, ;moss I<" to compvnsate for high-frequency loss, Pa65P, having 14 studs, calibrated in steps of thc ferdhack is rcdr~cedby abor~t0-1 db at 10 kc-s 5 tlb, with an off position. This is followcd bj. and by about 0.2 dh at 15 kc,s. This recluctior~ two resistance-capacitance coupled stages which is effected by shunting CH and R20 across R2. use high-gain pentodrs. \'arinl)lc series feedback is usrd over V2, V3 and Tl~eoutpr~t stage to the rectificbr AIKI metctr ~xovides;i ready means of ndjusting thc working circuits consists of a triode, workcd at low anodc :rain iinr.l for lining up the meter. Tl~cfecdback \.oltage. This res~iltsin the \xlvc working near \-oltngc is rlevelopetl in thc cathoilt- circr~i.tnf \.I! tlit 01-crload point : to minimisc. instabilit!. of th~~across R1.5 and R8 : 118 is variahlc to provides :LII Instruction S.4 Section 1 adjust-gain control ; its ~~ljustmentvaries the performed carefully ant1 cannot be hurried, sincc output current of V3 which flows throiigh the the mcter movemcnt is rclativcly slow. rectifier-meter circuit. i'C.leas/tri)~,qLevels Gain Controls It should be clearly untlcrstood that the AD/I The calibrated gain controls used for taking was designed primarily for the measurement of tone rneasurcments consist of the two potentiometers level as distinct from programme volume, for 1'/65P and P/64P. The P/65P is variablc in steps of which latter purpose a test programme meter is 5 db over a range of - 50 to + 10 db ; thc P/6?P more suitable. To check a given level, set the is variablc in stel)s of 0.5 dl, over a rangc of calibrated controls to correspond with this level, O to -- ,5 db. wherever possible setting the 0-5 dl) control to zero. Adiust thc fine control until the meter reads as near tb mid-scale deflection as possible. The Meter Circuits level is then obtained by the algebraic sum of the The anodc of the third stage fecds into a 1 mA rcadings of the two controls and the scale reading Westinghousi: bridge-connccted metal rectifier of of the meter. the meter type via a 20-kilohm rcsistor R14 and For cxample, if it be required to measure a level ;I I-microfarad capacitor C6. Thc output of this which slioultl normally be + 10 db, set the coarse recliticr is taken to thc terminals of an appro- control to + 10 tlb and the fine control to zero db. priate tylr of indicating mcter. If the meter ,reads well off mid-scale, adjust the Two typcs of indicating meter can be used, the fine control accordingly. If the nearest adiustmcnt !'limpson type, with normal veriical scale requiring is at - 2 db, on the fine control, and the meter ;L fectl of 1 mA for full scidc tlcflcclio:~, or the reads j--0.4 tlb, then the actual level is + 10 - 2 Elliott edgewise typc as usctl on the Trar~smission + 0.4 =:: -1- 8.4 dl). Measuring Set TM/l. Thc Simpson type meter is coniicctcd direct to thc rectifier output, but thc Elliott typc rcquircs to be shunted by a 30-ohm Supplies rcsistor. The ultcrnativc connections are shnun The unit is designetl to work from a 250-volt or i11 Fig. I. a 300-volt battery or a rcctitied h.t, supply. The Uoth meters arc calibrated for zero at mitl- 1.t. supply may be taken from batteries or from scnlc deflection, the scde being cxtencld to cover the mains, except in cases of iiistruments having a %ll, swing cither side of zcro, the calibration serial numbers below 136. Thcse carlicr models bcbin:y in stcps of 0.2 db. 1-Icnct. it is possiblc to have an unscreenetl input transformcr and arc obtain readings thc accuracy of which can be subject to a certain amount of hum pick-up. nio:lsured to within + 0.2 tlb, provitlcd that the Present practice is for ncw installations to be ii~ctcrreadings be correctly inttqolatcd betwecn provided with a mains unit, Typc MUIIG. thr rcatlings of thc calibrated potcntionctcrs.
Valve Data Operation Anode Screen \1'hencvcr possible, the amplifier tlctector should Current Current FiL Fil. I)c switched on for at least ten minutes be for^ use mA . nzA 1,'olfs Amps. in order to ensurc stability. Stage 1, Calibmtion AC /SP3,1 1.2 0.35 4 1 Sct tlic Adj!jr(stGain potentiometcr to its extreme Stage 2, anti-clocl
General Data Ampllfier Detectors AD/4A, AD/4B, AD/4C, l'otentinmslsrs AD/4D No. of Loss per All these models use thc samc basic circuit as Tybr' Rssistancs Sl~rds Stud the AD/4, but are moun ted on 9-inch panels instead Coarse of the nornial 64-inch panel. Adjustment P165P 10 kIl I4 .idb. AD[?A is a converted AD/2. Fin, AD/4B is a converted ADIS, with Muirhead Adjust-ment P/64P 100 kt2 I I 0-5dh, potentiometers. Adjust Gain AD/4C is a converted AD13 with fainton potentio- Type, Reliance TW. meters. Rssisto trcc, 0-100 R. AD/?D is an AD]?, modifid for %inch panel Impcdon ces mounting. Input Z - 31) kt1 Instruction S.4
SECTION 2 VARIABLE ATTENUATOR AT119 The Variable Attenuator AT119 is designed for 5 db and has a range of 0-50 db. use on the standard a.c. test hay in conjunction The at tenuator controls comprise two Painton with the variable tone source TS/7 or TS/8, and potentiometer switches and have rotating scales,
I Fig. 2.1. Foce el AT/ 19
has an attenuation range of 0-55 db. The atten- calibrated directly in decibels, the total attenuation uator is normally connected to the output of the in use being given by the sum of the readings tone source by means of break jacks. of the two controls. Each control is fitted with
Fig. 2.2. Circuit AT/ 19
Circuit Description (Fig. 2.2) detents, or stoppcrs, so that the mechanical setting The attcnuator consists of two 600-ohm variable at each calibration is accurate. square networks in series. The first network is The use of the attenuator in conjunction with the arranged in steps of 0-5 dh, and has a range of tone s'ources TS/7 and TS/8 has been explained in 0-5 dh. the second being arranged in steps of theoperationinstructionappertainingtothoseunits. Instruction S.4
SECTION 3
WHEATSTONE BRIDGE RC. 1
The Wheatstone Rridgc I3G/1 was designed to three positions. In the central position it arranges permit the following types of measwemcnt I--- the circuit for simplc resistance measurements and (a) Resistance, e.g. Line loop. in the I .arlq position for measuring resistance (b) Resistance unbalance of lines hv thc \';~r- ~~nhalancc.The third position is used in conjunc- ley loop test. tion with t he switch designated Battery Od?, on the (c) Internal resistance of batteries. right of the panel, for arranging the circuit for the The bridge can be used for measuring resistances measurement of battery resistance. The latter key of any value up to 1.11 1 megohms to an accuracy when operated substitutes equal resistances of above 100 ohms of one part in 10,000, and below fixed value, namelv 2.5 kilohms and of adequate -- 100 ohms to 0.01'ohm. Used as a resistance box current-carrying capacity, in place of the variable the variable standard resistance enables any value .resistances in the ratio arms, and also connecrs the w between 1 and 11-11 kilohms to be obtained. Thc resistance of the Increme Sensitivity control, internal resistance of batteries, of voltages up to located at the top right-hand corner of the panel, 200 volts, can he directly measured to within and a fixed resistance of 3 kilohms in series with about 10 ohms. the galvanometer. A &volt d.c. supply is provided for the normal Circuit Description (Fig. 2) operation of the panel, either for the measurement The resistances in the ratio arms of the bridge of resistance or for the Varley test, and a 100-volt are controlled by rheostats designated Mwltipl~, d.c. supply for battery-resistance measurements. and Divide respectively, located at the top of the Fuses are provided in the 100-volt positive lead panel on either side of the galvanometer. The and in the galvanometer circuit and are located on switches each have three positions designated 10, the hack of the panel. Access to them is obtained 100 and 1,000, the designations indicating the hv removing the hack cover. value of resistance in circuit in ohms. The variable standard resistance is provided, like a resistance Measurement of Resistance (Fig. 3.1) box, with four decade switches, one each for The resistancz to be measured should be con- thousands of ohms, hundreds, tens and units. The nected either to the X1 and X2 terminals or to one galvanometer is a central-zero instrument reading of the jacks in the Test Ray jackfield to which up to 30 on either side. The battery and galvano- meter keys are located at the bottom of the panel - to the right of the centre and are of the push type. They are designated B and G respectively, and are w each provided with a locking device. Three pars of terminals are provided at the bottom of the panel on the left. The central pair, designated XI and X2, are for connecting the resistance or line to be measured and are extended to two jacks in the Test Bay jackfield, wired in parallel hut with their tip and ring connections reversed so as to provide a ready means for reversing the line con- nections. The pair of terminals on the right, designated R1 and R2, are connected directly across the four decade resistances and connection is made Fig. 3.1. Resistance Measurement Circuit to them when it is desired to use them as a standard resistance box. The pair on the left, designated these are wired. The switches should be operated B - and B +, are provided for the connection of a as follows :-- battery of which the internal resistance is to be T'arley-Butt Kes key in mid position. measured. The key in the centre of the panel has Hatt~rvol@ kev to o//. Instruction S.4 Section 3
,. 1hc circuit is now arranged as a simple Wheat- the resistance to be measured is less than stone Bridge with the galvanometer connected, in 100 kilohms or to 10 if it is greater than series with its switch, between the junction of one 100 kilohms. of the ratio arms P and the variable resistance arm (Ir) For resistances between 100 and 10 li and the junction of the other ratio arm Q and kilohms set both Divide and Multiply controls the unknown resistance arm X. The 6-volt battery, to the sdme value, namely, 100 if the in series with its switch, is connected between the resistance to he measured lies between 100 junction of P and (? and that of R and X. Thc and 500 ohms, and to.1.000 if it lies between variable resistance I< is adjusted to a value such 500 and 10 kilohms. that when the battery and galvanometer keys are (c) For resistances under 100 ohms set Il.lultiply depressed there will be no current through the to 10, and set Divide to 100 if the resistance galvanometer. This condition obtains when the two to be measured is greater than 10 ohms or to ends of the galvanometer circuit are at the 1.000 if it is less than 10 ohms. same potential, that is to say, when the ratio P/R Before making the test, the value of the is the same as the ratio Q/X. From this it follows resistance to bc measured should be estimated and *. the decade dials initially set to the corresponding that X =I<----Q I' I' value. When making a test for balance the battery - On the panel the variable resistances Q and P key B should always be depressed before the galvanometer key G, because otherwise, if the are designated- Mvlliply- - and Divide, respectively. ' ~l/lzcllip&'selliig resistance under test has either an inductive or The rule therefore becomes X = H capacitative component the galvanometer needle ' Divide' selling will kick even .in the balance condition. If when a The most sensitive condition is obtained when test is made the needle deflects to the left, the the resistances in the four arms of the bridge are resistance in circuit should be increased, and all equal, or in practice, when the resistances in the conversely, if the needle deflects to the right the ratio arms are equal to one another and of the same resistance should he decreased. order as the resistance to be measured. Obviously, however, this condition can only he met for Varley Loop Test resistances which fall within the range of adjust- For measuring conductor resistance unbalance, ment provided for the resistances in the ratio arms, the switches should be set as follows :- namely between about 10 and 1 Idohm. Further- Varley-Boll Res key to Varley. more, it is only possible to equate the variable Battery only key to Ofl. resistance R to the unknown resistance X over the The line to be tested should be connected range of adjustment provided for the former. either to the XI and X2 terminals or to one of the Therefore, when the resistance to be measured exceeds 10 kilohms the unity ratio can no longer he used and Q must be made larger than P to enable a balance to be obtained, that is to say, the setting of the M,zclliply control must be greater than that of the Divide control. Likewise if the resistance to be measured is fairly small it will be impossible to obtain an exact balance with P and Q equal unless it is an exact number of ohms, because the fourth dial of the variable resistance does not give fractions of an ohm. In such a case therefore greater accuracy will be obtained if P is made greater than Q, that is to say, if th-setting of the Divide control is greater than that of the Fig. 3.2. Varley Loop Test Clrcuit Multiply control. The foregoing remarks can be summarised in the following practical rules :- jacks in the Test Bay jackfield to which these arc (a) For resistances exceeding 10 kilohms, set wired and at the distant station the conductors Mullifil~to 1,000,and set Divide to 100 if should be looped together and earthed. , . . 1 hc circuit is now arranged as show il; Fig. 3.2, 'l'hc circuit 1s then arrangeti ;is sl~ownin Fig. 3.3 with the galvanometer connected as before in The Mlrllifih and Divide resisttmcr in the ratio series with its key, betwcen the junction of P antl R arms arc cllt ant of circuit and fixed and equal and the junction of Q and X, ancl with the 6-volt rvsi5tanc.m I' rollnc.ctcrl in thoir plac~s. The 100- battery connected in series with its key between thc junction of P and 9 and earth. 'The battery is thus virtually connected between the junction of the ratio arms mcl the earthed loop at the distant station so that the It arm of the bridge includes the variable resistance and one leg of the line, antl thv X arm thc othcr Icg of the line. Unit), ratio sl~o~~ldbe used, 130th tllr J)iuiclc and Mrrlli~lj~controls king set at 100, and balance 01)tilined as hrfore by adj~~stmcnt~i the variable resislancr. The test should be started with a resistance setting of zero, because. u if the two conductors have exactly the samrh resistance, balance will be obtained wit11 the zero setting. !f, however, the galvanometer shows a Fig. 3.3. Battery-resistance Measurement Circuit deflection this can be balanced out by inserting resistance in the R arm. If the deflection increases when resistance is inserted the line shouId be volt battcry is comected in the X arm in oppositio~~ transferred to the other jack so as to reverse the to the battery to be tested, the negative poles of connections to it. When balance is obtained, the both .batteries being earthed. The ga1vanometc.r resistance unbalance of the line is given directly is connected in series with a fixed resistance and by the setting of the decade controls. with the Incrense Sensili~dycontrol bctwecn the In the event of two lines being looped A-A ancl junction of PI and R and the junction of 1'1 and X, B-B at the distact point, an overall test can be and the key G is no longer in circuit. The key B made by looping and earthing one of the lines and is connected across the junction of the two PI arms connecting the other to the Test jack. The result and that of the R and X arms, but the 6-volt of the test in this case will be the algebraic sum of battcry is cut out of circuit, so that when this key is the unbalances of the two lines. closed it mcrely placrs a short circuit across these The Varley test can also be used for locating an two points. Therc will be a resultant e.m.f. due to earth on a line, but in this case the two legs of the the difference between the voltages of the two bat- circuit are looped only and not earthed at thc teries, and this will cause a current to llow in either distant point. The resistance of the loop should first one direction or thc other round the closed loop . be measured by the method clescribecl for resistance provided by the four arms of the bridge. A differ-
j measurements, and then the rcsistance unbalance ence of potential will thercfore exist between the measured by the method given above. If R1 is the ends of thc high-resistance shunt containing thc loop resistance and R2 the resistance unbalance, galvanometer and a deflection will bc obtained. thc distance of the earth in ohms from thc station at This is adjusted as nearly as possihle to full scale which the measurement is made is then given I)? deflection. in order to obtain the most sensitive R1 - R2. condition by means of the I7rcrease Se~~silivily the cxpression 2 control. If now the rcsistancc in the R arm is i~tljustcclto thc samc value as the rcsistnnce in the Battery Resistance S arm, that is, if R is matlc clqt~alto thc intcrnnl For this measurement, the battcry of which thc* rc~sist;mccof the batter?- rlntlcr tc,st (tlw resistanc'c. ~ntcrn;llresistance is required shoukl hc connc~ctcvl of this 100-volt accllnn;l:l~or1):tt tt*r!. in scrivs with it to terminals B - and B f in the correct polilrit!.. being ~w,!i$gil>l(,j, it ~YIII1~. .S~IO\VII t 11:~i l)l;u511g;L The Imrease Se~~sihdycontrol shoultl bc t~~rncdas short-rircuit iLcro.; tl~r1wi11t.; to n.11it.h tlic conti~cl.; far as possible in an anti-clockwise d~rcction.Thv of thc kcy H arc. conncctcd will not alter tlw I/avley-Ball Res key should be thrown to Bnll Rct. potential applied across llict sln~titcont:iini~~g 1h- and the Baller? only key to On. galvanometer. Instruction S.4 Section 3
Ii~)wrvc:l.,rt+i~~lrs ac:c.~~ri~tc IO withi11 i~l)ollt10 I)~IIII> cxn bc> obtni~ic.rlwit11 wrrf~~lUSC. If. \VI th the lrlcrzirsz .);~nsifiai{ycorilrol stbt for rr~inim~~mscwsirl\.it\., tho gi~lvanometergoes hard uvrr against its stop, this will indicate that eithcr
the volt;qc: of 1I)ty 100-volt I)nt tery is low or that that of 111(. 1latt1.1\ 111111cs is greater than 800. and th: differonco of,potc~~~tiill across t IH. S~IIIIt containing the galvanom~trrwill Iw . 111 the Iat tc-r c;~wtl~, I)nttvr\r can eithcr he tested ill two swtions or, if iin accumulator of suitablr \,oltagc is ;~\~~ilnl)li.,this can bc connected in series opposition with thr httery under test in order to If key U is now closed, the arm 12 and the left-hand obtain a net \'oI~ag(.slightly less than 200. A arm PI arc short-circuited, and the potential similar c.xpcdit~i~tcould be ndoptcd iri thc unlikely at the.left-hand end of the shunt containing the casc of thv v~lt;~gt:of the hattwy under test being galvanometer is the same as that at the upper exactly the salncs ;IS that of the test bnttery so that end of the right-hand arm 1'1. The cllrrent no galvanometcsr deflection is obtilinetl. In this flowing rorlnd the loop will now be case, of courst!, it wo111d not matter if the accumu- lator, which nc:c.tl only he about 2 volts, were connected in sericbs opposition or in series aiding. and the potential difference across thc shunt con- In eithcr case thr error due to the small additional tainin~the galvanometer will be resistance introtluccd bv thc accumllla tor can hc ignored. (1'1 i2JJlwhich as before ---. - 1'1 + x Resistance Box Having adji~sted for approximately full-scalc -1.0 usr thc decade rcsistancc*~forming the vari- deflection by means of the Increase Sensilivil.~ ;ible resistance of the bridge as a resistance box it control, the process is therefore to adjust the vari- is only necessary to make connection to the ter- able resistance to a value such that when the key minals RI and HZ and adjust the controls to the B is depressed no movement of the galvanometer vall~creq~iired. .Any vxact number of ohms be- needle can be detected. ' Over a wide range of t wcm 1 alld 1 1 ,I 10 is available, but thc current adjustment of the decade dials the change in the ill the circuit shodd hc restricted as follows :. meter reading will be extremely small, amounting Units 'dial only in usc . . . 0.5 A to little more than a quiver, and unless the needle Tens and Gnits dials ohly in use 0.2 A is closely observed whcn the key is actually being Hundreds, l'ms and Units operated, it will be difficult to determine thc point dials in IISC ...... 60 mA of balance with any rlegrcc of accllraci.. In ~)ractic.c-, .All (liiils in IISI: ... .,, ?Om:\ Instruction S.4
SECTION 4 CALIBRATION UNIT CALil 'fhc Calibration Unit CAL;I is clesigncd to -1 tE being 2.45 = .775. provide standard sending levels of either .+ 10 db -14.62 or zero db with reference to 0.775 volt at a fre- When put to the + 10 db position, the output line rluency of 50 c/s. The calibrating voltage is is connected across both resistors, the full output
ADJ. LCVEL L 1 Fig. 4.1. Face Panel CALI1 taken from the 50 cls mains supply. The unit is voltage of 2.45 being available at the output used for lining up programme meters and amplifier terminals (20 log,, 2-451.775 = 10). detectors, and should be regarded as the basic A rectifier-type a.c. voltmeter is connected unit for checking low-frequency measurement. across the potential divider, the face of which is uncalibrated except for a single white line which Circuit Description (Fig. 4.2,) indicates an r.m.s. voltage of 2-45 volts. 'lle unit is designed to work from '200/!?50 volts 50 CIS supply mains. The mains input is fed, via Operation n 0-75 amp fuse in each leg and a double pole on-OH To operate the unit, close the " or]-ofi" switch switch to a transformer, Type M35.4. and adjust the variable resistor labelled Adj.
+IOdb
1 0-711 MAINS INPUT A c 2001150 V I VOCTUETER
I 2.45V MID6CALE
-I LP~NELr- FR bUE Fig. 4.2. Circuit CALI l
:\ 0-7-ohm variable resistor is ~rlsrrted ill Level until tl~emcter reads 011 thc whitc 111~'. series with one output lead from the secondary Having obtained this condition, the output sending winding for adjusting the outlmt level to the level will be + 10 db or 0, depending upon the correct datum voltage of 2.45 volts r.1n.s. A position of the selector switch. potential divider consisting of two card-would In the 10-db position, the level will be correct resistors (4.62 and 10 ohms) is coilrwcted across for any load impedance. the output leads on the line side i~tthe vari;iblv 111 the zero position thc output iinpedance is resistor which provide the ~1t;lgc.drop for approximatelv 3 ohms, which is sufficiently low supplying the two tixetl output lcvt*ls of + 10 dl) to make the error when feeding into a 300-ohm and zero. Connections arc takvn from these hxctl load impedance only 0.1 tlb compared with the resistors to a selector switch of thc toggle tvpc. level into an open circuit. For load impedanccs which, when put to the zero positinn, connccts tht, above 300 ohms, the output level error is less than output line across the 4.N-ohm resistor only. 0.1 dl> and may be ignored. Instruction S.4
SECTION 5
HARMONIC ROUTINE TESTER FBP/3
'The Harmonic Routme Tester FIHP/3, when (ii) I-11~;sPosition used in conjunction with a variable-frequency \Vhen the key is put to the I-kc/s position oscillator and an amplifier detector, permits thc (Fig. 5.2), the high-pass filter is introduced into measurement of total harmonic content a1 [he the circuit. The output impedance of the filter two fundamental frequencies of I00 c/s and is 6 kilohms and its termination is provided by I kc/s. The unit is essentially a high-pass Kt and H, in parallel, i.e.. 6 kilohms. The repeat- filter, i.e., a filter which rejects low frequencies ing coil is, therefore, terminated by CJ~olims and allows high frequencies to be passed through shunted by 6 kilohms, i.e., 600 ohms. it, the frequencies at which cut-off begins to take CI C3 place being determined by the values of the constituent prls. In the FI-IPj3, these cut-off I frequencies are fixed at 1.5 kc/s and 150 CIS approximately.
Circult Description FHPj3 [Fig. 3) Referring to the circi~itdiagram, it will bc sccn r that the unit comprises an input jack, repeating Fig. 5.2. Filter Circur: when Meosuring Harmonics at coil, high-pass filter, terminating resisters, change- I kc/s over key and output jack. The theorectical dia- grams (Figs. 5.1, 5.3, 5.3) show the circuit arrange- The filter is designed to reject I kc/s and all ments for each psition of the key. frequencies below. It is a two-section filter, one cjf the sections being the simp1est type of high- ]KISS section, the prololype ; the other section is (t') Cetrtral Positiou (Futrdrrmcilftrl) In the central position, thu circuit is as shown one derived from the prototype by a process in Fig. 5.1, K1 ;111d K2 being conr~ectedin parallel known as ill-dwiiliizg, which will give resonant and shunted across the secondary of the repenting arms. and introduce very great loss over a narrow- coil ; when the 30 kilohm inpul of the AD/4 hand of frequencies. %I-derived sections may be is connected to the output of FHP/3, therefore. designcd so thal they have an impedance which, the repeating coil has a 600-ohm load. in nearly all the pass mnge, is a constant resist- ance ; for this reason they are very suitable for putting at the ends of composite filters where they provide rt better termination than worlld the ordinary prototype, For this reason the 31- derived seclion is cul into two halves and the halves placed one at each end of the filter with the prototype in the middle. The total loss of the Fig. 5.1. Filter Circuit when Measuring Fundamentof lilter will thc~be, for about 95 per cent of the pass mnge, the sum of the attenuation of the Let Kg = input impctlance nf AD.4 - 30 kl!. individual sections. In the slop range, however, We then havr there may be serious impedance mismatches at the input and output of the filter, and for this H,K, 7,500 .; 30.000 -- = Ci kll rtbason, total losses introduced by Lhe fi!~. .,': H, H, - 7.500 + 30,000 be less than the sum of the attenuatir , ,. This 6 kilohms is shunted by H 1 = 6ti7 !! therc- individual scctions. fore, the repeating coil is loaded by Thc ntteiiualions of the XI-derive(! . . . of the prototype section, and the appj. . total loss of the filter, are shown in Fig. 5.;. . .
.4. H and C res1)ectively. The resultant ch, . ' . . Instruction S.4 Section 5 kticsshuw that thcrr is a high loss at all frccluti~~cirs ci~l>~~itt~rs('4, (:5, C6 ar(. ~~ari~ll~lcdilCroS5 (:I, t,cluw 1.5 kc,'s and a partis~ilarlyhigh loss in thc. 2.3. 'I'hc effect of this is tqr nir~ltiply thc immediate neighbourhood oi I kc/s. l'hc loss capacitancc in cach casc by 100; thc cut-of1 at all harmonic frctlucncics is low. frequcncy. and thc filter output irnpcdance 2, In thr I kc!s position of the key, thc reading are, therefore, both divided by 10. The filter is obtained on thc arnplificr dctcctor will represent thcn tcrminatcd by 667, 7,500 and 30,000 ohnis thc total ot thr harmonic content at I kc!s. in parallel, and its terminalion irnpcdance is rnabling the pcrctBntagedistortion to be ohtainetl 600 ohms. I-ry a simple calc~rlation. Input Impedance Figure 5.4 shows that in the 100-c:s positiori, thc repeating coil is not directly shlinted by R1 ; the input impedance is therefore high at the fundamental freqi~ency. If, however, the appara- d tus under test is designed to work into a 600-ohm load, a 600-ohm 10-db attenuator should bc .- con~zcctedbclwcen thc oulpul of lhc apparatus and the input to the FHPI3. In the 1-leek position, the input impedance is liniited by the inclusion of RI across the repeating coil and the attenuator is not essential. Kormally, however, tc.its at both frequrncics are required and there is no object in switching thc attenuator in and out of circ~~it.It is recommentled, there- lore, that tl~eattcnuator be uscd for a11 nonnal routinc lests.
Ftrqutwy, CIS -APPARATUS d lOdb - Fig. 5.3. Lou Curves for Filter, I -kc/s ~osi'tion T1S UNDER 600 600 FkIP/J *ID TEST [ AT 7 - 'I'hus, if l1ic lcvcl of the fltndamental -- I 4 dl? Fig. 5.5. Test Circuit : Block Schematic and that of the total harmonic content = - 26 dl), by finding the voltage ratio for - 30 db and rnul!iplying this ratio by 100, thc percentagc Operation (Fig. 5.5) harmonic conlenl is obtained. 111 the exarnplc (i) Switch on variable oscillator, amplifier detector and ;ipparatus to be tested, quoted ' thc voltage ratio = .031ti. Hence thc pcrcenta~charmonic contcn t -=. 3.16. (l'hc oscillator, Type E)TS/9, is no/ srritablc for usc with the FHP13, since its harmonic conlent is likely to be greater than that of the apparatus under tcst,) (ii) Chcck oscillator for zcro bcal and calibrate amplifier detector in accordance with instructions given in Sections 9 and 1. tiii] PJag the tone sourcc lo the apparatus or line input ; insert a 600-ohm attenuator set for 10-db loss between apparatus or lint: output and FHP/Y input. Plug the Fig. 5.4. F~lterCircuit when Measuring Harmonics uutp~ttof FHP,!3 to the high-i~n~cdancti at I00 cls input of thc t'D/4. (iv) With key in central position, measure (iii) 100-cis Posiliarr [Fig. 5.4) thc level of thc fundamentn1 with thc In the 100-cls position, s~mil;lr condilicms appara t 11s under test sct lor normal output obtain ;IS thow intlicated al~ovr,csccl)t that thc level. Instruction S.4 Section' 5 . Pugc IL'I ss14t.d JIuy 1966
ivj \\it11 h.).III I- kc-:^ IIO~IIIOII.IIIV.ISII~Y [IN levtl of the total harrnol~iccontcnl. (vl) Hcpt (iv) ;it 100 cis, thrn fv) w~tti 1111: key in the IOU-c:s posir lull. For normal ro~~tir~rt(sts, it is snfficrent 10 tnrrrer the levels upon the apprupriate forms. I11 cases where thr r:sults art: tu 1x: expressed in tcrrns of percentage distortion, thc method of c;~lcula~~o~r described in the text of this Ii~struction shollld br used.
Checking Filter Characteristics If. for any reason. 11 I)eco~n~qsneccsa;Irg IU cl~c-ck the filter characteristics, it is vsscntial ll~irlpn- cautik are taken ~IIchnin;itc l~arn~onicsfrom !he tone source. I The following tahlc. indiua~c.~1~'plcilI losb fre(411encychnractrrist ics for hot h rangrs.
FHP/3A and PFHPI3A Thcse units and the FHP13 are identical clec- trically but the A versions are physically smaller. The PFHPj3A is a portable form which is mounted in a wooden carryiug case. Instruction 5.4
SCALE 1:2 Fig, 6.1. Face Pariel OBT.2
Fig. 6.2. Atrenuator Circuit OBT 2
ti. I Instruction S.4 Section 6
Attenuator Panel. Fig. 6.2 which is 600 ohms. When using this circuit for This consists of a fixed 50-db attenuator, measuring levels from source impedances other permanently in circuit, followed by four other than 187 ohms (600/240 x 75), slight errors in the attenuators of values 4, 8, 20 and 40 db. Any one indicated levels will obtain because of the changing or two of these can be switched into circuit by meter impedance referred to previously. These operating the appropriate key; thus the total errors are not likely to exceed .25 db at the attenuation can be varied from a minimum of extreme scale readings. 50 db (both keys central) to a maximum of 110 db. Because the meter impedance has been adjusted to 240 'ohms when accepting a power level of The Measuring Circuit. Fig. 6.3 + 4 db, this reading will be accurate whatever The meter of the measurin~circuit is scaled to the source impedance mav be. This is not auite give accurate levels in term; of power when a true for the 660-ohm circht, for while the chhnge source impedance of 75 ohms is connected to the from 240 to 600 ohms impedance is arranged to
f@-, T? BE ADJUSTED ON TEST
Fig. 6.3. Measurlng Clrcuit OBT/2
240-ohm input jack. This meets the requirements be accurate (by a suitable change in transformer for measuring the power output level of the OBA/8 turns ritio), 'aWslighterror occu;s because of the amplifier, which has an output impedance of extra winding resistance of the transformer ; for 75 ohms. normal purposes, however, the error is too small to The meter itself is a rectifier instrument, the he taken into account. impedance of which changes when the power To summarise, the measuring circuit of the accepted'by it changes, i.e., the meter impedance OBT/2 gives accurate power-level readings ivhen falls as the current through it rises. To minimise the output of an OBA/8 is plugged to the 240-ohm this change in impedance with different applied jack, and approximate power levels when source power levels, the meter circuit is built out with impedances other than 75 ohms are plugged to the swamping resistances. When using the 240-ohm 240-ohm jack, or when the 600-ohm circuit is circuit, the impedance transfer in the transformer used. is such that the apparent meter impedance is .When using the unit, two facts should be 240 ohms exactly, when accepting a power level remembered. First, if the programme-meter of $: 4 db. When accepting other levels, there is circuit of the OBA/8 is correctly calibrated, with a slight change in meter impedance which varies the output switch at + 4, the programme meter from 268 ohms at - 4 db to 230 ohms at + 12 db. will read 4 when the meter on the OBT/2 reads This change in meter impedance does not affect + 4 db. Under these conditions, the output the accuracy of the calibrated meter readings so zloltage level will be zero. long as the source impedance is 75 ohms (which is Second, when the 600-ohm meter circuit is used the normal output impedance of the ORA/8 for testing other amplifiers, the indicated power amplifier). level will agree with the working output voltage To extend the use of the instrument, a second level only when the normal load impedance for circuit is provided, the nominal impedance of that amplifier is 600 ohms. Instruction S.4
SECTION 7
FIXED-FREQUENCY OSCILLATORS
OSCILLATOR OSj9 output signal to negative by mcans of a The Fixed-frequency Oscillator OSj9 was \Vcstinghousc metal rcdificr, Type WXG, designed to supply 1-kc,'s tone at a fixed levei the rcclified signal being smoothed by the of zero db for LirJng up programme metcrs, filler circuit comprising C3, CH, R5, the amplificr detectors and programme chains. Its signal being fcd to the rcctificr via C7. frequcricy has now been changed to 900 c,k, to (iii) Xegative Feedback. avoid confusion with the trar~snlitt er frequcncy- Kcgative current feedback is obtained checking tone which has a frequency of 1 kc/s. by orrksion of the decoupling capacitor Moreover, its frrnction has become IargcIy that from the bias rcsistor R7. of a general-purpose tone source of fixcd frequency *- L and reasonably corlstan t output level. Oul/wt SCngc The oscillator design includes feat urcs which The oscillator stage is rcsistancc-cnpacitmce ensure that both frequency arid output level are co1.1pled to the output stage, gain control being almost en tirely independent of fluctuations in efiected by the pre-set potentiometer 1213. h'cga- supply voltages or of valve changes. In addition, tive voltage feedback is applied by means of a the ontpr~timpedance is sufficiently low to ensure third wilding on the tral~sforrnrr,thc purpose of that the output level remains sensibly constant the feedback bei~igto rcduce thc output impedance irrespective of the Ioad impdame. It is thus of the valve to 8 kilohms. possible to feed the output of the osciIlator into The transformer ratio is such that the effective any amplifier whatever its input impedance may output impedance is 1.35 ohms. With this low be, without the stabilised zero level of the output impedaiicc the oscillator, if set to deliver oscillator being affected. zero level in to an open circuit, wiIl suffer a change oionly 0.1 db, approximately, for n load impedance Circuit Description (El$. 4) . of 100 ohms. Since the oscillator i:; ~nlikrlyto be The oscillator comp~isestwo stages, the oscil- loaded with an impedance lower thiirl 100 ohms, lator proper and the output stagc, each stage ' the output Icvcl, once sct at the rcquired value, using a single valve, Type ACiSP3B. remains effcctivcly constant. 111 addition to maintaiiiing a leu-impedance Oscillntor Slnge output, the feedback to this stirgc malies the The oscillator VI employs resistancc-capacitance stage gain substantially il~tlep~nrltmtc~f changes . feedback coopling between grid and anode in voltagc supplies nr c>f v;~lvercpl;~ccm~r~ts. ._ circuits. The anode is coupled to the grid via the capacitors Cf to C4, the circuit being com- H.7'. cwd L.T. S.ufip!ie~ pleted via shunt resistors RI to R4. R2 takes the The OSj9 is dcsipwrl to w)rk from tkilher fo~m~nf a preiset variable 0.5-megohm potentio- battery or mains suppll*. For Iwttery s~rpplj-,the meter the function of which is to provide 1i.t. is taken from a 250 or 3013-volt bat tcry and irequeiicy adjustment on test. 1.t. from a &volt accnmi~lator. The following circuit arrangcmcn ts are used to 111 cases where the unit is rnomtcd on an a.c. maintain stability of frequency and of output level test bav, supplies arc takrri from a stidarrl mains of the oscil1,itor stage :- wit MU/16. (i) Constant Screen Volts. In the event of n c11i1ng.c-over from I~i~tlcryto The voltage to the screen grid of the ~nainssupply, the L.T. circuit of tl~c;~nlldifier must oscillator valve is corrtrolled by a neon be modified, resistor R2-I being short-circuited voltage stabiliser. hy the adjustmcur of its slider a~iilrr,sistors R25 (ii) Grid Volts Control. and R26 joincrl 1,:. strirpping tags 17 mtl 18, the Ncgntive bias for the colitrol of grid volts junctior~lwing cstimtlcd to tng 13 (lu~fi;~livv11.1.) is ohtninrd by rvstifying thr nssillatnr lo pmvidr an t*nrthtd cvntr~pni~if. Instruction S.4 Section 7
Meter Circuits cautions having been taken lo maintain stability Facilities are provided for switching the meter of frequency and of output Icvrl. across sl~untsto measure the foUowing feeds : V1 Anode, V1 Screen, V2 Anode, V2 Screen, Total Circuit Description (F1g. 5) Feed, Filanlent Volts (d.c. only). Referring to Fig. 5, it will Ilc srcn that the In some cases, where a portable meter is pro- oscillator stage is of the ferdback type and is vided at the statibn, the osciUator is not fitted similar to that of OS/9, anode to grid coupling with a fecd-meter, lhe meter terminations being consisting of the feedback chain CIL-C5, R1-R4. taken to a jack, An Elliott >ilniature edgewise The working principles of the feedback oscil- 1.5 rnilliamp type should be used for plugging into Iator have been described on page 7.1. In this this jack. section it was stated that the cor~trolbias for the oscillator grid was obtained hy rectifying Valve Data the oscilIator output signal and returning it to A node Screcn grid. In the OS/l&use is made of amptified *, Cawart Crrrrc~zf Fil. Film delayed control, the cont iol voltage being taken lrnh nrA mA Volfs An@ from the anode of the ontput stage, thus ensuring - Stage 1, that the control compensates for level change over AC 1s P3B 0.4 1-75 4 1 the whole circuit, and not merely for levcl changes (incl. neon) in the oscillator itself. Stage 2, The delay voltage is obtained from the. resistors AC/S P3B 4 4 1.75 4 1 across the neon tube, and the signal obtained from Total Feed, 84mA. the output via C 1.4 is balanced against this delay H.T. Supply, 250 or 300 V. voltage ; the poten.Lia1 difference thus obtained is L.T. Supply, 4 V ax. or 6 V d.c. rectified and used to control the grid voltage of the oscillator VI via the resistor R5. General Data Additional oscillator st ability is obtained by the Neon Sttlbiliser, -ED 1454. screen grid volts being stabilised hy the neon Pilot Lamp. Typ P.O. Xo. 2, 4 V. and by introducing negative current feedback by means of the bias resistor R6. Polcntimlers The output levrl from V2 can be adjusted by Adj. Frequency (R2). means of the variable resistor R14, adjustment Type, Morganite Stackpolc MNAP 50.150. of which has the effect of varying the delay voltage. Resistnrrce, XMI kR. It has been found that with this arrangement the Pre-set Gain (R13). frequency is virtually unaffected by adjustments of 'Typc, Morganite Stnckpole JiNAP 10450. the gain control, the maximum frequevcy varia- Rcs-islancz, 100 kfl tion over the lull range of the gain control being Mtttr, Miniature Edgewise ED. 1456. of the order of 1 per cent. Meter Sxitck, Yaxlcy, Type A, 2 bank, 9 pasition. 'l'he rectifier comprises three Westectors, Type -. . - -- - , ., .. WX6, connected in series, this arrangement being OSClLLATORS OS/I 0 AND OS/I OA necessary in order to maint.ain correct impedance Thc Oscillator OS,'10 is a modification of the conditions, having regard to source and load OSj9. It was originally designed to have a impedances hxcd by other parts of the circuit. frequency of 1 kc/s and an output level of Furthermore, in the initial stages of development, f 4 dh, but modern conditions have necessitated the oscillator lacked stability under certain modifications of these values to 900 c/s and temperature conditions, and thc present arrange- zero db, respectively. AC/SP3 valves are used in ment of the rectifier has considerably improved both stages. The normal output load is 60 ohms. stability at all working tempcrati~rcs. The oscillator is in general use at recording Since the output-level stability is dependent centres, where its main function is to provide upon the delay voltage, this stability ultimately a standard fixed level for the lining up of recording depends upon the neon tube prodr~cingthe delay programme-input bays. voltage. The Mullard Neon, Type 7475, was ai In general, the performance of the OS/lO is an first found to be most suitable for the purpose, improvcnlent on that of the OS!9, special pre- but suitable Osram Neons are now available. Instruction S.4 Section 7
The normal load impedance for the OS/10 is Total Feed, 11.7 mA. 60 ohms. This is made up by using. ten 600-ohm H.T. Supply, 300 V. resistors in parallel, but since under normal L.T. Supply, 4 V a.c. or 6 V d.c. conditions, the oscillator is required to feed a number of recording room input impedances of General Data 600 ohms, one of the ten shunt resistors is removed Neon St~2biliser,Mullard 7475 or Osram ED 1454. for each 600-ohm load required, the tone source Potentiometer jack in' each recording room having a 600-ohm Type, Morganite Stackpole-MNAP 10450. load across its inners to maintain correct impedance Resistafice, 100 kil. conditions at all times. OSCILLATOR OS/1 OA Operation and Maintenance Fundamentally this oscillator is similar to the Complete operating instructions for the OS/10 OS/10. The points of difference are shown in are contained on a chart attached to the bay on the circuit diagram (Fig. 6) and may be summarised which the oscillator is mounted. For the sake of as follows :- , a' economy in materials, no feed meter or jack has 1. Frequency adjustment : Provision is made been provided, but provision is made for checking for adjusting the frequency to 900 c/s by the feeds when necessary by the removal of the means of a variable potentiometer R2. accessible links connected between + HT and the 2. Metering : A meter switch is fitted instead various stages. of the loops used with the OS/10. This provides for measurement of anode and Valve Data screen currents on both valves, for total Anode Screen feed and for filament volts. The moving C,urrent Current Fil. Fil. arms of the switch are brought out to a Valve mA mA Volts Amps. feed-meter jack to be used in conjunction Stage 1, with a portable meter, 0-15 milliamps. AC/SP3B 0.7 5 4 1 3. Grid Resistor : A stabilising resistor is (incl. neon) inserted in series with the grid of V2. Stage 2, 4. Output Transformer: This is Type AAL/ AClSP3B Anode + Screen' 4 1 14RA instead of Type SA42, resulting in a Current 6 mA lower output impedance. Instruction S.4
SECTION 8
VARIABLE-FREQUENCY OSCILLATORS
OSCILLATOR TS15 fixed-frequency oscillator, with the other two The tone source TS/5 provides a source of a.c. capacitors set at zero, as an initial adjustment. test current at any desired frequency between zero The setting of capacitors 1 and 2 for any desired and 10 kc/s. frequency is then given by the calibration chart. The dials fitted to capacitors 1 and 2 are of a Circuit Description (Figs. 12 and 12A) special slow-motion type. The slow-motion knob The oscillator is of the heterodyne type and is engraved in fifths of a main dial division and consists essentially of two oscillators, one of fixed rotates once for two main-dial divisions. The drive and one of variable frequency, followed by an is a positive one and, with the large step-down gear anode-bend detector stage ; this is followed by a ratio of 100/1 care should be taken not to forcethe three-stage amplifier having two AC/Pl's in capacitor beyond the stops at either end of the U the output stage. The frequencies generated by main scale. The slow motion can be disengaged by the fixed and variable oscillators, respectively, are pulling out the slow-motion knob and re-engaged by both impressed upon the anode bend detector and pushing upwards the catch below this knob. The a component at the difference frequency is pro- arrangement of the gears is such that the slow- duced at its output. motion always re-engages correctly, relative to the The oscillators are of the self-biasing type with engraving on the main and sub-dividing scales. tuned anode circuits inductively coupled to their The output circuit of the detector is coupled to own grid circuits and to the grid circuit of the the first stage of the amplifiers via a low-pass filter anode bend detector. The coupling of the fixed circuit which is designed to suppress components frequency oscillator is made via a filter circuit at the original frequencies. which passes the fundamental frequency but Resistance-capacitance coupling is employed rejects the harmonics. -4 variable capacitor between the first two stages of the amplifier and designated Filter is included in this circuit. This resistance-capacitance-transformer coupling be- capacitor is adjusted on installation but the setting tween the second stage and the push-pull output should be checked periodically', especially after the stages. The valves are of the indirectly-heated removal of any of the screening boxes, since slight type and grid bias to the detector and amplifier deformation of these may effect the inductance of stages is obtained automatidally from resistors the coils. included in each h.t. return circuit. Volume- To carry out this adjustment TS 0111 should be control potentiometers are provided in the grid . connected to Amp. Det. In. The tone source should circuits of the first two amplifier stages for adjust- then be set to any frequency and the level adjusted, ing output. The potentiometer in the input of the \- either by means of the tone-source volume control second stage provides coarse adjnstment and that or the level switches of the amplifier detector, so in the inpnt of the first stage fine adjustment. that, with the Galvo key on the transmission A 100,000-ohm rheostat in series with the poten- measuring set thrown to Meas. Cct. or with the tiometer in the grid circuit of the second stage is Amplifier Detectov key of the thermo-couple panel set so as to give 15-volt maximum output when thrown to Adjust, the galvanometer reads approxi- the tone source is used with a transmission mately 2 scale. The filter tuning capacitor should measuring set, TM/l, and 10-volt maximum output then be adjusted to obtain a maximum reading on when TS/5 is adjusted to give 20-volt maximum the galvanometer. output into 600-ohm resistance load with the The frequency of the variable oscillator is 100-kilohm rheostat all out. adjusted by varying the shuiit capacitance in its tuned anode circuit. Three variable capacitors Valve Data are provided in this circuit, one being designated Anode Zero and the others 1 and 2 respectively. The Zevo Cuvvent Fil. Fil. adjustment capacitor is used to bring the variable- mA Volts Amps. frequency oscillator into synchronism with the Fixed Oscillator AC/P 4 4 1 Instruction S.4 Section 8
A?s ode OSCILLATOR TS/7 Current Fil. Fil. The Variable-frequency Tone Source TS/7 is mA Volts An+. a commercial product. It is a beat-frequency Variable Oscillator oscillator having a frequency range from 30 c/s to AC/P 13 4 1 14 kc/s and will deliver a maximum output of Detector AC/HL - 4 1 + 20 db into a load impedance of 600 ohms. Amplifier Stage 1, The unit comprises a fixed oscillator and a AC/P 6 4 1 variable oscillator, a portion of the output of each Amplifier Stage 2, oscillator being fed into an h.f. amplifier. AC/P 6 4 1 The h.f. amplifier is coupled to the grid of the Amplifier Stage 3, detector stage, thus producing a beat note, the 2 AC/Pl's 40 4 2 frequency of which is determined by the setting Total Feed, 69 mA. of the variable oscillator control. The frequency H.T. Supply : of the beat note will be equal to the difference in Rectified a.c. 250 V. frequency between the two oscillators. Battery Supply. The high-frequency components, comprising the *- L.T. Supply, 6V (adjusted to 4 V by series oscillator frequencies and that produced by their - resistor). sum, are rejected by the low-pass filter following the detector stage. The detector stage feeds into two 1.f. stages, Volume Control the second of which feeds the output transformer. Type Resislance No. of Loss Loss on Sending levels are set by means of an H-network Sluds per stud lowest slud attenuator connected across the output trans- P.8 100kQ 21 0.1 db 2 db (Total) former. The network has a range of + 20 db to P.4 100kQ - 21 2.0 db Infinite - 50 db. (down to H.T. and 1.t. supplies are derived from a mains Stud 6) unit supplied by the makers of the tone source. Studs 6-5 3 db The unit is designed to operate from either 2001250 5-4 4 db or 110/115-volt single phase 50-c/s supplies. 4-3 6 db The tone source utilises six valves, Type 3-2 7 db AC/SP3B, which operate as follows :- 2- 1 Infinite V1 Fixed Oscillator connected for triode working. V2 Variable Oscillator connected for triode Impedances working. Qutput No. 1 Z = 300 L1 V3 H.F. Amplifier connected for pentode , . No. 2 Z = 600 R. working. Maximum output available into 600 R (harmonic V4 Anode-bend Detector connected for triode content not exceeding 1%) = 25 db approx. working. - V5 L.F. Voltage Amplifier connected for pentode Operation working. To adjust the tone-source oscillator to send at V6 L.F. Power Amplifier connected for pentode any desired frequency, the following operations working. should be carried out :- (1) Set the frequency dials I and 2 at Zero. (2) Adjust the zero dial, either by listening on headphones plugged into the output jack, Circuit Description (Fig. 13) or more accurately, by observing the milli- Fixed Oscillator ammeter in the detector feed circuits, until The grid and anode circuits of the fixed oscillator the frequency of the 'beats' does not valve V1 are inductively coupled through L1, L2, exceed one per second. a fraction of the output signal being fed to the h.f. (3) Set the frequency dials 1and 2 to the reading stage V3 via L3. Tuning is effected by C4-C7, C5 indicated in the calibration table corrc- being vxriahlc to covcr the beat frequency range of sponding to the desired frequency. 0-600 CIS. Fig. 8.1 shows two separate tuning Instruction S.4 Section 8 controls, labelled L.F. Scale arid H.F. Scalz of thc first 1.f. stage, V5. The low-pass filter is resl)ectively, the L.F. Scale control bcing associated designed lo reject the h.f. conrponenls of the with the so-called ' fixed ' mcillator. The reason detector output and to pass the 1.f. beat fre- for this arrangement is to give widc scale adjust- qi~ericicsto the 1.f. stages.
- Fig. 8.1. Face Panel TS/7
ments at low frequc~~cies: if Ihcrc wcrc no A.F. Stu~es adjustment of thc ' fixed ' oscillator tl~e h.f. The two a.f, stages are worked under pentode scale wouId be too cramped at thc lower end. co'nditions and are resistance-capacitance coupled to each other. Negative voltage feedback is injccted into the cathode circuit of VS from the I'ariabld Oscilidor anode of V6 via R2S and C36. Negative current Thc Vnriablc Oscillator, 12, c.lnploys ;i circuit fecdback is also applied by the omission of decoup- similar to that of the fised oscillator, l'hc anodc ling from the cathode resistor R26. coil L5 is loosely coupled to LG so that a fraction of In some cases it has been found ntlcesyary to the output of V2 is coupled to the grid of the h.f. introduce frequency correction at this point by stagc V3. connccling a small capacitor with series resistance across R36, the effect being to reduce the feedback H.F. Slugc at the nppy frequencies, The h.f. Stage. V3, comprises n straightforward The outyjut of V6 is choke-capacitance coupled to aperiodic amplifier, cholte-capacitance coupled to thc output transformer TI. The secondary wind- the grid of the detector stagcb. ing of the output transformer is tapped to give output impedanccs of 75 ohms and 150 ohms. Under normal conditons the 75-ohm tap is used. Ddeclor Slagc The Detector, V4, functions as an anode-bend Orrf~rtlMzter and Sending Circrril detector. The anode circuit is coupled to thc The secondary of the output transformer is hrst 1.f. stagc through a low-pass filter com- switched to a rectifier-type voltmeter, so that the prising L8, 1.9, C21, CY!,C23, It%, R'B. levcl at this point, at all frequencies, can he kept R23 acts as a volume control in thc grjd circuit constant at 15.5 volts ($- 26 db). Instruction S.4 Section 8
Across this point is shunted a 1.2 kilohm fixed Both oscillator and mains unit are provided resistor, R31, and an H-type 600-ohm variable with 4-pin sockets, connection being made by at tenuator, R34, built out to 1.2 kilohm impedance means of double-ended 4-pin plugs. by means of a 300-ohm resistor in each leg (R32 and R33). 'The output transformer is thus Valve Data loaded with 600 ohms and the loss between this A node Screen point and the attenuator input is 6 db, making Current Current Fil. Fil. the level at the attenuator input + 20 db relative Valve mA mA Volts Amps. to ,775 volt r.m.s. Stage 1, This arrangement is adopted so that the volt- AC/SP313 2.6 4 1 meter reads the true source or opep-circuit voltage, Stage 2, the source impedance of the unit remaining at AC/SP3U 2.8 - 4 1 600 ohms under all conditions. The attenuator is Stage 3, calibrated from + 20 db to -- 50 db sending AC/SP3B 2.2 0.75 4 1 levels into a 600-ohm load. These levels are Stage 4, absolute, provided that the level at the output of AC/SP31?, 1.03 - -1 1 a. the transformer is maintained at + 26 db and the Stage 5, load resistance at 600 ohms. In order to check AC/SP3B 1.65 0.6 4 1 these conditions, the output voltmeter can be Stage 6, switched to the secondary winding of the output AC/SP3B 15.0 7.0 4 1 transformer. Total Feed, 33.7 mA. Metering Arrangements H.T. Supply, Stages 1-5, 230 V ; Stage 6, 290 V. L.T. Supply, 4 V a.c. The meter switch is incorporated on the front panel. In position 1 it enables the output meter to Total Harmonic Co~tent be utilised for reading the h.t. volts from the mains unit and in position 2 the detector anode feed < 0.5 per cent. at 100 cis and 1 kc/s (output in milliarnps. In position 3 the meter is placed at + 20db). across the tone source output via a Westector Operation rectifier. Turn down both tuning dials to zero and adjust The full scale reading for d.c. volts is 400' (scale to zero frequency by means of the Adj. Zero readings to be multiplied by 20). controL For this operation, the meter switch Mains Supply ~n'it should be set to position 2 (measuring detector The mains Supply unit, supplied by the makers current) and the control adjusted for zero beat. of the tone source, is designed to work from 110-115 Care must be exercised that the meter indicates volts or from 200-250 volts, the primary of the true zero beat, since a condition can be obtained transformer being suitably tapped to accommodate where, if the setting is a long way from zero .these voltages. On installation the transformer frequency, the meter will appear stationary. This voltage tap is set to the voltage obtaining from may be checked by switching the meter to position the local power supply. 3 (measuring a,(:. output volts), when it will read A valve rectifier of the UUj4 typc is used in the zero for the true zero beat frequency setting. An h.t. supply circuit, the supply being split into adhtional check can be made by listening to the two sections. For anode and s.g. supplies to the output on he~dphones,with the output attenuator output stage, choke-capacitance smoothing is used, set to give masimum output. both anode and screen being supplied with the full Set frequency to 1 kc/s, check the a.c. volts output voltage, except for the voltage drop across and adjust the a.c. volts control until the meter the smoothing chokes in the mains unit, which will reads on the red line at 15.5 volts. Under this be small. condition a level of + 26 db is available at the Additional resistance-capacitance smoothing is output of the transformer. For very accurate introduced in the h.t. supply to the remaining work, the voltage should he adjusted to 15.5 at each stages. frequency. L.T. supply to all valves is taken from the For frequencies below 600 c/s set the h.f. 4-volt winding of the mains transformer. control to 0 and select the required frequency on Instructior! S.6 Section 8
'I'he to~wsnllrcr consists of a variablc oscillator, thc o~itl~~t.oi wllich is fed in push~ln~llinto the gritls of a pir ol tlotec,tors, ;uld ;I fisctl oxillator \t.l~icl~is It,(! ~:I~IJtl~c grid of n buffer stage, :i::: OII t p~tof \rli~clris connected to thc detcclor gri!; ill parallcl vi,l the centre tap o! thc G~I~PH~cfi;: a? tlw vari;tble o.;cilli~tor. Coupling brtwccu tIw te~ooscillntorsis, th~m.biri d'fectec-1 in thc dctcctor stagc, producing a bcst note, thc frcqucncy of which is dctcrrnined by I:ic sc~tingof the t~mingcapacitor of the vari:rb!r? oscillator. 'The intnotc is passtd to the output stagc via a v-fir 'l'hc ~>ush-~)~illo~~tl~~t stagc is c.;lp;iI:lc oi delivering lone to line at lcvcls oL I f 20 db, f 10 db, ;:r zcro, with regard to 0.773 voll r.m.s., the frrq~lcncyrangc bring from 10 c/s to 10 kc 5. Thc tonu source ulilises ACiSP3 valves unrlcr 1x31to& worlting conditions. t hc powcr suppl?: bcing taken from a standard mains unit, Type lIWl6.
Circuit Description (Fig. 11) l'hc VRli(lb&cOsci/htor The V;wiablc Oxillator, V 1, i- i (;,It as a separatc unit within its own screening bas ,illd co~npriscs an orthodox induct~vcly-couplrd;iri-T; and grid circuit. Instruction S.4 Section 8 provided for zero setting. The grid circuit is consist of a variable oscillator fed direct to the aperiodic. detector grids in push-pull and a fixed oscillator, .The transformer TI has an additional winding fed to the same grids in parallel via a buffer stage. which provides inductive coupling between the anode coil and the grids of the push-pull detector The Detector Stage, V4, V5 stage. A screen is inserted between this coil and the The detector stage valves V4, V5, are arranged anode coil to prevent capacitance coupling. The in push-pull. The output of the detectors com- anode circuit is decoupled by R1, Cl, additional prising the audio beat-frequkncy (produccd by decoupling being provided by C41, the high the difference between the variable and fixed capacitance of C4 1 being necessary t9 prevent inter- frequencies) and the unwanted higher frequencies, ference from the 50-c/s mains supply. C41 is is fed to the low-pass filter which is designed to mounted outside the screening box becnuse of reject all frequencies above 30 kc/s. limitation in space. In order to compensate for the small loss's introduced by the low-pass filter at the higher The Fixed Oscillator, V2 audio frequencies, the equalising networks C37, The Fixed Oscillator, V2, is mounted in a R31, and C38, R32 have been included. *' separate screening box, its circuit arrangement being similar to that of the variable oscillator The Out$z~t Stage, V6, Y7 except for the omission of the variable tuning The input to the push-pull output valvrs V6, capacitor. A small variable capacitor is fitted to V7 is taken direct from the output of the fi:tcr. provide a frequency increment of & 50 c/s. This The anodes are connected to the h.t. supply provision supplements the normal tuning arrange- via the split primary of the output transformer T4, ments to the extent that frequencies lying outside the secondary winding of which is connected to the the calibrated scales of the main tuning dial may output terminals via the contacts of the Loss be obtained by suitable adjustment of the Pad key and the top travellers of the meter increment capacitor. Its application is explained key. The cathode return circuit is taken via the under the heading ' Operation.' centre-tapped feedback winding of T4 and the The oscillator is tuned at a frequency of 100 kc/s bias resistances R29, R30. Since R29, R30 are by the capacitor C12 and the primary of the shunted by the small capacitance by-pass capaci- transformer T2, the output coil of T2 being fed tors C33, C34, current feedback as well as voltage into the grid of the buffer stage. feedback is applied to this stage, the current feedback being reduced at the upper frequencies. The Bttjfer Stage A rather flat acceptor circuit, L5, C35, is con- The inclusion of a buffer or separator stage in nected across the output winding of T4, the a beat-frequency cscillator is an effective means circuit being made to resonate at approximately of separating the two oscillators and thus pre- 100 kc/s in order to supprcss any remaining venting direct coupling between the two. interference from the primary oscillators. The output of the fixed oscillator is connected to the input of the buffer stage, the output of the The Ozd+at Circuit latter being connected to the gr' s of the detector The secondary winding of the output trans- valves V4, V5.. Since a valve is interposedu' between former, T4, is taken to the travellers of a two the fixed and variable oscillntor outputs, direct position Loss Pad key. With this key in the coupling between the two oscillators is eliminated. central position the maximum output level, as The grid of V3 is connected to the output coil determined by the setting of the buffer stage of the fixed oscillator via the potential divider Adj. Level control, is available for sending to line. R15, P1, the latter acting as a volume control. With the Adj. Lmel control in the maximum The anode of V3 is tuned to the frequency of the position, this level is -1 20 db. fixed oscillator and connected to the detector A 10-db loss pad is connected across one pair of grids through the centre tap of the output coil of contacts of the Loss Pad key. An output levcl of the variable oscillator. Anode and screen grid zero db may be obtained by setting this key decoupling is similar to that in the oscillator stages central, adjusting the Adj. Leoel control for a meter except that the additional anode decoupling has rt,ading of + 10 db and thcn setting thc key tc, not been included. The circuits so far described 10 db Loss. Instruction S.4 Section 8
The Meter Circuits Operation The output of the Loss Pad key is taken to the To operate thc tone source, switch on the mains top travellers of the Meter key which, in the unit and proceed as follows :--- central position, is connected to the output 7'0 Check Zero : Sct the main tuning and fre- terminals. In this position, the meter itself quency-increment dials to zero. Set Meter key to is disconnected. In the V600 position of the mcter V-600 ohms and Loss Pad key to 0 db. Adjust the key, the output of the tone source is connected Adj. Zero control for zero beat on the meter. across the .meter rectifier, the output terminals To Check Level: Set the main tuning dial to being disconnected. 1 kc/s, leaving the meter and Loss Pad keys in the The meter is calibrated for output levels of V-600 and 0-db positions. Adjust the Adj. Level either + 20 db or + 10 db, so that with the control to the buffcr stage to obtain a reading Loss Pad key in the central position and the Meter of + 20 db or + 10 db on the meter as required. key in the V600 position, either of these levels may be obtained by suitable adjustment of the Adj. Level control. As previously stated, an output Sending Tone to Line level of zero db may be obtained by placing the Carry out adjustments as indicated above.*' 1 Loss Pad key to 10 db Loss and adjusting the Adj. For a sending level of 4- 20 db or + 10 db set W Level control for a reading of + 10 db on thc the Loss Pad key to the central (zero attenuation) meter. In the Feeds position, the meter is con- positio~~.Set the Meter key to 7'-600 and adjust nected to the Meter Selector switch without the buffer stage Adj. Level control until the interrupting the output circuit of the tone source. required level is indicated on the meter. In this position, therefore, the meter can be Re-set the Meter key to Lirre position. connected across the shunt resistors corresponding For a sending level of zero db set the Loss Pad to the various anodes and screen grids, by the key to 0 and adjust the Adj. Level control for a manipulation of the Meter Selector switch. meter reading of + 10 dl). Thcn set the key to 10 db Loss. Re-set Meter key to Liwe position. Valve Data It is important to note that the above adjust- Anode Screen ments obtain for an output load of 600 ohms only. C~rrertt Current I://. Fil. In cases where the tone source is required to feed Valve mA wzA I-011s Amps. into a'load other than 600 ohms, the output level Stage 1, should first be obtained 011 an amplifier detector or AC/SP3B .9 .3 4 1 other suitable measuring instrument. Stage, 2 For scnding levels between 0 db and - 55 db AC/SP313 -9 .3 4 1 use should be madc of the variable attennator Stage 3, AT119. Plug the TS/8 to the AT119 input. Set AC/SP31J .9 -3 4 1 the AT119 controls to the attenuation required. Stage 4, - u For levels below 55 db, it is necessary to .SC /SP3H 2.6 .8 4 1 check the output level on the AD/4. Set the TS/8 Stage 5, LossPad key to 10-db loss. Plug thcoutput jack to -1C ISP3B 2.6 4 4 1 the AD!4 600-ohm input and adjust thc level Stage 6, coiltrol on the 'fS18 to obtain the additional -1C 6P3B 150 50 a 1 attenuation required. For example, to obtain a Stage 7, level of - 70 db, reduce the level of thc TS/8 AC 'SP3R 15.0 5.0 4 1 until the :II)/-I reads mid-scale with its control set Total Feed, 504 mA. nt -- 15 tlb. Kow plug the ontput'of the TS!8 to H.T. Supply, 300 1.. tlitb AT.19, wit11 thc AT'19 set for n~oximunl L.T. Supply, 4 Y n.c. attcbnuntion, i.c., - 55 dl,. ,. 1hc main tuning dial hxs two sets of calibr;~tions, cmc in the form of an ivorinc scale, calibrated for Tobal H(lrmoicic Co~rttnb standard frccluciicv rulis, xntl thc. 0-450 scalc. c~~gra\wl tlw chi itsr.li. 1:or 1wr1na1frcq~~tmc\- rms, thc I;wqrrrrrr\~I~icrrnrrrrt did shol~ldremain Instruction S.4 Section 8
at zero, tuning being effected by the main cali- Checking Feeds brated tuning dial. Set Meter key to H.T. Current and Meter Switch For miscellaneous frequencies or, where great to the appropriate setting. A table for these accuracy is required, the 0-450 degree scale must settings is given below :- be used in conjunction with the calibration chart mA. provided with the tone source. A. Fixed Oscillator Anode and S.G. Current 1.2 To obtain frequencies not listed on the chart, B. Variable ,, ,, ,I 1.2
set the 0-450 degree scale to the nearest listed C.Buffer ,, ,, I) 1.2 frequency and adjust the Frequency Increment D. Anode Current Detector, V4 2.6 dial accordingly ; e.g., for a frequency of 256 c/s, E. ,, ,, V5 2.6 supposing the nearest listed calibration to be F. S.G. ,, V4 0.8 248 CIS,set the main tuning dial to the listed scale G. ,, ,, v5 0.8 reading, and the increment dial to 8 on the red H. Anode ,, Output V6 15 scale. (For a-frequency of 244 CIS, the increment I. ,, v7 15 dial'would be set to 4 on the white scale). J. S.G. . . V6 5- K. 8. , v.7 3 S' Instruction S.4 Section 8
OSCILLATOR TS/9 Introduction amplifier section V1, V2, and an amplifier section, The TS/9 is a rack-mounted RC oscillator V3, V4. The oscillator-amplifier section is designed intended primarily for use in test rooms and con- on Wien-bridge principles, with amplitude limita- trol rooms. The frequency range extends from tion by a negative-feedback circuit, employing a 40 c/s to 40 kc/s and is continuously covered special resistance lamp, connected in the cathode in three ranges, but the third range is regarded as circuit of the first valve. This circuit operates in satisfactory only up to 20 kc/s., The output the same way as that of valves V1 and V2 in the. level can be adjusted between - 50 db and + 20 db PTS/13 and referencc should be made to p. 9.15 by two variable attenuators, one operating in for further details of the circuit. 10-db steps and the other in 1-db steps, and a The output of V2 is coupled by the variablc continuousky variable control with a range of gain-control to an amplifier section comprising
Fig. 8.3. Tone Source TS/9 : Face Panel
6 db which, in conjunction with an output meter valves V3 and V4. This section has considerable calibrated in 0.1-db divisions, can be used to give negative feedback to give good linearity and the accurate output settings. The output impedance output is connected to the two variable attenuators, is 600 ohms. one having 10-db and the other 1-db steps. (Fig. 8.4.) .Mechanical Design The oscillator is built on a 68-inch standard Circuit Description panel 22 inches wide which carries the fvequency Oscillalor-ampl#er Seclion (Fig. 19) control, range control, adjusl oulpzd control, This section consists of an EF50 (Vl) conven- b oulpul level attenuators, feed-measuvzng switch, tionally RC-coupled to an EF55 (V2), both valves output meter and indicator lamp. (Fig. 8.3). being connected as pentodes. These two valves The valves and most of the components are are capable of considerably more than the 10-db mounted on a conventional horizontal chassis voltage gain necessary to overcome the attenuation which extends for about two-thirds of the panel of the frcqu;:ncy-determining network and to width, leaving a space at one end for the variable maintain oscillation. The gain is, however, reduced capacitor (frequency control), the range switch and to the required amount by the negative-feedback other components in the frequency-determining circuit connected between V2 anode and Vl network, all of which are totally enclosed in a cathode and consisting of R11 and the resistance rectangular screening box. lamp. The resistance of this lamp increases greatly An external mains unit is requircd and the with increase of the current through it ; in this oscillator output, h.t. and 1.t. connections are way, the lamp limits the amplitude of oscillation made via a tag strip behind the panel and at one built up by positive feedback before this limitation end. is imposed by the curvature of the oscillator valve Electrical Design (Fig. 19) characteristic. In addition to the voltage feedback The TS/9 consists of two parts, the oscillator- V2 also has about 6-db current feedback due to the Instruction S.4 Section 8
bias resistor 1115 .which is not deconpled. The quency sweep over each range. On each range, large amount of total feedback gives this oscillator- C is trimmed to give a 10 : 1 changc in capacitance amplifier section linearity of amplitude/frequency (and hence in frcquency) and the values of R also characteristic and output waveform, and minimises decrease in 10 : 1 steps as the range switch is harmonic distortion of the signal generated at advanced towards the higher frequencies. By V2 anode. careful trimming of the variable capacitors and by
60-db FREQUENCY OSUI-LATOR AMPLIFIER 10-db DETERMINING ' ATTENUATOR ATTENUATOR - 0 NETWORK SECTION SECTION 7 10-db STEPS I-dbSTEPS OUTPUT Fprn;; I NEGATIVE] g~g~1 NE~EI FEEDBACK FOR FEEDBACK FOR AMPLITUDE GOOD LINEARITY STABlLlSATlON Fig 8.4. Tone Source TS/9 : Block Schematic *'
Frequency-determining Network use of close-tolerance resistors, a single scale on The basic form of the frequency-determining the tuning dial holds accurate for all three ranges. network is shown in Fig. 8.5. Each of the capaci- The range switch is calibrated as a multiplier tors, C1, C2, consists of two 500-ppF variable and values of R for individual ranges are as below. capacitors ; i.e., C1 = C2 = 0.001pF max, and Switch Selling Range Value of R the whole four units arc ganged to form the main x I 40 CIS - 400 CIS 3.3 M a frequency control. x10 400 C!S - 4kc/s 330 k a xl00 4 kc/s - 40 kc/s 33 k L2 1- 1- V2 ANODE In the Wien-bridge circuit (Fig. 8.5) the frame of the variable capacitor must be connected to the grid of V1. Thus the frame must be insulated from VI GRlO chassis and the inevitable frame-chassis capacitance appears in parallel with C2, i.e., between grid and earth;upsetting the equality between C1 and C2. EARTH This difficulty could be overcome by trimming Flg. 8.5. Tone Source TS/9 : Basic form of C1 but this might so reduce the ration of maximum Frequency-determinlng Network to minimum capacitance that the 10 : 1 frequency ratio could not be obtained. To avoid this, the The ~esistors,R1, R2, are used for frequency frame chassis capacitance is considerably reduced range control ; each has three different values, by use of an electrostatic screen, consisting of two selected by the range switch, but for all positions 16 s.w.g. conductors in thc shape of a V placed of the switch, R1 = R2. between the capacitor frame and the chassis and There are, therefore, two circuits, each having connected to the cathode of V1 which is, of course, similar component values, and in the explanation at signal potential. Note. The theory of this that follows, C and R apply to both circuits. circuit is atlalysed on page A. 1. The oscillation frequency f is that for which the network gives zero phase-shift between the anode Amplifier Section of V2 and the grid of V1 ; it is given by The amplitude of the signal at the anode of V2 1 f=-- is of the order of 20 volts, too large to be applied 2 TRC directly to the grid of V3. It is therefore attenu- Frequency may thus be varied by altering the ated by the 47-ka resistor R18, the fixed 1-ka value of either C or R, or of both C and R. The resistor R17 and the 1-ka continuously variable circuit diagram, Fig. 19, shows that R is variable gain-control R16. These resistor values ensure in three steps to give three frequency ranges, and that the signal fed to V3 is not too large and that that C is continuously variable to give the fre- the range of R16 is limited to approximately 6 db. Instruction S.4 Section 8
V3 is an EF50, used as a pentode, and has a high of the four valves. The meter scale is calibrated in value of anode load (100 kQ) to give high voltage 0.1-db intervals from - I db to +- 1 db. with a gain. V3 is coupled by C16 and R21 to the output mid-scale zero. The meter switch is normally left at pentode V4, an EF55, the output of which is choke- the meamye outpzd position and the values of R32, capacitance coupled to the output transformer T1. R33, R34 are chosen so that when the adjust Voltage feedback is applied over the amplifying oa4l$ul control is set to give a mid-scale reading on section by means of the resistors R28 (100 kQ) the meter, the output of the oscillator is at zero and R22 (1 kQ), R22 also functioning as bias level. A 250-pF capacitor C11 is connected across resistor for V3. To keep the value of the feedback the meter to make the readings steady at low voltage correct down to zero frequency, no capaci- frequencies. tance is included in the feedback hop, and a small In position I, V1 .feed, the feed-measuring direct current therefore flows through the loop switch connects the meter across RIO in the h.t. from the h.t. supply; the steady p.d, produced feed to V1, and the value of R10 is chosen to give by this current in passing through R22 is used to a mid-scale reading., when the feed is normal. Simi- bias V3. 'TO make good certain losses at the high larly the feeds of the other valves are measurgd frequencies R22 is shunted by a 0.001pF capacitor by operating S2 to'connect the meter across other C17 which reduces the efiective value of at high feed-measuring resistors, the values of which are frequencies and maintains a level response over the chosen in each case to give mid-scale reading for wanted frequency range. Some current feedback a normal feed. is applied to V4 by the bias resistor R29 which is When the meter is reading d.c. feeds, R35 is not decoupled. connected across the rectifier ; this is to keep the The output of V4 is coupled to a bridge-type total anode resistance of the last valve constant. copper-oxide meter rectifier W1 via an RC network and thus prevent a high .voltage occurring across comprising C20, R32, C21, R33, C22 which is neces- the rectifier. sary to ensure accuracy in the meter readings at the higher frequencies. Valve Data Valve V 1 V2 V3 V4 -4 ltenualor Secliox TYpe EF50 EF55 EF50 EF55 The secondary winding of the transformer T1 is in two equal parts connected to provide a balanced Powev Supfdies Required source for the attenuator which follows it. The two H.T. 300 v at 50 mA. halves of the winding are connected together via a L.T. 6.3 at 2.6 A. parallel network R31, C23, the values of which are chosen to ofiset the high-frequency loss due to the General Data leakage inductance of the transformer. This loss Ozrtp7c.f Impedance is not corrected by negative feedback because the Output Level Output Impedance 'transformer is not included within the feedback i-20 db 600 i 60 loop. $- 10 db 600 i 18 The two attenuators are of the bridged-H type, zero 600 6 having a ccmstant and balanced input and output Amfil~itrtdeCharucferislic Refirred lo 1,000 c/s impedance of 600 ohms. The first attenuator, 81, 40 c/s to 10 kc/s, within 1 0.3 db. has 8 studs 'giving 0-60-db attenuation in 10-db 10 kc/s to 20 kc/s, within 1.0 db. steps and a position of infinite attenuation. The second attenuator, A2, has 11 studs giving 0-10-db attenuation in 1-db steps. Thus by use of both Harrnonic Distovfion attenuators, the attenuation can be varied in 1-db Total steps over a range of 70 db. Frequency CIS Harmonic Dist,ortion 40 1% Meter Facilities 60 0.5% The oscillator is fitted with a milliammeter 100 0.3% which by operation of the meter switch, S2, can bc 1-20 kc/s 0.2% used for measuring output level or 11.t. feeds of any See also page A. 1. Instruction S.4 Section 8
TONE SOURCES TS/10 AND TS/lOP Introduction The mains unit is mounted behind the right-hand Tone So1lrc.e TS/l.O is a variable-frequency RC end portion of the panel, to the right of the output oscillator of the Wien-bridge type intended for use attenuators, and the fuses and indicating lamp are in test rooms. and control rooms. It has three the the panel. frequency ranges covering 20 c/s to 20 kc/s and a normal maximum output of + 18 dB into 600 Circuit Description (Fig. 28) ohms which may be increased when required to Oscillator Stages 4-20dRover most of the frequency range. Anoutput The RC oscillator is basically of the Wien-bridge
..,.I. Flg. 8.6. Tone Sources TSIIO and TSIIOP: Face Pa,-tel attenuator controlled by a three-position switch type described in Appendix A and used in other gives 40 dB, 0 dB, and 20 dB reduction in output BBL Lone sources. Valves Vla (4 LL455) and L'2 level, and 18 dB variation in 2-dB steps is provided (CV2127) are used for this purpose and the output by a stud-type variable attenuator. The output of the oscillator is fed via an amplifying stage Vlb level with no attenuation in circuit can beaccurately (4 CV455) to the phase-splitting push-pull output adjusted to + 18 dB by means of a continuously- stage V3 (CV455). variable control in conjunction with a calibrated Positive feedback is applied from the anode of V2 red mark on the feed meter used as an output to the grid of V 1 a via the series-reactance arm oft he meter. bridge as in nther tone sources and as described in The unit is mains-operated and is mounted on a Appendix A. Negative feedback to limit the 54 in. x' 19 in, panel. For portable use it is fitted in amplitude of the oscillations is applied via a a case and is then coded TS/lOP. thermistor THl instead of using a lamp as the TS/lO weighs 19 lb and the TS/lOP weighs non-linear resistor; the fixed resistance arm of the 224 lb. bridge is provided by the resistor R19 connected in the cathode circuit of Vla. This is the arrangement General Arrangement used in the 'PTS/16, and reversal of the 1inear and The layout of the front panel is shown in Fig. 8.6. nnn-linear re~ictanrcarm< frnm th~nrranccmpnt The general construction is similar to that of the shown in Appendix'A is necessary because a TS/9. The components used in the frequency-deter- LIICI'II~I~LUI. has a Iiegallvt: resistancelten~perature mining network are mounted in a screened coefficient whereas that of a lamp is positive. compartment behind the dial of the two ganged The output from the bridge is fed to the amplify- variable capacitors which is calibrated 200 to 2000. ing valve Vl b via a second fiemistor TH2 and a Multiplying factors are introduced by the frequency resistance network. This thermistor helps to range switch on the left of the dial to give the correct variations in output level caused by the three frequency ranges of 20 c/s to 200 c/s, 200 c/s effect of ambient temperature changes on THl, and tn 3 kc/s, and 2 kc/s to 20 kc/s. t lic two t hcnnistnrs arc therefore mounted clnse Instruction S.4 Section 8
together so that they have tlie samc ambient the output attenuator. The resistor K57 in series temperature. with the bridge rectifier is adjusted on test so that The resistance network includes two variable the red index line on the meter scale corresponds to resistors R22 and R23 which are used to adjust the an output level of + 18 dB when the tone source is maximum output of the tone source to exactly terminated by 600 ohms and the attenuator switch + 18 dB. With the ADJUST LEVEL control resistor and the variable attenuator dial are set to read R23 set at maximum, R22 is pre-set to give a zero and + 18 dB respectively i.e. the attenuation maximum output of + 21 dB into 600 ohms; a is zero. maximum output of' + 18 dB is then obtained by adjusting R23 so that the pointer of the output Outfiut Imfiedance Arrangements meter coincides with thc calibrated red line on thc A purely resistive output impedance of 600 meter scale. ohms is obtained from the two resistors R58 and R59, and the meter circuit indicates the e.m.f. in Frequency-delermi~lingNelwork series with it. Because of the impedance of the The theory of the frequency-determining network amplifier output stage this e.m.f., and therefore the is described in Appendix A. The resistors in the meter reading, will depend on the impedance *. , two arms are adjusted on trst to give values of presented by the attenuator and the external load. 7.15 WZ,715 kR, and 71.5 kd2 for the three positions At sending levels of zero dB and below this - of the range switch. impedance is almost constant at 600 ohms irrespec- The trimmers of the two ganged variable tive of external load variations. At higher sending capacitors CI and C2 are adjusted on test, with the levels, however, when sending to a load impedance range switch in its mid position, to give constant which varies with frequency, as for example, a output level over the required frequency range of line, care should be taken to re-adjust the level 200 to 2000 c/s, and the frequency dial is then control when necessary to keep the meter indication calibrated to read 200 to 2000. at the red mark to maintain the correct level and Multiplying factors of 1 :10 and 10:l arc in- output impedance conditions. troduccd by the othcr two positions of the range switch. General Data Amfilijier and Outfiut Stugrs Frequemy Kangcs The amplifier and output stages incorporating valves Ylb and V3 are virtually identical with those employed in .hplifier ('19 and described in Technical Instruction S3 Scrtion 21. Frcque~r~yAccuracy I 'ulve-Feed and Ozclpul Mekr Over the middle range tlie frequency should be The meter circuit is arranged to give readings at accurate to within + I %. the red indes line on the meter scale of the following At any frequency between 40 c/s and 10 kc/s the valvc modc cllrrcnts 20 % for an H.T. voltage of range-change switch should give frequency changes 300 + 15 V and an L.1'. voltage of 6.3 + 0.15 V : in the ratios 10:l and 1 :10 relative to the middle range to an accuracy of + I %. Between 20 c/s and 20 kc/s the error should not csceed + 2 %. Iiu~mouicConled At a niasimum output level of + 18 dB into GOO ollms (incter reading to red indcx mark) the total Ilarirwnic content should not exceed: Instruction S.4 Section 8
Output Level Output Balance LVithout readjustment of level the output level The o~~tputcircuit balance, with no attenuation, should remain constant with frequency over the should not be worse than : range 40 c/s to 15 kc/s to within 5 0.15 dB. - 80 dB at 1 kc/s With readjustment of level to the red mark on the meter scale the output level should similarly be - 60 dB at 10 kc/s constant to within 5 0.05 dB. Stability ri/h Temperature .arrd iIlni~~~-I*'dtr7~c Noise Changc LVith oscillation stopped by disconnecting the Over a period of one ho11: fi\.e rnin~~tesafter positive feedback circuit at C10 the noise output switching on from cold, the output Ic\.el should with no output circuit attenuation, measured on an change by not more than 0.1 dB and the freqi~ency amplifier detector should not exceed: by not more than 0.1 %. At anv frequency setting in the middle or upper A mains voltage change of 5 10% should chylge the output by not more than 0.1 dB. The cf'icct on U' ranges: frequency should be negligibly small. - 60 dB (a signal/noise ratio of 78 dB). At a setting of 200 cis on the lowest range (the worst case) : - 50 dB (a signal/noise ratio of 68 dB) Instruction S.4
SECTION 9 PORTABLE OSCILLATORS OSCILLATOR PTS/9 range of 0 to -5 db, from a source impedance of The Portable Tonr Source PTSJ9 was designed 75 or 600 ohms. for the testing of lines or equipment at placcs such as O.B. points, where a rack-mounted Circuit Descriptfon (Fig. 9) tone source is not available, The unit is compact A simplified schematic diagram, showing essen- in construction and takes its supply from 200-240 tial components only, is given in Fig. 9.2. In this
L 1 Fig. 9. I. Face Panel PTSi9 volt ax, mains. The frequency range is limited diagmm, the components are numbered to corre- lo a number of fixed frequencies from 50 c/s to spond with the equivalent component numbers in 8 kc/s, selection of which is effected by a rotary . the circuit diagram (Fig. 9). switch. The output level available is limited to a
Fig. 9.2. Simplified Circuit PTSJ9 instruction S.4 Section 9
The Oscillatov Stage In order to delay the action of the ;eta1 rectifier The oscillator stage comprises a two-stage WM8 while oscillation is building up, negative bias resistance-capacitance coupled amplifier, in which is applied to the rectifier by virtu(, of the current oscillation is produced by feeding the output of passing through R13 (Fig. 9). The control circuit V2 back to the grid of V1 through a suitable also has the effect of reducing harmonics and feedback resistor, R2, and the H.T. blocking maintaining an effective low' output impedance capacitor, C17. he frequency determining net- which is reasonably constant. work comprises the tuned circuit, L, C, which is included in the grid circuit of V2. The required The Out+ut Civcztit frequencies are obtained by including in the tuned The output circuit is transformer-fed from the circuit a number of variations of the L/C values anode circuit of V2. The output impedance is which are selected by the switches S1, S2 (Fig. 9). approximately 25 ohms, built out to 75 ohms by Fig. 9 shows the circuit arrangement of the R17, R18. An alternative output impedance of inductors and capacitors. From this it will be 600 ohms is provided by ~23,'R24. When a S' seen that either a single tapped coil of Standard 600-ohm load is connected to the 600-ohm output Telephones & Cables type, or a pair of BBC jack, the output level is reduced by 5 db, with coils are used, the tappings being brought out to respect to the level indicated on the meter during the switch S1. The oscillator was designed to line-up. work from the former, but in some cases supply A number of refinements ale incorporated in the difficulties have necessitated the use of the latter. amplifier to ensure constancy of output to within The tuning capacitors are selected by S2, and 0.25 db at all frequencies. Referring to Fig. 9, it frequency correction is introduced by S3. The will be seen that the capacitors selected by the three switches are ganged and operated from -a switch S2 tune the primart of the output trans- single control. former at 50, 100 and 250 c/s. This provision is necessary because of the inefficiency of the tuned The Contvol Civcziit circuit and the low impedance of the transformer The efficiency of the oscillator circuit varies to primary at the lower frequencies. Without this some extent with frequency, hence a control provision, the initial output of the oscillator, circuit is provided in order to maintain a constant ignoring control, would be too low. output at all frequencies. This consists of an 'The output level at 8 kc/s is also low. To anode bend detector V3, which is biased beyond compensate for this, the feedback resistance (R2) cut-off by R20. A fraction of the output of V2 is is lowered by shunting R25 across it when the fed into the grid of V3 through C18, the input frequency switch is in the 8 Itc/s position. voltage to V3 being determined by the setting of Additional levelling of the output characteristic the level control PI. is achieved by the correction network C24, R16 Before bscillation is set up the anode circuit of across the level control P1, affecting the frequency V3 has no anode current and the impedance of range from about 5 kcjs upwards, and by the the metal rectifier WM8, included in the anode value of the coupling capacitor, C18, affecting the circuit, is very high. The rectifier is shunted across 50 c/s and 100 c/s range only. the grid of V1 via C20, but since its impedance is high it has little effect on V1. As oscillation builds The Metev Civcuit up, the input voltage to V3 increases suffitiently The rectifier-type voltmeter is normally con- to make the valve operative. V3 therefore rectifies nected across the 75-ohm output circuit. For the signal, and the presence of current in the lining up the output level, depression of the anode circuit lowers the impedance of the rectifier V-600 key disconnects the meter from the output WM8. This in turn decreases the impedance of jacks and inserts a 600-ohm load, R9, across the the grid circuit of V1 and hence limits the output meter circuit. from V1. Provided that without control the output at all Power Supplies frequencies is well in excess of the required Filament supplies are taken from the centre- con trolled output, the latter will remain sensibly tapped L.T. winding of the mains transformer and constant throughout the frequency range of 50 c/s rectified by the metal rectifier L.T.8. Smoothing to 8 kc/s. is effected in two stages, choke capacitance L4, C28, Instruction S.4 Section 9
C3Q, i~ndrcsista~~cc ca1)nt itanre, 1121, IC2, C29. lcvel or -- 5 clb nu recl~~irecl.'L'he clepressio~lof Th.? working voltage of 1.5 is ubtaiwd from the tllc linc-up kcy tlisconnccts thc tonc sorrrce dropping rrsistors R21, R22. from ihtr output jacks and placcs an artific~al H.T. supplies are tnkm from thc 100-volt load of 600 ohm (119j across the or~tput of C'2. winding oi thc mains trn11sfc)rmcr ant1 rrc!ificd (3) Cllcck thc o~iiputlcvcl at all frcqunicius. If :~ythe H.T. rcciiftrr J30. Resistant v-capncitnncc tl~colltprlt level is not consi!,tent to wiihin il mooil~ingis cffcc1c.d bl- R19, C25, C26. tcrlera~wcof $ ,25 db, a rrnrrang~mentof thc The grid bins supply to thc rcctilicr \xltrc. 1'3 is wlws mnjr providc n IwtteY cornhination lor provided by 1120, which is ctmnccletl bclwccn improving co~isi~tcncy.IYO idvt mtsl ever bc L'r and - HT and shrmtett by (37. w~nm~srfiL~itlrol~/ /irsf si~ikhiq II)~thc nscill(ttor.
Voltages and Feeds Vults Fsctls AC~OSSC35. . . . 200 Through l< 19 ., . I ,:l mA 'b , C26 ... 110 ,, I1 I ... 0.3 ,, ,, C25 ... 8.5 ,, TI ... 1.0 ., , C30 ... 6.8 ,, \VSI8 ... 0-0.5 .. , , (29 . . . I *5 (whcli oscillaling) Total I-Iarmorlic Content, 2 pr ccnt approsi- mnteIy at 100 c/s and 1 kc,:s.
Operation (Ij Adjust m;~il~s-tri~nsfor~ncrtap to nearest comet mains voltage. :2) rklwss '!-GOO key :lnd, \\.it11 ll~elrequcncy switch set to 900 c,'s, ntljl~st cu~tpl~tto zcro ~nstructionS.4 Section 9
PORTABLE OSCILLATORS (conlinued)
PTS/lO . Principles of Operation The PTS/IO, which was developed for use by General Lines Department, is a battery-operated portable A simplified diagram of the PTS/IO is given in oscillator with continuously-variable frequency, Fig. 9.4 and the complete circuit in Fig. 20. from 25 c/s to 15 kc/s approximately. It has The output of the two-stage amplifier, V1, three different output impedances, 75,300 and 600 V2, is coupled back to the input through the Wien ohms, and a number of output levels in discrete bridge, in one arm of which is a lamp with a non- steps up to a maximum of + 12 db with 1 per linear currentlvoltage characteristic. The phase- cent distortion. The instrument is thus suitable shift through the amplifier is approximately~80 L for use over the complete range of line tests, the degrees per stage, or altogether about 360 degrees, only limitation on its performance being the while the bridge introduces zero phase-shift necessity to avoid long periods of continuous duty between the amplifier output and input at a fre- because of the small size of the self-contained quency fixed by the component values. Positive batteries. feedback therefore takes place at around this The oscillator employs a resistance-capacitance frequency, and pr~vided that the gain in the frequency-controlling network which takes the amplifier is sufficient to offset the loss in the bridge, form of a Wien bridge, and is connected between oscillations are generated. the output and input of a two-valve amplifier Due to the presence of the lamp, the loss occur- whose gain and phase characteristics are stabilsed ring in the bridge is dependent upon the value by negative feedback. The generated signals are of the applied voltage, and thus, for a certain gain maintained at a level well within the handling in the amplifier, the circuit may be adjusted so capacity of the amplifier by the inclusion in the that the oscillations are maintained at an amplitude bridge network of a lamp whose resistance varies which can be handled without an undue amount steeply with current. The frequency of oscillation of distortioli. If the oscillation amplitude tends is controlled in each range by varying the resistive .to alter for any reason, the loss in the bridge also elements of the bridge, and the range required is alters and lends to compensate for the change ; selected by switching the capacitive elements. the output therefore remains sensibly constant The three ranges are :-(I) 25 c/s to 250 CIS, in spite'of any variations which may occur in the (2) 250 c/s to 2.5 kc/s, and (3) 2.5 kc/s to 15 kc/s circuit external to the bridge.
' approximately, above which frequency the level The oscillatory voltage at the anode of V2 is - rapidly falls off. applied to the grid of the output valve, V3, and The single-valve octput stage feeds into one thence via a resistance bridge, used for lcvel arm of a resistance bridge, the load being connected calibration, and a variable attenuator to the across one of the bridge diagonals. The e.m.f. output terminds of the oscillator. of the generator is measured across the other The full mathematical details relating to the diagonal ; with this arrangement the value of design of tlie frequency-control and output net- load impedance does not affect thc generator works are contained in Lines Department Report e.m.f. ,and the measuring instrument is left con- No. 81.1. A simplified treatment of the Wien tinuously in circuit while testing. bridge is given below. Power is supplied to the oscillator via a non- locking plunger-key which prevents unnecessary Wien Bridge drain on the batteries ; these latter are mounted This bridge has two adjacent arms containing as a separate unit and can be replaced without resistance only; and two containing resistance disturbing the rest of the instrument. The three together with capacitance. Referring to Fig. 9.3 valves employed are all of the same type, and a the resistive arms consist of the fixed-value spare is provided. The total weight of the oscilla- resistor R, and the non-linear resistance RL pro- tor in its attach6 type carrying case is about vided by tlie lamp. One of the reactive arms 23 Ib. comprises a resistor in series with a capacitor Instruction S.4 Section 9 and the other a resistor in parallel with a capacitor ; Clrcult Description (Fig. 20) the values of these elements, denoted by R and C, Oscillator Amplifier are kept the*same for both arms at all frequency This unit comprises V1 and V2. Vl is an 1.f. settings. pentode, connected as a high-gain amplifier with a resistive 'load, and is capacitance coupled to V2. In order to stabilise the gain and to minimise the distortiori produced for a given output power, negative feedback is applied to V2 via R11. A transformer, T1, in the anode circuit of V2 steps down the valve impedance to approximately 40 ohms, with a phase-angle varying with fre- quency. The gain from the grid of V1 to the secondary of T1 is about 26 db. The filaments of V1 and V2, taking 100 mA each, are connected in parallel and are supplied from a single 1-5-volt cell.. Except for the p.d. a' across the filament, no negative grid bias is applied to V1. V2 is biased by R18 de-coupled V by C12. Flg. 9.3. Wien Bridge Incorporatlng Lamp Resistance Frequency-determiniq Network When employed as a frequency-controlling In explaining the action of this network, it will unit the bridge operates very near balance, and it first be supposed that the lamp constituting a is therefore useful to investigate the condition non-linear resistance is replaced by a normal under which balance obtains. If the branches linear resistor of value half R5. Under these containing reactance are denoted by P and Q circumstances the bridge is in balance and has then, in the notation of Fig. 9.1, the balance zero output at a frequency, f, = 1/2nRC, where condition is : R and C are the resistance and capacitance in each reactive arm. In addition, when the bridge is balanced, its phase-shift is zero. If the resistance replacing the lamp is now reduced by a small amount, there will be a certain definite output from the bridge, approxi- mately zero phase-shif t through the network. being still maintained. When an amplifier of suitable gain and zero phase-shift is connected Separating the equation into its real and between the output and input of the bridge, imaginary parts, oscillations are set up at a frequency of approxi- - mately f,. The phase-shift through the bridge -R6 - 2, and RL increases very rapidly as the frequency changes from 1/2nRC, and hence the effect of changes in oCR - l/oCR = 0. phase external to it is kept small; a further Whence advantage is that harmonics of the oscillatory frequency are fed back in a negative sense, and are thus reduced in amplitude. and since the frequency f is equal to o/2n, If the .lamp is now restored, an exactly similar condition can be set up by a suitable adjustment, with the added advantage that the circuit now tends to maintain the oscillations at a pre-deter- mined fixed level. The reason for this can be It follows that, providing R, has twice the explained by considering a possible increase in resistance of the lamp, balance occurs at a fre- the amplitude of the oscillations due, say. to an quency 1/2nRC. (See also page A. 1 .) increase in the gain of the amplifier. More current Instruction S.4 Section 9 then passes through the lamp, whose resistance Output Nelworks increases, bringing the bridge nearer balance and There are two resistance networks between increasing its loss, and thus tending to maintain the secondary of the output transformer and the the oscillations at their original amplitude. Similar output jack and plug (which are in parallel). The compensation occurs for a decrease in amplitude. first is a straightforward square-type attenuator In the F"TS/10, the Wien bridge consists of R1, with three loss settings : 0 db, 4 db and 8 db. The second is a network consisting of a resistance R2, R3, R4, C1-C6;R5 and the lamp. Rl and R2 are ganged wire-wound 100-kilohm variable bridge and appropriate resistors for changing the resistors. The fixed resistors, R3 and R4, are output impedance. connected in series with R1 and R2 in order to The bridge (Fig. 9.2) which is formed by the limit the minimum values of these latter to 6 resistors R43, R44, R45 and the resistance seen kilohms. A ten-to-one frequency change is avail- looking back toward the secondary of transformer able by varying Rl and R2, with an adequate T2, acts like a hybrid transformer in that the overlap between the three frequency ranges, which power delivered to an external circuit conneded are selected by switching the capacitances C1-C3 across one diagonal is independent of the power and C4-C6 in the reactive arms. delivered across the other, provided that the resistance ratio between the pairs of adjacent The lamp is a 6-v P.O. switchboard type No. 2 arms is the same for opposite pairs. Unequal manufactured by Ediswan, and other makes of arms are used, making the loss through the bridge lamp must not be used; due to its very robust from T2 to the output jack 2 db, and the loss to construction and the low temperature at which the meter rectifier 13 db. With the arrangement it operates it is unlikely to require replacement, employed the reading of the meter is proportional and is therefore soldered directly in the circuit. to the output e.m.f, of the oscillator and is inde- Since the temperature of the lamp does not vary pendent of the load conditions. over the time occupied by a half-cycle at any of the Both the output attenuator and the bridge frequencies generated by the oscillator, the lamp work at an impedance of 300 ohms. On the first. resistance depends on the r.m.s. v~lueof the setting of the output switch, an additional resistor, current flowing, and the lamp does not therefore R46, is connected in parallel to give an output introduce distortion. impedance of 75 ohms. The e.m.f. of the generator is adjusted so that when the meter is reading mid-scale a power of 1 mW is delivered by the Oulput Stage oscillator when a 600-ohm load is connected to the The output valve, V3, is an 1.f. pentode of the output terminals. In the second position of the same type as those used in the oscillator amplifier. output switch the bridge remains in circuit, but The filament supply is from a separate 1.5-volt the parallel resistance is replaced by a balanced-L cell and. the bias is obtained by means of R22 pad which raises the output impedance to 600 de-coupled by C15. To obtain a satisfactory ohms, and is of such a value that for the same frequency response and the required output level generated e.m.f. a power of +4 db above 1 mW with low harmonic distortion, negative voltage is delivered when a 600-ohm load is connected. feedback is applied between the anode and grid In the third position of the output switch the of V3 via R20 and C14. The grid of the valve is bridge is cut out of circuit and the transformer fed from the anode of V2 through a level-control secondary connected directly to the output ter- R34 and a fixed resistor R50 ; the purpose of this minals. In this condition the oscillator will fixed resistor is to make the degree of negative deliver + 12 db into a 300-ohm load from an feedback independent of the level-control setting, output impedance of 300 ohms. and also to ensure that, variations in the input The reason for the choice of an output impedance impedance of V3 do not affect the oscillatory of 300 ohms is that a suitable transformer, Type frequency. The transformer T2 in the anode AL/13R, is easily obtainable, enabling the oscillator circuit of the valve is connected to give an output to deliver its maximum power. It is considered impedance of 300 ohms. The value of C14 is that since the impedance of a line at 100 c/s and adjusted on test to compensate for a tendency 1 kc/s will rarely fall below 300 ohms, an oscillator towards a fall in output level at the extreme lower which will establish a power level of + 12 db in frequencies. that impedance will be adequate for giving a Instruction S.4 Section 9 reasonabk measure of overload on thc linc. 6'00 ohms, + 4, 0, -4 into 600 ohms. Rcflection loss, due to tlw impedance of the line The fourth setting ol thc output switch connccts being higher than 300 ohms, is small provided an cart11 loop to the outpul jack and plug. 'Two that the line impcdance does not exceed 1 kilohnl line jacks are providcd, each also connectcd to tlrc reflection loss with a 600-ohm line, for example terminah, so that the standard method of taking being 0.5 db. In rwing thc oscillator, it is important d.c. nieasurcments may he employed.
Fig. 9.4. Portable Oscillator PTSI I0 : Simplified Circuit
to reme~nhcr that when sending + t? dh the .Velcring '4 rj mi,tpn~.~ils mctrr is out of circuit, and thc level should thcrr- The mctcr, wh~chhas n 200-microanlp rnove- fore bc adjusted on either of the two lower settings mcnt, is insrrtetl at various points in thc circuit and the switch then turned to IIlc~lZ-dbscttin~. as required bv means of the 6-position meter Sincc thc outpnt attcnuator is connectrd bc- switch, S3. 111 the first position of the switch. tween the e.m.f. and its nreasnring circuit, thc the meter is conncctcd to the d.c. output of tlrc e.1n.f. must always be adjusted with the output instrument.rectifier across thc output bridge, attenuator set to zero. The output levels and and therefore rcads thr tone level. In tlic sccond impedances available arc then :- position the mrter is connected, through an 75 ohms, 0, -4, -8 into 600 ohm. appropriate rc.sistmce, to rncilsurc the voltage of 300 ohms, + 12, 8, 4 into 300 ohms the cell supplying the filaments of 1'1 and V2. Instruction S.4 Section 9
In the third posltion the voltage of 1'3 filament Valve Data cell is similarly measured, and in the fourth the H.T. Feed Fil. Fil. h.t.-battery voltage. In the fifth position the Valve Current mA Volts Amps anode plus screen current of V1 and V2 is measured, Stage 1 DL35 1 1.4 0.1 the current in mA being given by the mctcr reading Stage 2 DL35 3.7 1.4 0.1 multipliec! by 5. In the sixth position the anode Stage 3 DL35 2-7 1.4 0.1 and screen current of V3 is measured, the current in mA being equal in this instance to the meter Power Supplies reading multiplied by 2.5. H.T. supply, 133 V (from three 45-V batteries). L.T. supply, 1.5 V (from two separate cells, NUSE.-The multiplying factors shown on the one for V1 and V2 and the other for V3). face-plates of early models for positions 5 and 6 of S3 were incorrectly given as 4 and 2. Since Genera1 Data the meters and metering circuits on all models Frequevcy Ranges are identical, thesc markings on the early models Nominal ranges, ignoring overlaps. must be ignored. (1) 25 c/s to 250 c/s. *' U' (2) 250 c/s to 2.5 kc/s. (3) 2.5 kc/s to 15 kc/s approx. Frequency Accuracy Apart from the stability of the components Outfiut Impedances forming the reactive arms of the Wien bridge, Selected by output switch S1 there are two factors which affect the frequency 75 R. 300 R or 600 R. accuracy of the oscillator, one being the replace- ment of a valve, the other a change of filament or Oulpul Levels anode voltage. The components of the bridge are Selected by output-attenuator switch S2, and estimated to have a stability of within 1 per cent also dependent upon setting of output switch or better throughout the range of temperatures S1 and on load impedance. likely to be encountered under the conditions in Oulpul Ouput Load Oulfivt which the oscillator will be used. Impedance Allcnrtntion Impedance Level ohms d b olims d b Replacing a valve may change the frequency 75 0 600 0 because the varying input and output impedances 4 - 4 which different valves possess affect the phase- 8 - 8 shift in the amplifier ; this trouble is more likely to be serious -at the higher frequencies, since valve impedances become lower with increasing frequency. On the frequency ranges 23 c/s-250 c/s and 250 c/s-2.5 kc/s, a change of valve has negli- gible effect. On the 2.5-kc/s to 15-kc/s range 600 0 600 f 4 changing V2 has negligble effect, but changing 4 0 V1 may occasionally produce a frequency variation 8 - 4 of as much as 1 per cent at 10 kck. although the Potentiometers variation resulting from changing this val1.e is in Frequency control (Rl, R2) : Reliance dual general much smaller ganged, Type TW. Resistance of each half, 100 k62. Chwges of h.t. and 1.t. \,oltagl. ovcr the normal 1.c.vc.l control (1134) : .\lorganitv Stackpolc, range occurring d~~ringthe life of the batteries, Type 1,HNAP 104 10 24000. Kcsistance l.e.,h.t. from 135V to 110V,anrl 1.t. from 1.5V 100 kR. to 1.25 V, have negligible effect on frequency on ranges (I) and (2). On range (3) at 10 kcis an Szcilclres h.t.-voltage change of 10 per cent causes a fre- Output ($1) : Plessey rotary Typc A. quencj- drift of 0.5 per ccnt, thc drift nt lower Output attrnuator (S!?) : I'lossc~j.rotary Tyw 13, frequencies being proportionatelj, It.5~. 4-poll: 3-waj-. Instruction S.4 Section 9
Meter (S3) : Plessey rotary Type 13, 2-pole 6-way Width: 109 in. Range (S4) : Plessey rotary Type B, 3-pole Weight : 23 lb. 4-way. On-off plunger key: P.O. No. 229 (black). Test Data Orr~put-level/FrequencyCltccracteristic Meter From 50 c/s to 10 kc/s : * 0.2 db with respect 200-PA movement. E.D. 1467 to response at 1 kc/s.
Metal Rectifier Accuracy of Ouput-level Setting Westinghouse instrument-type, 1 mA. Absolute accuracy : within * 0.3 db of nominal setting. Lamp in Bridge Circuit Relative accuracy (i.e. variation with frequency) 6 V P.0.-switchboard type, No. 2. Ediswan only. -J= 0.1 db between 50 c/s and 10 kc/s.
Batteries Accuracy of Output-impedance Setting Housed in detachable battery unit. 75- R and 600- R settingi : within + 2 per cent *' H.T. supply : 135 V, obtained from three of nominal. 45-V Ever Ready deaf-aid batteries, Type 300- Q setting : within * 4 per cent of nomizal. B102, connected in series. L.T. supplies : 1.5 V, obtained from two Total Percentage Harmonic Distortion Siemens 'S ' cells, one for V1 and V2, the All output levels and impedances. other for V3. Frequency Distortion 50 c/s 3% approx. Dimensions and Weight 100 c/s < 2% Oscillator in carrying case. 1 kc/s < 1% Depth : 79 in. 7 kc/s 1% or less. Length : 164 in. See also Appendix A. Instruction S.4 Section 9
PORTABLE OSCILLATOR PTS/12
Introduction The frequency of oscillation is controlled in each The PTS/12 is a high-grade resistance-capacitance range by varying ganged capacitive elements of the oscillator designed for lines testing. It operates on Wien bridge, and the range is selected by switching 190/250-volt a.c. mains, but is otherwise an resistive elements. (The lowest range is an improved and portable version of the rack- exception to this, and is obtained by adding shunt mounted TS/9. The frequency is variable con- capacitance and making any necessary alteration tinuously from 20 c/s to 20 kc/s in four ranges, and of padding resistance.) The four frequency ranges the output level can be set to, any value between are : + 20 db and -50 db. To facilitate the testing of (1) 20 c/s to 40 c/s, transmission circuits incorporating narrow-band (2) 40 c/s to 400 c/s, J' filters, a series of small frequency increments and (3) 400 c/s to 4 kc/s, decrements can be introduced, graduated according (4) 4 kc/s to 20 kc/s. to the frequency range. The instrument has a Range (4) extends nominally to 40 kc/s, but is balanced output with a source impedance of 600 satisfactory only up to about 20 kc/s. The small ohms. frequency increments and decrements, the values
Fig. 9.5 Oscillator PTS/I2 : Face-plate Drawing No. EK 7245
The face-plate of the oscillator, with the various of which are constant within each range, are controls, is shown in Fig. 9.5. A simplified circuit obtained by additive and subtractive switching of diagram is given in Fig. 9.6 The essential features capacitors in series with the ganged capacitors. are (a) a two-stage oscillator section, embodying a The frequency settings of the Increment switch are : resistance-capacitance network of the Wien-bridge type, which has zero phase-shift at the oscillatory Frequency Deviations frequency and incorporates resistance-lamp ampli- Range (CIS) tude stabilisation, and (b) a two-stage amplifier (1) and (2) f 0.2 f 0.4 with transformer output followed by balanced (3) f2 f4 attenuators. (4) ,1 20 f 40 Instruction S.4 Section 9
Of the two output-level attenuators, one has a ever, explanation is adopted, however, it will be range of 60 db in 10 db steps, and the other a range clear that the actual behaviour of the circuit is the of 10 db in 1-db steps. Finer adjustments can be same. The mechanism of frequency control and made using a continuously-variable Adjust Level amplitude stabilisation is examined in somewhat control, placed between the oscillator section and greater detail in AppendixA, page A.1. the amplifier, in conjunction with a centre-zero The PTS/l2, apart from its portable nature, output meter, calibrated in 0.1 db steps from --I differs from the TS/9 by possessing the following db to + 1 db. . additional features :
OUTPUT rMETEI
OUTPUT VIA , Iii- 41TLNUATORS
I I fRCOUENCV - OUTPUT CONTROL OSCILLATOR - AMPLIfICR CIRCUIT HCTWOAK
I
I A I I 2 POSITIVE fII01AC* PATH
Fig. 9.6. Oscillator PTSI I2 : Simplified Circuit
Electrical Design (Fig. 21) (i) Extended frequency coverage down to General 20 CIS, The circuit of the oscillator is shown in Fig. 21. (ii) Frequellcy-increment switching, Electrically, the PTS/l2 is virtually the same (iii) A bviit-in mains unit. design as the rack-mounted TS/9; this latter instrument is fully described in Section 8 to which Extended Frequency Coverage reference should be made. The frequency-deter- The extension of the frequency coverage by the mining and amplitude-stabilising networks are of provision of a 20-c/s to 40-c/s range is accomplished a type now widely used, and their methods of by means of the 900-pF capacitors C37 and C38. functioning, considered as separate entities, are These are switched in shunt across the main tuning explained in Section 8. Together, the twonetworks capacitors C1A and ClB, thus nearly doubling the constitute a Wien bridge, and their operation on maximum available capacitance in each variable this basis is explained on page 9.5, wi'th reference arm of the Wien bridge, and halving the minimum to another similar oscillator, the PTS/lO. Which- frequency of 40 c/s obtainable on the next lowest Instruction S.4 Section 9
range. The frequency-range switch also selects General Data appropriate padding resistors RIC and R4C. Frequency Ranges and Incremeut Settings The frequency selected is read off on a dial See page 9.1 1. attached to the ganged capacitors CIA and CIB (see Note on Fig. 21), a slow-motion drive being Output Level provided for accuracy in setting. The frequency -50 db to + 20 db. dial is engraved with two separate scales, each occupying a little less than 180 degrees, and Output Impedance provided with separate gauge-marks situated on 600 R, resistive. opposite sides of the ciial. One scale is calibrated from 20 to 40 c/s and refers to range (1) while the other, calibrated from 40 to 400 c/s, is common to Test Data the remaining three ranges ; this latter scale is Output-level/Frequency Characteristic direct-reading on range (2), but the appropriate With respect to level at 1 kc/s. multiplying factor of 10 or 100 must be introduced 50 c/s to 15 kc/s, h0.1 db. for range (3) or (4). 20 c/s to 20 kc/s, h0.2 db. *' w Frequency-increment Switching Accuracy oj Frequency Setting The full details of the frequency-increment Accuracy 20 minutes after switching on. switching arrangement appear k Appendix A, Within f 1 %. and no explanation need therefore be given here. ~ccurac~oj Output-level Setting Mains Unit From + 20 db to - 10 db, f 0.1 db. The built-in mains unit is of a conventional type. Below - 10 db, depends on precautions taken The primary of the mains transformer is tapped for against longitudinal currents. (See note an incoming supply of 205, 220 or 235 volts ; the below.) lowest tapping can be used with a supply as low as 190 volts, and the highest tapping with a supply as Longitudinal Output Voltage high as 250 volts, without affecting the normal At output-transformer secondary. functioning of the oscillator. Of the order of 0.1 volt from 0.005 pF.
Mechanical Design NOTE :-The transverse component of this volt- The instrument is contained in a standard age appearing across a reasonably well-balanced duralumin box, measuring 19# in. by 9 in. by 96 in. load is negligible at output levels down to - 10 db. All the controls, together with the output terminals At low output levels, particularly at higher fre- and two output jacks, are mounted on the face- quencies, a further guard such as a screened plate, which is protected by a detachable hinged repeating-coil must be used if good accuracy is cover. A canvas carrying case is provided. The required. weight of the oscillator is 31 lb. Separation oj Total Harmonic Content jrom Signal Valve Data Level Heater Heater Valve Volts Amps Frequency Separation Stage 1 EF50 6.3 0.3 100 c/s to 20 kc/s better than 50 db Stage 2 EF55 6.3 0.95~ 50 c/s to 20 kc/s better than 40 db Stage 3 EF50 6.3 0.3 I kc/s 55 db Stage 4 EF55 6.3 0.3 100 c/s 52 db Rectifier UU6 4.0 14 20 c/s satisfactory on C.R.O.
Power Supply H,um Levd 190 to 250 volts, 50 c/s a.c. More than 50 db below signal at all output levels. Instruction S.4 Section 9
Oscillator PTSl13
Introduction 1. The oscillatbr section which includes valves The PI'S113 is a portable oscillator designed V1 and V2. primarily lor lines testing, The frequency range 2. The section including thc components HI, covered is from 40 cls to 10 kc/s and the output C1, HJ, C2; these represent the main level is adjustable to ccrtain definite valucs frcquency-dctcrmining elements. required in lines work. 'Thesc are 0 2nd + 4 db 3. Thc amplifier scction consisting of the valvc (for normal transmission measurements) and v3. + 10, + 12, + 14 and -f 20 db for: special tests 4. 'The final section including the ontput- and overload measurements. impedance and output-level switching.
Fig. 9.7. Portable Tone Source PTSI 13 : Face Punej
Two alternative output impedances, 75 ohms The Oscillator Section or 600 ohms. can be selccted by means ol a switch. The oscillator section consists of the two valves The tonc source can be operated from 50-cjs mains V I and V2 which arc convcntiorlally HC-couplcd ; using a built-in rnains unit or from external h.1. both are pntodes but V2 is strapped as a triode. and 1.t. batteries. Osciljation occilrs duc to positive fccdback obtained Thc oscillator is contained in a wooden box by colipling thc anode of V2 to the grid of V 1 via measuring approximately 164 in. x 6 in. X 9 in. one of the frcqncncy-dctcrmining networks, eg., and fitted-with a carrying handle. The wcight is K I, (11, KJ, C2. Oscillation occurs at the frequency 25 Ib. at which thc network gives zero phase shift and at A spare for each type of valve uscd in the this frequency thc network has a minimum loss oscillator is carricd inside but no spare special of 10 db ; hence the amplifier must give more than resistancc lamp is providcd bccause if this fails 10 db gain to maintain oscillation, Thc amplifier the oscillator will rcquirc adjustment when the section is capable of giving about 70 db gain but lamp is replaced. this is rcducrd to the required valuc by negative feedback, obtaincd by coupling the anode of V2 Genera1 Description of the Circuit to thc cathode of V1 by the cornponcnts C3, R17, Very briefly the P'1'S/l3 consists of a two-valve the fccdbuck voltage appearing across thc lamp KC oscillator with amplitudc stabilisation by a in the cathode circuit of V1. This lamp carries negativem lamp, the output of the oscillator being thc steady cathode current of V1, about I mA, amplified by a third valvc. For the purpose of which provides grid bias; it also carries abont explanation the simplified circuit diagram shown 5 mA ax. wher~thc amplifier is oscillating normally, in Fig. 9.8 can be convcnicntly dividcd into the and provides about 60 db negative fccdback following four sect ions which will be separately making the amplifier output-input chnractcristic considerh. practically straight and reducing waveform die- -- .- - .- Feedback circuit wli~chincludes a spwiirl rcsistancr. tortiori to a low levcl. Instruction S.4 Section 9
The lamp filament has a large tlicrni,~lcot4icicnt Thc rcstrictctl co~.cr.;r.p011 r;ingc C is obtaincd by of resistance and if, for any rcason (for example putting fixed ca1)acitors in parallel with C', and Cp. an increase in h.t. voltage), the output of 1.2 tends The operation of this oscillator circuit can also to increase, the current in the lamp also tends to be explained by regarding the componenls R1, C1, increase ; the resulting increase in lamp resistance R4, C2, R17 and the lamp as constituting a Wien causes an increase in thr negative feedback voltage bridge ; this explanation is given in an article and a decreasc in amplifier gain. In this way the by Clifford.* (See also Akperrdix A, page A.1.) lamp tends to maintain the output of V2 at a constant Icvcl irrcspective of thc frcqucncy The Amplifier Stage gcncratcd ; it also keeps the output intlependcnt The oscillations at the anode of V2 are applied of changes in supplj- voltages and of cllanges in to the grid of V3 via a potentiometer R18, the valves. The component values arc so chosen that level control, and a series resistor R20 is included the signal level rnaintaincd at tllc anode of 1.2 to isolate the output stage from the oscillator is within the power-handling capacity of V2. section. A large amount of negative voltage e 1.imitation of output is an imllortant fcature in fccclbacli is applied to V3 to enable it to deliver an oscillator required to give a11 undistortetl the maximum desired output (+20 db) with little - wa~.eform; witho~~tthis circuit or some other distortion. The feedback is applied in two ways : forni of cxtcrnd amplitude limitation, the oscilla- by returning the cathode circuit to a tertiary tions ~vo~~ld1)uiltl up uiitil the valves overloaded winding on thc output transformer T1 and by,direct and introduced harmonic distortion, i.e., ainplitude coupling by C9 antl R23 between anode and grid would be limitcrl 1)y non-linearity of the valve circuits. This large amount of feedback causes rharactcristics. 1.3 to have a vcry law effective anode a.c. rcsist- ance ; the output load has therefore a vcry small The Frequency-determining Network cffvct oli the a.c. potential at the anode and the l'hc frcq~~ciicy-dctcrniining:lc.t~\.orli i*; f~~r~tla- output Icvel measuring meter can be lcft in circuit mcnt?.lly an L-tjp, thc scrics ;Irm collsi~til~gof irrespccti~x: of thc output load. rcsistancc antl capaci1;wcc in ~;r*ricts,anrl thv sl111nt arm of rcsistancc. :~iid C;LI):IC~~;IIICCill ~)arallvl Output Impedance and Output Imel (Fig. 18). Zcro pliasc-shift occllrs at t he frcvluc.iicy Switching for which The sccondary winding of TI is in two sections 1 which arc connectt:d in series in one position of the .- - - - - . '-~T,/(R,~.~K~~'~I. output impedance switch and give an output impedance at the sccondary winding of about 180 ohms. This is raised to 600 ohms by the components R31, R32, and R33. The series ca1)acitors C30 and C21 are included to minimise the high-frequency loss caiised by leakage induct- where I< --. R, = Kp and C = C, - ('P. .111ce in the ou!pi~t transformer. The 600-ohm 'The frcclwncy of oscillatio~i is thus i~l~xm(*!!. autput across 1133 is connected to a 600-ohm proportional to K ;lnd C. Two similar vi1rial)lt. l-);tlil~cdattcnuator, the output of which is capacitors ganged I)!. an iusulatetl coupling arcs conncctrd to trle output terminals and output used for (', antl Cp and, for :l hxcd ~.alucof I(, jnc.1; in parz~llei. the capacitors arc trilnmetl to give ;I 10 : 1 changc At 111e second position of the output-impedance in frerll~cwcy. l'hp fiqucncy range is furthcr >witdl the scco11da.ry windings are connected in cxtcndctl by cha1:ging thc value of R a~idthe 1)arnllel to gi\.c an o"tput impedance of about 50 circuit corist,lnts of the PTS!13 are chosen to give ohms. This is ~ncreasetito 75 ohms by resistors thc followir~g rai1gc.s :- liX9 ant1 1-30, which arc followed by a 75-ohm
I\'trttgc I-reqttctt~;v I .trlttc o/ K attcnuator gangctl. - uith the 600-ohm attcnuator A 1-10 kc s 330 kc2 and insrrting th~*same loss as the 600-ohm attcn- J3 100 c s- 1 kc '5 3.3 Ali! t ' 40-100 c G 1.7 Jl!! Instruction S.4 Section 9
Metering Facilities the 1.t. trrminals. The mains-battery switch shoultl 'The front panel of'the oscillator carries a 2OO-pA bc used for on-qlj' switch in^. meter, the resistance of which is built out to Operation. 1 kilohm by R34. By operating thc six-position 1. On Mains. Set meter switch to ,WJ. sralss meter switch thc meter can br rlsrd to give and adjust mains switch to give the highest the following mc:wmmcnts :- - meter reading nearest to 100. H .T. L'olts On Batteries. Connect the batteries to the The nieter is conncctcd in series with R16 ant1 appropriate terminals and srt the maiw- across the h.t. supply. Thc readillg should bt, batter^ switch to BATTERY. multiplied by 2 to give the h.t. voltagc. 2. Set the range switch to the desired frcquenc~. Cutlrode Curre~rtl'3 rangr and the frequency control to the desired l'hc metcr is connected across 1128 in the cathode frequency: circuit of \3. 'lh(, rc.ading shou!fl I)r dividwl :I. Kt.! the meter switch to TOKE LEVEL and alljust by 10 to give the catllodt~current in rnilliamps. t11~~le~d control until thc meter wads 175. The meter syitch can be left in this posi&m. C'uthorlc Ctrrrenl 1.2 4 hi,: the ')i~:pll:imfedance switch to thr tlcsirrtl 'Thc meter is conncctcd across R15 In the cathod~ va!ue. circuit of V2. The reading should be dividctl hi- 5. Set the otttput lwei switch to t1w dcsiretl \~~IIIP. 10 to givc the cathode current in mil1iamj)s. (:rtthode Crrrrent 1.' 1 Valve' Data The mctcr is connected across R11 in the cathotlt. Valve Type C'trihode Curre~rt circuit of \.I. 'l'hcb rvading should be divid~dI)\ \' 1 EF51 1.0 m.i 10 to givc the cathode current in rnilliamps. V2 EF55 10.5 nlA hlnins ivoltagr A4jrrstmelit V3 EF5-I 5.5 m.-\ V-I U [!ti . - Thc meter is connrcted across the d.c, sick of ;L 'l'otal 1l.t. feed : 17 mA ;~t290 \.1,11s. bridge-type rrctiher M2, the a.c. side of which is connected across thc valvc heater supply. The General Data resistor R36 in series with the rcctificr source input Outpd Impedance is so adjusted that the metcr reads mid-scale when \Vith output switch set to 600 olri~rs, the tapping switch at thc mains trnnsformer 600 ohms i 10 ohms over the whole out put-lcvrl primary is correctlv sct. at te~~uatorrangc. Tone Level - - With output switch set to 75 ulrms, 'Thc meter is conncctctl across the ~.l.c.si(le of ;t 13 ohms 5 5 ohms over tl~cwhok outl)ut-lc\.el bridge-type rectifier 311, the a.c. side of which is attenuator rangc. connected bctween thc anodc of \'3 and earth. Amplitude Characteristic The resistor R25 in series with thc a.c. side is so LVithin rf 0.25 db of the 1 kc s value frorn ~2 adjusted that the meter reads 175 when the oscil- 50 cis to 10 kc/s. lator is deliwring thc power indicated hy the Icvcl . switch. .-lccftracy o/ Measftred 0utp11tI,rvel Within 0.5 db of the level indicatul by thc Power Supply mctvr and level switch from 50 crs to 10 kc's. When the mains-battery switch ib sct for maills Meter Error due to Load operation, h.t. is obtained from a IJU6 wctificr Xegligible at all settings except -!- YO where o and smoothing circuit and 1.t. from a winding on maximum error of 4-0.2 dl) occurs with a 100-ohm the mains transformer T2. There are a numbcr of load. primary tappings on T2 and the correct one can be selected by a switch which has intermediate Harmonic Distortion off positions between the various tappings ; these (measured at zero level) should be used for on-o//' switching. Frequcnc? GOO-ohms out~rrt 75-ohms output When the mains-battery switch is set for batter!. 50 CIS < 2% 27" operation, a supply of 20 milliam~sat 300 volts is 100 CIS -- 17; . 1% required at the h.t. terminals ad1.3 A at 6 \-nlts at I kc/s 1 %, 1 y, Instruction S.4 Fig. 9.8 Portable Oscillator PTS, 13 Simplified Circuit
75n ATTENUATOR
SECONDARY WINDINGS IN PARALLEL FOR 7M OUTPtll
I FREOUENCY ' ' OUTPUT .- - 2 VALVE OSCILLATOR I OUTPUT ATTENUATOR 8 - CONTROL AMPLIFIER WITH - IMPEDANCE I LIMITER 7 - VALVE - SWITCHING - I I1 - POSITIVE FEEDbUK PATH i Fig. 9.8 Pcrtable Oscillator PTSI13 : Simplified CIrcuIt Instruction S.4 Section 9
PORTABLE OSCILLATOR PTS:15
Introduction sent to line is decicletl hy tlic setting of thc Ol3A!8 The PTS/I5 is a miniature battery-operated volume control, the Ilattcry can continue to be oscillator designed for use on O.B. work, in con- used as long as oscillation is mailitailled. junction kith an OBA/8 amplifier, for sending line-up tone to the controlling studio centrc. It has Circuit Description a fixed frequency of 900 c/s, and an output of about Fig. 9.9 shows the circuit diagram of thc -54 db into 300 ohms. oscillator, with details of the valve base and battery It uses a single subminiature pentode in a 4- connections inset. Positive feedback between section resistance-capacitance oscillator network. anode and grid is applied via the frequency- To provide an adequate output, the valve is driven determining nctwork C1-C.5, R I-R6, which rotates rather hard, and the harmonic content is therefore the phasc of thc grid signal by 180 degrees ex%ctly high, although this is not of great importance in at one particuhr frequency determined by com- 'd the application for which the instrument is ponent \.alucs. I'rovitlctl that the gain of t he valve dcsigned. is not less than tlic loss in the fceclbaclc network,
VALVE BASE 12 345
BRITISH BSA
C6 0.02
WIRING SIDE 1 OF SOCKET
IOk
A
Fig. 9.9. Portable Oscillator PTSI I5 : Circuit Drawing No. EA 7788
The small receiver-type battery provides 69 volts oscillation occurs at this particular frequency. h.t, and 1.5 volts l.t., the 1.t. supply being switched Accurate adjustment to 900 c/s is effected by the on only while there is a plug in the output jack. trimming capacitor C5. With normal use, the battery should last for Grid bias is derived from the filament-voltage several months. dropping rcsistor R7, which reduces the 1-5-volt As the battery deteriorates, the output level supply to U6!!5 volt. The total anode load is of gradually falls, and finally reaches about-67 db the ordcr of 500 kl2. The output impedance is before oscillation ceases ; since, however, the level 10 kc2 and is intended to feed into a 300-i2 load. Instruction S.4 Section 9
Mechanical Construction Test Data The instrumcnt is mounted in a rectangular case Test Conditions measuring 5&in. by 4 in. by 29 in. The bottom and H.T. voltage, not less than 66 V. sides of the case are constructed from a single L.T. voltage, not less than 1.45 V. bent-up duralumin shcet : screwed to this are two Load resistance, 300 R. laminated-plastic end-plates, to which in turn is attached a top panel carrying the output jack. Fr equency Two furthcr panels, which are a sliding fit in 900 cls ; limits of adjustment, & 18 c/s. grooves in thc end-plates, are accessible when the top panel is removed ; one of these sliding panels divides the batterycompartments from theoscillator Ozib;6ttt Level compartment, the other forms a withdrawable -54 db &3 db. chassis, on which the oscillator components, including the valve-holder, are mounted facing inwards, the wiring being run on the reverse side of Percentage Total Harmonic Content the chassis and brought out to individual com- Less than 10%. ponents through small holes. Flexible lcads from the oscillator chassis and Maintenance output jack are connected to a 4-pin plug, which Battery Replacement engages with a socket wired to the battery Remove the two screws at each end of the top- terminals. The battery plug and socket are panel, and lift the panel clear of the case. Withdraw accessible when the top panel of the caseis removed. the plug attached to the oscillator from the sockct attached to the battery, and slide the battery out General Data of the case. Attach a new battery to the socket, Valve referring to the inset sketch (Fig. 9.9) for the Mullard DF66 on British B5A subminiature method of wiring. Insert the battery in its com- (flat) base. partment, connect plug and reassemble the case.
Supplies Valve Replacement H.T. supply, 69 V, 80f2OPA. Remove the top-panel, unplug the battery, and L.T. supply, 1.5 V, 15k1.5inA. slide the component-mounting panel out of the case. When replacing a valve, take care that the Bat!er.v red spot on the valve-base is adjacent to the red Ejther Evcr Ready B114, or Exide Drymex 514. spot on the panel. Replace the rubber band securing the valve. Im;bedances Output Z=lO kQ Frequency Adj~rstment Normal lozd Z=300 Q After a change of any #:component,including the valve but excluding the battery, the frequency Cmnl)onent Types should be re-checked and, if necessary, re-adjusted Trimming capacitor C5, T.C.C. Type TCK 0540. to 900 c/s. (The trimmer capacitor can only be Screen-decoupling capacitor C6, T.C.C. Type reached when the component-mounting panel is CP33N, f 20%. withdrawn from the case.) Since the h.t. and 1.t. Other capacitors, Hunt Type L1/2S, f 27,. voltages have a slight but appreciable effect on the Cathode-bias resistor K7, Welwyn Type SA 361 1, frequency of oscillation, it is desirable to use a 0.125 W, 52%. reasonably fresh battery for the test. Other resistors, Erie Type 9, 0.25 W, f 5%. To measure the frequency, feed tone from the Output jack, S.T. and C. Type 4114B. PTS/I5 through an OBA/8 amplifier, and beat the Battery connector, 4-pin Carr ' Fastener ', Cat. output with a supply from a calibrated 900-c/s No. 2745. source. Valve socket, McMurdo Type XSM 5 US. G.H. 1253 Instruction S.4 Section 9
PORTABLE OSCILLATOR PTSj16
Introduction ing network is of the Wien-bridge type, as can be The PTSl16 is a small six-frequericv ojcilIator for clearly seen from Fig. 9.1 1. The use of a Wien line testing at 0.R. points. External powersupplies bridge in this application has been analpxd in are necessary, and can conveniently be dmwn from Appendis A. a SUP16 unit as used with O.B. equipment If Fig. 9.11 is comparcd with the diagram on OI1A/9. (See Instruct ion S.3, Section 20.) page A. 1, it will be seen that whereas the frequency- The output Ievel of the oscillator into 301) ohms is determining arms C1, R1-R6 and C2, R7-R12 of about -58 db, with a small range lor adjustment the bridge in the PTSjl6 are conventional, the on either side. The six fixed frequencies are 90 arrangement of the amplitude-limiting arms TH 1 and 250 cis, and 1, 3, 5 and 7 kc/s. The output and R 13 is the inverse of that previously employed. impedance is approximately 300 ohn~s,unbalanced. In earlier Wien-bridge oscillators, like the PlS/lO,
Fig. 9.10. Oscillator PTSI 16: Circuit Drawlng No. €A 8284
The overall dimensions are 34 in. by 31 in. by ESj12 and PTS113, the anode of the second 5$ in., and the weight is 24 lb. The instrument can stage was connected back to the cathode of the first thus be accommodated in the ORAjg+quipment stage via a resistor, the earthing of the cathode. spares box if desired. being via a lamp. In the PTSl16, on theother hand, the second-stage anode is connected back to the Circuit Descrlptlon first-stage .cathode via the thermistor, TH 1, and Fig. 9.10 is a circuit diagram of the oscillator. the cathode is earthed via the resistor R13. Since. with details of the valve base inset. The double- howevei, the resistanceltemperature coefficient of triode valve, Type CV455, has a centre-tapped the lamp is positive, whereas the corresponding 12.5-volt heater, connected as shown for use with coefficient of the t hemmistor is negative, the net the normal 6-3-voIt 1.t. supply. effect of either arrangement is the same. The frequency-determining and amplitude-limit- The capacitors C1 and C2 of the frequency- Instruction S.4 Section 9 determining network are fixed, and the resistors General Data R1-R12 are switched to give the six frequencies Zmpedan ces required. The thermistor has a resistance when cold Output Z = 300 1.2 approx. (unbalanced) of some 50 kilohms, falling to.about 10 kilohms for Normal load Z = 300 SZ a current of approximately 1 mA when.the oscil- Oulfizrl Lecel lator is working. The operating value of the -58 4-4 db into 300 ohms with h.t. supply of thermistor resistance, together with the resistance 270-230 volts and level control set in mid position of R13, gives a negative-feedback ratio which sets approximately. Total range of adjustment, 4 the gain from input grid to output anode of the &1 db. Variation of putput level with frequency, maintaining amplifier to 10 db, the value required +On5 db referred to level at I kc/s. for the type of positive-feedback network used, in which,Cl is equal to C2, and the two resistances Oidpul Frequencies selected from R1-R6 and R7-R12 are also equal. 90 and 250 c/s, I, 3, 5, 7 kc/s. All &2%. Percenlage Tolal Harnzonic Dislortirjtt Less than 1% at 1 kc/s with 270-230 volts h.t. Componenl Types 6 Cl, C2, L.E.M. Type 2010 (jll%). C3 and C4, T.C.C. Types CP34S and CE47L. Resistors, Welwyn ( f1%) : R1, R7, Type SA3623, R2, R8, Type SA3622, R3-R6,R9-R12, Type 361 1. Resistors, Brie Type 9 : R14, R15, &200/,, R16, R18, R19, R21, &lo%. R13, Welwyn Type SA3611 (+.5%), or Painton Type 72, or Erie .Type 9. R17, Erie Type 2 (&lo%). R20, Morganite LHNAP 50350 26800. Fig. 9.1 1. Oscillator PTSI 16: Simplified Circuit Switch, N.S.F. Oak Type H, to Drwg. EPA 7901. Power-supplies connector, F. & E.Type EM-6-14. The output of the oscillator is brought out to a Thermistor, S.'I'. & C. A54 12/100. jack connected across the 15-ohm resistor R15, which forms part of a potential divider across V1B Operating Instructions anode load. The output-level control, R20, Where OBA/9 equipment is used, a power supply operates by altering the ratio of this divider be- should be taken from the batteries of the SUP/6 tween limits of 15 ll/100 kQ and 15 Q/150 kll. unit, using the power cable of the spare OBA/9 The 270-ohni padding resistor R21 in series with amplifier. The r~utputjack of the PTS/16 should Ee the jack brings the output impedance of the plugged to the input of the working OBA/9 ampli- oscillator up to the region of 300 ohms. fier, or, alternatively, tone may be fed into a ribbon- n~icrophonechannel of the mixer; in the latter Valve Data circumstances the appropriate fader must be Anode Healer Healer fully up, since there will otherwise be a danger of Valve Currenl Volls Amps frequency-response errors due to the unbalanced ll2 A nature of the tone-source output circuit. V 1A 0.8 6.3 0.15 The output level may be set to within 1 db using 4 .O 6.3 0.15 the gain control of the OBA/9 amplifier, and a fine adjustment obtained with the PTS/16 control. Supplies Where OBA/8 equipment is employed, the H.T. supply, 250 volts, 4.8 mX & 20%. necessary h.t. and 1.t. supplies for the tone source L.T. supply, 6.3 volts, 0.3 amp a.c. or d.c. can be obtained from separate external batteries, NOTE:-These supplies obtainable from SUP16 via an OBA/9 battery cnblc. (See S3, Fig. 53) unit where available. G.H. 0354 Instruction S.4 Section 9
APPENDIX A
THE ZERO PHASE-SHIFT OSCILLATOR WITH WIKN-BRIDGE CONTROL
General Application (2) The loss introduced by the bridge must not Reccnt BBC designs of variable a.f, oscillators exceed the gain obtainable from the valves consist basically of ;( two-valve zero phase-shift in the maintaining circuit. maintaining circuit, the input and output of which In practice, both fcquirements can easily be met are connected across oppos~tediagonals of a Wien by the circuit shown ; the bridge then operates bridge providing frequency and amplitude control. very near balance, and the balance conditions will Such an arrangement is exemplified by the TS/9, therefore be derived. PTS/IO, PTSII2 and PTS/13. The principle of operation will be explained here with reference to Wien Bridge Network C the PTSII2. Conditions JOY Balance w A simplified circuit diagram of the PTS/12, is The four arms of the Wien bridge (Fig. A.1 given in Fig. 9.6. The first two stages, comprising comprise a series KC combination, Z,, a para1r' el the maintninine network with Witwhridp cnntrol. RC combination, Z,, a fixed-value resistor R,, and are redrawn in Fig. A.1. Each stage introduces a a lamp with non-linear resistance RL. The values of the resistive and capacitive elements R and C are variable, but in this application are kept equal in both arms 6f the bridge at all settings. At balance,
Substituting the admittance, Y,, which is the reciprocal of the impedance, 2, :
Separating the eql~ation into its real and imaginary parts.
= 2, and Fig. 9.1.1 ; PTSIIZ Oscillation-maintalning Circuit RL
with Wien-bridge Frequency and Amplitude Control ,
phase-shift between grid and anode of approxim- Whence w~L =- atcly 180 degrees. and the output from V2 is C2R2 therefore nearly ii not quite in phase with the input and since the frequency f is equal to wpm, to V1. Without further examination, it will be clear 1 ,that the foUowing requirements must be fulfilled f =- if oscillation is to occur : 2nRC (1) Positive feedback mtlst be applied via the bridge from V2 anode to V1 grid, the voltage It follows that, providing R,, has twice the fed back having a maximum value at some resistance of the lamp, balanceoccurs at a frequency definite frequency. 113nRC. Instruction S.4 Section 9
pin^; 1: :III~IC \v0111(1 b~ numcric;dly the same. tlirough ;IN, ni:lintx~~~ingXIII;)I~II~T 1- :~n.~~rxt~l~. I'hll.~. 1161 11t.1 siglul \vordtl be :~ppli(dbetween V1 ~.robct\\'oc~~ \'1 grit1 inpl~t:~t 1: and \2 IIIII~H~~;Lr. grid :;I:;: c;~tllodc..;md osci1l:~tioncould not occur. A : the \xl\.cs \\.ill tl~cnbuild r:p oscill;~t~o~~s.b~;t It is tllcrc,forcs necessary to arrarigc illat, under uilij, ;it tllc lrequcncy for which the. 1)ll;ist. 01 thc: opwl';iilngcontlitiolls, Ii,. ~1~~11bc /lw t11m half I?,,. , . fcr!dbacl; voltage applicd at B is thv siinw ;w that llic rvsistancc of thc lm1p incrciws with the qt A. The reason for the restriction is tlmt ally cnrrcnt passing through it. (This current includes quadrature component possessed by the fcc.tlb;~ck not only that supplied through N,,, but also the voltage will be degenerative, since it cannot bc cathode current of V1, the a.c. component of which reinforced by further amplification ; from another contributes a measure of current feedback as standpoint, an input component in quadraturc distiilct froni the voltage feedback applied via with the output makes no contribution, when R,,.) Thc change in Iamp resistance with current amplified, to the power required to replace the causes the dcgree of unbalance in the bridge to circuit dissipation. vary as an inverse function of thc oscillation level, Thus, the reactive branch of the Wien bridge and thus to stabilise the level at a value depending alone would be capable of controlling the frequency on the circuit dcsign. of oscillation, and with zero phase-shift through J the maintaining circuit, the oscillatory frequency Applicalion to Oscillator Control would be li2nRC. However, small changes in In Fig. A.1, the diagonal corners of the bridge phase are likely to be caused by the coupling are labelled for rcference A,B, C and D. A feedback capacitors, and by other effects of minor import- voltage from V2 anode is applied between points ance. For regeneration to take place, a small A and D, thc latter of which is at earth potential, phase-change might thus necd to be introduced by Since the branch ACD, containing R,, and RL, is the control network ABD, and the frequency of purely resistive, there is no difference of phase oscillation would depart slightly from that pre- bctween the voltage at C and that at A, measured viously stated. This effect is minimised by the use with respect to the earth point D. When the bridge of current negative feedback in the maintaining is balanced, the voltage at B must be precisely the amplifier. same as that at C, both in magnitude and in phase ; from this fact two inferences are to be drawn. Amplitude Linitaiion Firstly, it has already been shown that for The argument contained in the preceding sub- balance, RL must be equal half R,,, so that with section is applicable irrespective of the presence of respect to earth, the voltage at C is one-third that the resistive arms of the bridge R,, and RL, at A. It can now thcrefore be stated that the although if these arms were absent V1 cathode voltage at B is also onc-third that at A. would of course have to be earthed with respect to Secondly, sincc the voltqges at B and C are equal ax. Since, however, the gain required from the not only in mngnitudc, bllt idso in phase. the maintaining. amplificr is only about I0 db, some voltagc at B must thus haw thc siimc phasc as that form of limitcr is necessary to preven t the amplitude at A of oscillation from increasing until finally restricted It follows that thc branch .4BD divides the by valve-characteristic curvature or other non- feedback voltage applicd from V2anode to V1 grid linear means. The addition of the resistive branch in the ratio of 3 to 1 without change of phase. The is a convenient method of providing the required loss to be made good by the maintaining circuit, amplitude limitation. It should be mentioned that, V1, V2, is 20.log 3, or 9.54 db. This is truc, although the completed network ABDC takes the irrespective of the presence of the othcr branch form of a bridge, under oscillatory conditions the ACD, always providing that the operating frequency voltage at point B in the reactive branch ABD remains at 1/2?rRC. always remains one-third that at A, both voltages The important property of the network ARD is being measured with respect to the earth-point D. that it does in fact introduce zero phase-shift only ,The voltage feedback at B is i-pplied in a positive at the single frequency lI2nRC. At any higher sense to VI grid, whereas that at C is applied in a frequency, capacitive reactance decreases, and negative sense to V1 cathode. If the lamp resist- capacitive currents rise, causing the voltage at I3 ance RL were fixed at exactly half R,,, the bridge to lag on that at A ; conversely, at any loww would be in balance, and the voltages fed back to frequency, thc voltage at B leads on that at A. Instruction S.4 Section 9
Any slight rise in level which may be caused by a change in external conditions increases the current through the lamp and brings the bridge nearer to balance, so that V1 input falls and the level drops The variation in capacitance alters the frequency back toward the stable value. Any slight fall helow from f to the stable level correspondingly reduces the lamp current ; RL thus decreases, and the degree of unbalance is increased, causing the level to rise again. When stability is reached, the 10-db loss in ABD, The resulting frequency change is equal to the plus the gain reduction due to the total negative difference between the new and the original feedback applied at C, must together equal the frequency, and is givcn bv gain of the maintaining amplifier, V1, V2, with the fredback bridge network disconnected.
... (i)
TUNING CAPACITOR
PART OF 'INCREMENT' Fig. A.2. Wien Bridge with increment Capacitors SWITCH SWZ Fig. A.3. Frequency-increment Switching Productlon of Small Frequency-Changes Two srm~larcircuits, one for each tuning capacitor With any Wien-bridge controlled oscillator, small frequency -changes of pre-determined value can conveniently be obtained by inserting or removing Practical Arrangement a subsidiary capacitance 6C in series with each A practical arrangement. applied to the PTS/l!?. main illnine capitancc. (,' of the hrider circi~it is eiwn in Fie 3 11~1ic :IIW ql~mvnin vjnci~l~~ shown in Fig. .-\.?. 'I'!HY>i~uxtmc~rlts a1111 tltucb- in Fig A. 'I'his conbists of a s(vitc11, S\\"L, mcnts of frccluc.nc:y of ust, ill tt~)ting thc, by rneans 01 which ;illy one of a group of capaclturs rt8sl)onscof 11arrou-har~tlcircuits. . can be inserted in cach reactive arm of the bridge in series with the corresponding tunilg capacitor. It car1 be seen from equation (i) of the preceding subsectiori that, for a constant value of R, the The oscillation frequency, /, under normal frequency change introduced by a given capacitancc conditions is given by 6C is independent of the value of the tuning capacitor C. Thus, the frequency increments and decrements in any given range are fixed. When tht. range is changed, R is varied in the ratio of 10 : 1 When 6C is inserted, the;csultant value of each or 100: 1, and the frequency incrcnients ;in(\ capacitance in the bridge changes from C to decrements vary correspondingly. Instruction S.4 Section 9
The frequency Incremenl control of the PTS/12 remaining settings with increment capacitors of has five set'tings, corresponding to frequency varying values. changes of -0.4, -0.2, 0, +0.2 and +0.4 c/s The changes introduced on range (3) are 10 on ranges (1) and (2). The setting of -0.4 is times, and on range (4) 100 times the above obtained with no increment capacitor, and the figures. G.H. 12.53 Instruction S.4
SECTION 10
TRANSMISSION MEASURING SET TM/1
The transmission measuring set TMII is used in connection to the sending circuit to be made via a conjunction with the variable-frequency tone repeating coil, otherwise the results obtained with source TS/4, oscillator amplifier OA/1, variable the testing equipment may be seriously inaccurate. attenuator AT12 and amplifier detector AD/2. The gain-measuring circuit consists of two It provides, among others, facilities for : branches with change-over switching enabling (0) Sending tone at any of five absolute levels, either to be connected between the Gain Input and with reference to a zero of 1 milliwatt in a Gain Output termmals. One branch includes Test 600-ohm resistance, e.g. for the calibration In and Test Out jacks, between which the apparatus of programme meters and amplifier detectors to be tested is connected, and loss pads on eithc'r and for making level measurements in the side of the test jacks controlled by key switches. b a.c. testing of lines. Provided that the circuit under test has input and (b) Making 600-ohm test-gain measurements on output impedances of 600 ohms, the loss pads L.F. amplifiers and for determining their together introduce losses of zero, 30 db or 60 db, 600-ohnl test frequency responsc charactcr- according to their key setting. The other branch istics. of the circuit includes the variable at tenuator AT12 (c) Making 600-ohm test loss measurements on The meter is calibrated (red scale) over a range equalisers, filters and other networks, and of * 2 db, the divisions indicating approximately for determining their 600-ohm test frequency 0.2 db. The reading denotes the correction to be loss characteristics. applied to the attenuator reading, and should be added to the attenuator reading when the deflec- Circuit Description (Fig. 10) tion is positive, and subtracted when it is negative. The transmission measuring set is made up of two distinct circuits, namely, a se~zder circuit Range and Accuracy comprising that part of the circuit included he equipment, as ordinarily connected, enables between the Sender-I@ul and Sender-Oulptcl jacks, readings-to be obtained to an accuracy of * 2.0 and a gain measurin~circuit comprising that part db over the following ranges :- of the circuit included between the Gain-Infiut Loss measurements ... 0 to 62 db. and Gain-Ozclpul jacks. Gain . . . 0 to 60 db. The sender circuit works in conjunction with the Level ... + 10 to - 55 db (with tone source and enables the output to be adjusted respect to a zero level to certain definite levels with reference to a zero of 1 milliwatt in a power of 1 milliwatt in a 600-ohm resistance. For 600-ohm circuit). this purpose, a sending network is provided con- These ranges can, of course, be extended if sisting of a low-resistance potentiometer (122 desired by mak.ing suitable connections. ohms) tapped at suitable points and including resistances which build out the sending impedance Meter and Controls at all levels to 600 ohms. Provision is made for The meter provided on the panel serves three sending at levels of + 10, + 5, zero, - 20, - 40 purposes, as follows :- and - co decibels. (a) D.C. milliammeter accurately calibrated with A thermocouple circuit is provided for adjusting its shunt to read 40 mA at its mid-scale the value of the current and provision is made for deflection. calibrating the thermocouple on direct current. (b) Galvanometer for indicating the output of The centre point of the 122-ohm resistor is the thermocouple. earthed through a I-microfarad capacitor. It is (c) Level indicator for indicating in the output therefore, important when the circuit under test of the amplifier detector, used primarily as is unbalanced, e.g. earthy on bne side, for the a galvanometer but having a decibel scale Instruction S.4 Section 10 (marked in red) covering a range of * 2 db First key on the left Couple Cal. with divisions 0.2 db. ,411 other keys Mid Position. The controls provided on this panel are as The galvanometer should now be reading mid- follows :- scale. If it is not giving this reading exactly, the Two 3-position keys controlling the galvano- meter and thermocouple switching. One 3-position double key designated - 60 db, and'0 and - 30 db, controlling the attenuation introduced by the loss pads in the test branch of the circuit. One 2-position key (with green handle) designated Atten and Loss Pads, for performing the change-over switching for making gain or loss measurements. Three 3-position keys, designated Sender Level, for controlling the sending level. Two rheostats designated, respectively, Cali- hate Galvo and Adjust Input, for calibration purposes. Fig. 10.2. Calibration of Galvanometer SENDER CIRCUIT Calibration of Thermocouple Cal Galvo rheostat should be adjusted with a The arrangement of the circuit for calibration of screwdriver to make it do so. the thermocouple is shown in Fig. 10.1. Direct The direction of flow of current through the thermocouple heater should next be reversed and the galvanometer reading checked. First key on the left Couple Cal. Second key from the left Couple Rev. All other keys Mid Position. If there is any appreciable difference between the readings with the two methods of connection, the Cal Galvo rheostat should be adjusted to obtain 40 as the mean reading.
Adjustment of Sending Level Testing ax. at the desired frequency should be Fig. 10.1 Adjustment of D.C. applied to the Send~In$utjack of the transmission measuring set. current is fed from the 24-volt battery terminals The tone source and the oscillator amplifier through the sending potentiometer, galvanometer should be switched on and the tone source adjusted shunt, thermocouple heater and Adjust Input to send at the desired frequency. The T.S. Out$ut rheostat in series. The galvanometer is connected and O.A. Input jacks must be connected with a across its shunt. cord, but external connection between the Sender First key on the left Cou$le Cal. In$d and O.A, Out$z~t jacks is unnecessary since Second key from the left Galvo fo Shunt these jacks are permanently connected via their All other keys Mid Position. inners. The Adjust Input rheostat is then adjusted to With central Sender Level key at + 10 and all obtain a reading of exactly 40 mA (mid-scale) on other keys in their mid-position, with the circuit the meter. under test connected to the Sender Outpd jack, The circuit is then arranged as shown in Fig. the circuit arrangement is as shown in Fig. 10.3, in 10.2, the galvanometer now being connected in which the circuit under test is represented by the scries with the Cal G~lvorheostat in thc thermo- 600-ohm resistor connected across the sending couple output circuit. network. Instruction S.4 Section 10
To calibrate the sender circuit all that is neces- the level regulated by the operation of the Sender sary is to adjust the current in the sending network Level kevs. to a value of 40 mA, that is to say, to obtain Where, as in the case of the transmission mid-scale deflection on the galvanometer. equivalent line-up test, it is desired to transmit tone to the 600-ohm input circuit of a B amplifier at levels separated by steps finer than those provided by the Sender Level keys, the output may be taken via the Att Out jack and the sending level regulated by suitably setting the keys of the attenuator AT/2. The B input provides the proper termination necessary for the attenuator to ensure accuracy of its readings. Sender Level keys for sending at zero or any other convenient level. Key with green handle at Atten. Keys of attenbator for introducing the d&red loss. All other keys at Mid position. Fig. 10.3. Calibration of Sender Circuit In this condition tone is sent (see Fig. 10) via the Sender Output and Gain Input jacks, the inners of To avoid over-running the oscillator amplifier, which are permanently coinected, via the change- the Adjust Input rheostat should be set to maxi- over contacts. via the T.M.S. Att In and Att In mum and the preliminary adjustment made with jacks, the break contacts of which are permanently the Volume Control on the tone source ; the final connected, and via the attenuator and the Aft Oul adjustment to obtain mid-scale deflection of the jack to the B amplifier input. galvanometer should, however, be made with the If it is desired to send into 600 ohms, but to Adjust Input rheostat, since this provides a rather retain the attenuator in circuit for the purpose of finer control. adjusting the voltage applied to a measuring device Levels at the Sender Output jack of + lOdb, with a high-impedance input circuit, as, for ex- + 5 db, zero, - 20 db and - 40 db (with reference ample, when calibrating a programme meter, the to a zero of 1 mW in a 600-ohm circuit) can be connection should be made via the Cairz Outbut obtained by suitably operating the sender level jack, across which a 600-ohm termination for the keys and recalibrating the sender circuit to give attenuator is provided. The key settings are as for mid-scale deflection as follows :- the case just considered, the circuit being extended Zero All keys in Mid position. via the &ring connecting the break contacts of the +10db CentralSenderLevelkeyat +10db, ,411 Out and T.M.S. All Out jacks, and the othcr all other keys in mid-position. pair of change-over contacts to the Gain Out+ut + 5 db Left-hand Sender Level key at + 5 jack. db, all other keys in mid-position. The sender circuit may always be considered as a - 20 db Left-hand Sender Level key at - 20 generator of constant e.m.f. and constant internal db, all other keys in mid-position. resistance, namely 600 ohms. When the sender- - 40 db Central Sender Level key at - 40 level keys are set at zero, thc e.m.f. of the output db, all other keys in mid-position. circuit will be such that the voltage developed - 03 Right-hand Sender Level key at m, across a load impedance of 600 ohms, will be 0.775 all other keys in mid-position. volts. The value of the e.m..f is, therefore : The 600 Ohms to Line position of the right-hand 2 x 0.775 = 1.55 volts r.m.s. Sender Level key, in which a 600-ohm resistor is It will be observed that the actual power sent connected across the Gain Input jack, will not into the load will be equal to that indicated by the normally be used. sctting of thc sender level keys only where the impedance of the load is non-reactive and has a Connection of Output value of 600 ohms. Where the value differs from For sending tone in the normal manner, connec- 600 ohms, the power sent into the circuit under tion should be made to the Sender Output jack. and test will differ from that which would be sent into Instruction S.4 Section 10
a 600-ohm circuit, in respect of the reflection loss The attenuator is set so as to introduce maximum that occurs at the junction of unmatched impe- loss. Then, without altering the sending level or dances. In making test on lines, however, no disturbing the setting of the amplifier detector, the correction need be made for this, since the required change-over switch is operated and the attenuator line equivalent includes the working reflection adjusted to obtain mid-scale deflection as before. losses at each end. The attenuator setting will indicate the net loss in the loss pads branch of the measuring circuit. MEASURING CIRCUIT In the case of a loss measurement, seeing that Measurement of 600-ohm Test Loss and Gain the only loss in the Loss Pads branch is that intro- The method described is applicable in the case duced by the circuit under test, the attenuator where both ends of the circuit to be tested arc reading will be the required 600-ohm test loss. available for connection at the same station. The In the case of a gain measurement, this reading tone source and amplifier detector are used but it will denote the overall loss in the Loss Pads branch, should be observed that it is unnecessary for the from which the gain of the circuit under test can amplifier detector to be calibrated for the purpose be derived by taking the difference between the of this measurement. loss indicated by the setting of the Loss Pnds key *' For both loss and gain measurements the and that indicated by the setting of the keys of the - circuit to be tested is connected between the TestIn attenuator. Thus, if the loss introduced by and Test Out jacks (see Fig. 10) and is thus ter- the Loss Pads is x db, and that introduced by the minated by 600 ohms on both input and output attenuator is y db, the gain of the circuit under sides when the Change-over key is operated to the test is (x - y) db. Loss Pads position. Where either the input or A slight modification of the foregoing procedure output of the circuit under test is earthed on one is necessary in order to enable the 600-ohm test side, the connection to the Test In or Test Out jack gain of an amplifier to be measured, where this is should be made via a shielded repeating coil, and greater than 60 db. If such an amplifier is connected the result obtained from the measurement should between the test jacks there will be a gain instead be corrected for the loss introduced by the repeating of a loss in the loss pads branch of the transmission coil, which can be separately measured. The tone measuring circuit, even when the Loss Pads key is source is adjusted to send at the desired test thrown to the 60-db position. In this case, when frequency, which in the case of simple gain or loss the Change-over key is thrown to the attenuator measurements will normally be 1 kc/s. The branch it will not be possible to obtain a balance galvanometer is switched to read in the output becauke, even with the whole of the attenuation circuit of the amplifier detector. out of circuit, the output will be too low. The gain Loss measurements should be made with the of the amplifier can, however, still be measured, input level adjusted to a value approximately equal by using an increased sending level when the to that normally present under working conditions Change-over key is in the Attelz position, and by in the ibput circuit of the piece of apparatus under adjusting the result accordingly. test. Gain is normally specified for amplifiers with For example, a sending level of - 40 db can be their volume control on maximum. To avoid over- used when the Chanp-oi~erkey is in the Loss Pnds loading the amplifier, therefore, the input level position and, assuming the gain to be measurcd must normally be less than that present under does not exceed 80 db, this can be increased to working conditions in order to compensate for the - 20 db wher, the key is in the Atten position and increase in gain beyond the normal setting. When a balance obtained by adjusting the attenuator. adjusting the sending level it should be remembered If, for example, the attenuator reading were that half the loss introduced by the loss pads is 13 db, we should have a net gain in the attenuator included between the Sender Out and Test In jacks. branch of the circuit of (20 -- 13) - 7 db. The The method in both types of measurement is gain of the ampiifier would then be given by adding similar, since when gain is to be measured this is this to the 60 db represented by the loss intro- first converted into loss by suitable operation of the duced by the loss pads, giving 67 db as the gain Loss Pads key. Tone is sent via the loss pads of the amplifier. branch of the measuring circuit and the circuit The voltage gain of an amplifier connected as under test, and the level switches of the amplifier under normal working conditions (working voltage detector are set so as to obtain mid-scale deflection. gain) will only be equal to the 600-ohm test gain if Instruction S.4 Section 10
the input impedance of the amplifier and also the All other keys in mid-position. load into which the amplifier normally works are Adjust Galvo to give mid-scale reading. both 600 ohms. The voltage gain of an amplifier Key to Couple Rev and check reading. for any condition of working can, however, be (If this is not mid-scale, Adjust Galvo to give calculated from the 600-ohm test gain. 40 as the mean reading.) (3) (c) Calibrate Sender Circuit. SUMMARY OF OPERATIONS Set sender level keys for desired level Calibration of Sgnder Circuit (see d). (1) Switch on Tone Source, Oscillator Amplifier Adjust TS Volume Control and Adj~tstirrbqrt and Amplifier Detector. to give mid-scale reading. (2) Adjust Tone Source. (d) Send desired level into 600 ohms. Set Frequency dial to zero. Level Adjust Zero dial to give minimum frequency Keqqtired Sender Level Kej's of beats on detector feed milliammeter. Zero All at mid-position. Set Ran~ekey ancl Freq~tencydial according +I0 db Central key at 1- I0 db, othcrs to calibration chart for required frequency. at mid-position. e L Set Voltme Control at minimum. $- 5 db Left-hand kcy at 1 5 clb, others Connect T.S. Out to O.A. In. at mid-positior. (3) Adjust Transmission Measuring Set. -20 clb Left-hand key at - 20 clb, others ((I) Adjust d.c. at mid-positior. Key to Couple Cal. -40db Central keyat -4Odb,othersat Key to Calvo to Shunt. mid-position. All other kcys in mid-position. -co Right-hand key at a,others at Adjust input to give mid-scale reading. mid-position. (b) Calibrate Thermocouple. N.R. The calibration of the sendcr circuit Key to Couple Cal. must be checked for cach setting. Instruction S.4
SECTION 11
PEAK PROGRAMME METERS & AMPLIFIERS
PEAK PROGRAMME METER PPM/2 the meter. The control is labelled Adj. Law. The Peak Programme Meter Amplifier, PPM/2, A neon stabiliser is connected between the screen- is rack mounted and designed to work in con- grid circuit of V3 and a tapping point on the junction with a peak programme meter for the valve cathode resistance. The action of the neon, measurement of testing level or programme in addition to stabilising the screen-to-cathode volume. voltage, is as follows. When the mains-supply The PPM/2 is normally used for measuring levels voltage and hence the h.t, voltage varies, the
Fig. I I. I. Face Panel PPM/2
of the order of zero db, and has limited pre-set gain current through the neon changes, and as this control for calibration purposes. Arrangements current also flows through part of V3 cathode are made for dealing with levels of + lOdb, resistance, the valve bias is altered in such a way + 14 db, or, in special cases, - 23 db. as to oppose the change in anode current which would otherwise occur, and by a suitable choice Circuit Description (Fig. 7) of values the zero drift may be eliminated over a The PPM/2 comprises a single-stage amplifier, given range. The point on the cathode resistance V 1, transformer coupled to a double-diode detector, at which the neon current is injected can be V2, the output of which feeds into a pentode varied by the Zero Balance control, R38, and this output stage, V3, in the anode circuit of which is control must be adjusted with reference to the connected the peak programme meter itself. The particular combination of AC/VPl valve and neon signal input to the first stage is fed through a tube in use. step-up transformer, T1, which has an impedance The working principles of the PPM@ are simple. . ratioof 1 : 10. The applied signal is amplified by V1 and rectified by V2, the rectified signal being applied to the ' u lOdb negative voltage feedback is applied in this stage from the centre point of R18 and R19, grid of V3 in the fornl of additional bias. Since the connected across the output of V1. meter is of the right-hand zero type, it has maxi- Where the PPM@ is mounted on a recording mum left-hand deflection when this bias is zero, programme input bay, the feedback circuit is i.e. at ' no signal ' input. omitted in order to obtain increased gain for As soon as a signal is applied to V1, bias is dealing with the low line-up level of - 23 db. applied to V3 via V2 and R22 and the anode current The screen-grid volts supply of V3 is made vari- of V3 is reduced, causing a right-hand deflection of able by means of R25 in order to allow adjustment the meter in logarithmic proportion to the bias of anode current to maintain correct ' no signal ' voltage, which is, of course, dependent upon the zero on the meter. R25 is labelled Adj. Zero. input voltage to the amplifier. The bias resistance, which is not decoupled, is also variable by means of R23. This arrangement Meter Registration Speed permits adjustment of the cathode bias to allow the The meter registration speed is dependent upon valve to work on a suitably curved part of its the value of C5 and the total series impedance, charactcristic for correct logarithmic oncration of which includes R33, the reactance of T2 acd tllc Instruction S.4 Section '11 anode impedance of V2. The time constant of the Adjust Zero : Morganite Stackpole MKAP charging circuit is 2-5 milliscconds, and the 50350, 50 kil. instrument therefore reads 63% of the peak value Adjust Law : Morganite Stackpole MNAP of a square pulse having this duration ; for a 10250, 1 kil. 4-millisecond pulse the reading is 80% of peak, or Zero Balance : Morganite Stackpole LHAP 2 db low. The meter thus indicates the instan- 50250, 5 kR. taneous value of peak programme volume to a I?$,II~Imficdarice, 10 kQ. sufficient degree of accuracy to permit the uniform Suitches control, witi~outserious overloading, of all pro- Check Attenuator, Yaxley B, s.p., $-position. grammes irrespective of their dynamic chaiac- Meter, Yaxley A, 2-bank, 9-positio~. trristics. 'The meter return time, which is governed by thc Operation value of C5 and its discharge resistor, R22, is The PPMj2 can be used for input levels of 0, 3 secoiids for the 26-clb fall-back from 7 to 1 on the 1 10 or -1 14 db. Adjustment for the required scale. (It is to be noted that, although the scalc normal levcl is effcctcd by soldering the two markings represerit 4-db intercepts in generd, the flexihle leads on the inpt~t attenuator to the a, first intercept between 1 and 2 is 6 db.) appropriate termination tags, which are accessible To fi~cilithtethe comparison of different meters by removing the front covcr. The meter is cali- working on the same programme, provision was brated and adjusted to read normal level at 4, and made for increasing the return time by inserting maximum peak levcl at 6. the resistor 1137 in series with R22 by operation of Calibration. To ensure stability the amplifier the relay L34.43. 'I'llis method of comparison has should be switched on at least ten minutes before now been superseded, the present method being to calibration. integrate the peaks over :I period of about a second (1) Check ' no signal ' zero with no input. and thus to read ' average ' peaks. An external (2) Connect the output of the CALI1 to the meter is used, this being connected in series with a input and set the calibrating switch to 4 (normal). 10-kR resistor and the combination shunted hy an The meter should now read 4. If it does not, adjust 80-pF or 100-pF capacitor. The arrangement does to 4 by means of the sensitivity control. not affect other series instruments fed from the (3) Set the calibrating switch to 2 and 6 in turn. same programme-meter amplifier, and for normal The meter should read 2 and 6 respectively. If it peak readings the capacitor can he removed from does not, proceed as follows :- across the PPM meter and discharged through the ((1) To ' open ' scale (i.e. if meter reads above 10-k!l resistor by the operation of ;L switch. 2 and below 6). Valve Data (i) Unplug the input jack. Anode Screen (ii) Turn Adj. Law control till the meter C~rrreut C,rrrrent i. Fil. reads between 0 and 1. (iii) lieadjmt to zero by means of Adj. Zero Valve ' mA mA I'olls d mfis. Stage 1, control. ACjSP3B 7 3 4 1 (iv) Plug lip input jack and repeat 2 and 3. (6) Stage 2, To ' close ' scale (i.e. if meter on test 3 reads D4 1 -- - 4 0.3 below 2 and above 6). Proceed as in (a) Stage 3, except tLat undcr test (ii) set the meter to ACjVPl - - 4 0.65 read below ' no signal ' zero by means of Total Feed, 10 mA, appros. Ad'. Law control. H.T. Supply, 300 V or 250 V. Recalibration after Neon and I'alve Replacements. Replacement of the rectifier D41 or of the valve L.T. Supply, 4 V a.c. or 6 V t1.c.. ACISP3should not affect the cidihration in any way, General Data but after replacement of a neon tube or A~/vP-~, Neon Stabiliser , BBC S 1. the zero and law calibration including ' zero Pilot Lamp, P.O. No. 2, 4 V. balance ' must be checked and adjusted. For this l'otentimeters purpose it is necessary to provide a means of Adjust Sensitivity : Morganite Stackpole MNAP varying the supply voltage, and a tapped trans- 10450, 100 ki!. former or ' Variac ' should be interposed between Instruction S.4 Section 11
the mains-supply socket and thc mains unit General Nole feeding thc prcgramme mcter. A variation from Some modification to these instructions is thr normal working voltage down to about 15 liecessary where the PPM/2 is used on recording or per cent below normal (e.g., from 2-10 volts down transmit ter programme input bays. to 200 volts) should Iw obtainablr.. Adjust the mains-unit input voltagc to the PEAK PROGRAMME METER PPM;6 The portable Peak Programme Meter PPJI 6 nominal vahe, c.g., 240 volts, and with thc (Fig. 11.2) is a mains-operated measuring instru- Zero Balaucc! control- fully clockwise (i.e., ment which can be used in any apparatus room neon returned to earth), line up t lw pro- having a 200-250 volt, 50-c/s ax. mains supply. It gramme meter in the usual way. is particularly convenient for taking disk-repro- Set th~Zero Halmce control to its n~itl ducing response measurements and also for use in position, adrestore the mrtcr re;uling to conjunction with the portable oscillator P'1'S/9. zero by mrans of the A~j~itslLuil. control. In both cases. the use of s~ecialtie lines to an A.C.
(iii) l "Q::220 'p 240 SUPPLY VOLTS AC - Fig. 1 1.2. Face Panel PPM/6 (iv) Restore the mains-unit input voltagc to Circuit Description (Fig. 8) normal. rotate the Zero ~ulrrncecontrol a 'Thc circuit consists of a single amplifying stage small amoimt in the directio~las indicated using an .4('/SP3B, connected for triode working, in (iii) ahovr, and re-set tho metw zero by followed by the standard double diode and varia1jl~- means of the Adjzrsl Law control. mu pentodr circuit of the PPM/3. 'Thc normal (v) Reduce the mains-anit input volts once input circuit has high impcdance (12.5 kilol~ms)but again to 200, and proceed as abovc, until the provision of n second inp~~tjack givcs ;L 600-ohm varying the voltage between 240 and 200 input when required. A push-htton key C is produces no change in the meter zero. interposed brtwecn the input ,jacks and thr inp~~t (vi) The Zero Balame control should be left in trnnsformer. \Vhm this key is depressed, a 50-c.'s the position thus found unless either the calibrating signal, obtained from the mains supply. neon or the AC/VPl has to he replaced. is applied to the input of the first valw. Each lime the ax. voltage is varied, before The amplifier gain-control consists of an attenu- proceeding with the next step about 20 seconds ator, calibrated in steps of 4 db to cover a level should he allowed for the consequent change in range from + 20 to - 20 db. Measurement of peak valve-heater voltage to take effect. programme volume is obtained 1y srtting this Instruction S.4 Section 11 attenuator to the stop which gives a reading of 6 Zero Ralance : Xlorganite Stackpole LHAP on the meter, the volume being indicated by the 50250, 5 klo. attenuator switch position. Intermediate readings Operation can be obtained by interpolation, the meter being (i) Set the switch marked Sup$ly Volts A .C. to calibrated in 4-db divisions. Similarly, the range the ' Off ' position and connect the mains. of measurement may be increased within the limits Turn the switch to the stud position 250. of the meter scale. . Observe the indicator lamp immediately The accuracy of the meter readings depends above. If it does not light, switch off upon the correct initial setting of the calibrating at once-the mains may be d.c. and potentiometer. will burn out the mains transformer. When the plunger key V is depressed, the meter (ii) If the lamp lights up, depress the key is connected in series with HI8 across the mains marked V and adjust the supply switch transformer filament winding. The value of R 18 until the meter reads 4 as nearly as possible. is adjusted on initial test so that the meter reads 4 (iii) Release the key and allow a few minutes for when the primary of the mains transformer is warming up. Adjust the screwdriver control . worked on the tapping corresponding with the marked Zero until the needle rests on the mains supply voltage. white zero mark at the left end of the scale. To ensure that a meter reading of 4 indicates an (iv) Set the attenuator switch marked Input amplifier input signal of zero db, a special cali- Decibels to zero and depress the key marked brating circuit is provided. When the resistor C. The meter should deflect to 4 ; if it R5, in the 1.t. supply circuit, is switched across does not, adjust the Calibrate control. the input transformer T1 by depressing key C, the 50 c/s voltage across R5 produces zero level across Maintenance the primary of TI. The calibrating resistor 1?2 If the neon lamp N1 or the valve V3 has to be in the anode circuit of VI is then adjusted, with changed, il will be necessary to check the ' law ' key C depressed, until the meter reads 4. and ' zero balance ' of the meter. To check the ' law,' proceed as follows : Valve Data (a) Carry out tests (i) to (iv) as outlined above. Arzode Current Fil. (b) With the key marked C still depressed, Valve mA 1~'olls switch the attenuator to positions marked Stage I, + 8 and - 8 when the meter should read AC/SP3R 3 4 2 and 6 respectively. If adjustment is Stage 2, necessary, proceed as follows :- D4 1 -- 4 (c) To ' open ' scale. Stage 3, Release key and adjust Law control to bring AC/VPl Variable 4 the needle forward slightly. Rectifier Readjust zero by means of the Zero control. UU.6 - 4 Then check as in (a) above. (d) To ' close ' scale. General Data Proceed as in (c) above, except that the needle Neon Lamp N1, RRC S1. is first set back from the correct zero by Neon Lamb N2. HBC S3. means of the Law control. Decibel ~bilch,.Yaxley Type A, 1-bailk, 11- To check the ' zero ' balance ' :- position. (1) Connect a tapped transformer or ' L'ariar ' Supply Volts Switch, E'axley Type A, I-bank, between the mains-supply socket and the 7-position. ax. input to the meter. Then set the Slipply Potentiomelers Volts A.C. switch and Variac to the nominal Calibrate : Morganite Stackpole LHAR voltage of the supply, and with the Zero 25350, 25 kQ. Balance control fully clockwise, carry out Adjust Zero : Morganite Stackpole LHAR operations (iii) and (iv) and (b) to (d) above. 50350, 50 kR. 12) Now proceed as for the PPM/2 as described Adjust Law : Morgani te Stackpole LHAR under Recalibralion after Neon ar~J T'alve 10250, 1 kR. Replacements, operations (ii) to (vi). Instruction S.4 Section 11 TEST PROGRAMME METER AMPLIFIERS TPM/3 AND TPM/3A 'The 'l'td Programme Meter Amplifier TPM/3 is steps of 4 db from -+ 8'db to - 48 db. The designed for a general purpose programme volume control consists of a 100 kilohms potentiometer measuring instrument and works on the same having an accuracy of the order of f 1 %. principle as the PPM/2. It has an ~dditional The main level control is followed by a two-stage amplifying stage preceding the normal PPM,/2 resistance-capacitance coupled amplifier V1, V2. circuit, and is designed for rack mounting. A small amount of current feedback is applied to Fig. 1 1.3. Face Panel TPM/3 and TPM/3A The sensitivity of the instrument covers a range each of these stages independently, together with from -+ 8 db to -- 48 db in steps of 4 db, with a large amount of voltage feedback over-all applied an additional range of 2 db. Levels are read via R16. from the calibrated dials marked Level DB, a The bias resistance for V1 takes the form of given level being the algebraic sum of the readings a calibrated resistance-capacitance network PF/lA. - of the two calibrated dials when the programme which is the second level control. It comprises meter reads 4. two resistors, one of which is variable, so that the feedback is controlled by the setting of the Clrcuit Description (Fig. 11) variable resistor. Thc calibration of this resistor The input transformer has an impedance ratio of is from - 2 db to -+ 2 db in steps of .5 db. An 1 : 10, giving an input impedance of 10 kilohms, additional resistor, R33, is included in series with with a reactance of 7 kilohms at 50 c/s. A con- the other leg of the shunt to obtain the correct nection to the 250-kilohm potentiometer, R1, is value of mean bias. taken from the transformer secondary winding, It has been seen that the second level control, 'This resistor is the Adj. Sensitivity control and calibrated from -1 2 db to - 2 db is actually is used during calibration for bringing the pro- connected in the feedback circuit. In the -+ 2 db gramme meter to its correct setting for a givcn position (i.e. minimum gain setting) maximum input level. feedback is applied, reducing the gain of the The Sensitivity control is followed by the main amplifier to 50 db. In the - 2 db position Level DB control comprising the variable poten- (maximum gain setting) the feedback is reduced tiometer PN/.IAl, the dial of which is calibrated in and the gain increased to 5-1 db. Instruction S.4 Section 11 A small amount of current feedback is applied Valve Data to V2 by dividing the bias resistance (R17, T30) A~rode Scraers Filnrnerrls into two sections and leavirig one section, R17, Slage Vnlrae Curred C~rrre~rl undecoupled. Voltage, feedback is applied to mA mA l'olls Amps. both V1 and V2 via 1116. 1 AC/SP3H 3.0 1.1 4 1 The total feedback applied to the two-stage 2 AC/SP3B 3.1 1.1 4 1 amplifier reduces its output impedance to a value 3 I141 - - 4 0-3 suitable for feeding into the programme-metcr 4 AC/VPl - -- 4 0.65 circuit proper. In addition, the total feedback Total Feed, 15 mA. reduces the gain of the amplifying stages from 90 db Ncon Type Osram Sl. to 54 db and improves the general stability of thc H.T. supply, 250 or 300 V. amplifying circuit in relation to valve or h.t. I..T. supply, 4V. a.c. or 6V. d.c. supply changes. 'The output impedance of llie amplifier stage is 4 kilohms to 4.5 ldohms, according-to the setting General Data of the feedback potentiometer. InPi/l ImFeriance, 10,00011. The amplifier stage feeds into thc programme I'olewliom flers meter circuit, which is identical to that of the Level db (Coarse) : Paintoll PN/.IA I, 100 kfl. PPM/2. It comprises a double-diode rectifier, V3, Level db (Fine) : Painton PFIIAI. feeding into a &triable-mu pentode, V4, in the Adjust Se~isitivity: blorganite Stackpole MNAP anode-circuit of which is inserted thc peak pro- 25450, 250 kR. gramme metcr. Adjust Zero : Morganite Stackpole MNAP With no input signal, the anodc current of V4 50350, 50 kfl. deflects the right-hand zcro rncter to thc ' no Adjust Law : Morganite Stackpole MNAY signal" ' zero rnhrlc 10250, 1 ki1. On the application of sig~ialvoltagc to thc inl)ul Zero Halancc : 3Io1pnitcStackpole LHAP stage, the rectificd output of V3 applies bias to V4, 50250, 5 k12. decreasing the anode current and reducing the Ndard Iiey, P.O. No. 228. left-hand deflection of thc mctcr in proporti& to Jleler Swilck, Yaslcy 'fypc 11, ?-bank, 9-position. the strength of the applied signal. For further details of this part of the circuit,see under PPM/2. Operation Switch 011 the maills unit about 10 minutes Performance before usc. The 'frequency characteristics of the TPM/B are Set main Level DB control to Adj. Zero such that' for any setting of the main sensitivity, position and adjust zero by means of thc control, a sensibly linear rcsponsc is obtained for Adj. Zero control to thc ' no signal ' zcro all frequencies bctwecn 50 cis and 10 kc/s, mark. maximum variation of level being of the order of Plug thc output of thc CAI./l to the iiiput of the f 0-1 db. The sensitivity variation does not TPM13. Sct levcl controls to zero. The meter exceed f 0.1 db for a cliangv in h.t. volts from should read 4. If it does not. adjust to 4 by 300 to 250. means of the Adj. Sens. control. Set main level control to + 8 and.- 8. Thc meter should read 2 and G respectively. If Meter not proceed as follows :-- The rncter used with the TPhIj3 is idciitical to that described in thc Instriction on the PPM/2. (R) To open scalc (i.c. meter rcads above 2 and If an external inetrr is used with the TPM/3, it below 6) : rnust be placed in series with the existing meter, the (i) Unplug input jack. Set main lcvel wiring to hc effectcd ;it tags 11 and 12, which arc control to Adj. Zero (cxtrerne right) providcd for this parpose. When no external meter posit ion. is used, these tags will be shorted out. The meter (ii) Turn Adj. Lute1 control until meter connections arc, of course, at h.t. potential. reads above the zcro inark. Instruction S.4 Sectioh 11 (iii) I&-adjust to zero by means of A(j. (iii) Reduce the mams-uni t input to 200 volts.* Zcro control. If the meter reading rises, the Zero Balance (iv) Repeat tests 3 & 4, repeating Adj. Law control has not been rotated far enough. adjustment if necessary until correct If the reading falls, the control has been conditions arc obtained. turned too far. (b) To close scale (meter reading in test (1) (iv) Restore the mains-unit input voltage to below 2 and above 6). normal*, rotate thc Zero Balame control a Proceed as under (a) except that in (ii) meter small amount in the direction as indicated is set to read below zero. Some repetition may bc in (iii) above, and re-set the meter zero by necessary in these tests, but the error should be means of the Adjust Law control. reduced to a minimum by a process of elimination. (v) Reduce the mains-unit input volts once again to 2001, and proceed as above, until Recalibration after Neon and Valve Replace- varying thc voltage between 240 and 200 ments produccs no change in thc mcter zero Replacement of tlic rectifier 1111 should not readi:~g. affect the calibration in any way, but after replacc- (vi) Thc Zero &lame control should be lefr in ment of a neon tube or ACIVPI, the zero and law thc position thus found unless either the calibration including ' zero balance ' must I)e neon tube of the AC/VPI valvc has to he checked and adjusted. For this purpose it is replaced. necessary to provide a means of varying the supply *Note :-Each time the a.c, voltage is varied, voltage, and a tapped transformer or ' Variac ' about 20 seconds should be allowed to elapse should be interposed between the mains-supply beforc proceeding with the next step to allow sockct and the mains unit fccding the programmc the consequent variation in valvc-heater voltage meter. A variation from the normal working to takc effect. voltage down to about 15 per ce~ithelow normal (e.g., from 240 volts down to 200 ~,olts)should be Changes : TPM/3 and TPM/3A obtainable. TPMI3. Units having scrial numbers below 207 The procedure is as follows : are fitted with a retard-relay L34.43, operating (i) Adjust thc mains-unit input voltage to the from a 24-volt supply ; units having serial numbers nominal value, e.g., 240 volts,* and with the 207 and above are fitted with relay 31102 which Zero Balance control fully clockwise (i.e., may be operated from either a 24-volt or a 50-volt neon returned to enrth), line up the pro- supply. gramme meter in the usual way. TPMI3A. The only difference between this unit (ii) Set the Zero Balance control to its mid and the TPM/3 is that the retard-relay is an position, and restore the meter reading to liD 2500. This relay, operates from a 50-volt zero by means of the Adjust Law control. supply. Instruction 's.4 SECTION 12 VALVE TEST PANELS VALVE TEST PANEL VT/4 It is sufficient to measure the conductance at one General Description fixed point of the characteristic and to compare the The valve test panel VT/4 is designed for result with the known conductance of a good valvt: measuricg the mutual conductance of the various of the same type at the same point. types of small valves in use in the Corporation. A simple method of measuring mutual conduc- It also includes provision for measuring the cathode tance approximately is to change the grid voltage to heater insulation of indirectly-heated valves and by, say, 1 volt and to note the change of anode the emission of power rectifier and diode valves. current produced. The definition of mutual*con- The mutual conductance (g,) of all valves in .ductance given above, however, is only true, senrice should be checked.preferab1y once a month provided that (i) the changes in grid potential arc but at least every two months. Normally a valve small and (ii) the anode external impedancc is should be rejected when its mutual conductance zero, that is to say, the anode voltage is maintained has fallen to 50 per cent of the average value for constant. the type, as shown in Valve Instructiors, although Thc simple method of measurement mentioned in special circuits a valvc may cease to function is open to serious objection. In thc first place a satisfactorily before this hiit has been reached. I-volt change in grid voltage is too great, cspccially with modern high-gain valves, even approximately Principle of operation to satisfy thc first condition, while in thc second The amplification factor of a valve (p) expresscs place, to maintain the anode voltage unchanged the slope of the anode-volts grid-volts character- under the conditions of test requires either a istic in terms of the change in anode voltage per battery supply or readjustment of the ancde volts volt change in grid potential, i.e., on changing the grid voltage. A mains-operated equipment was, however, desired, preferably direct reading and sinlple to adjust. Another method of measuring mutual con- ductance, bascd on the application to the grid of a But the value of p is largely dependent upon the 50 c/s voltage is illustrated in Fig. 12.1 and is that mechanical construction of the valve and remains adopted in VT/4. substantially unaltered even though there may The valve is connected in the test circuit as have been considerable deterioration in the per- shown and the values of h.t. and of d.c. grid bias formance of the valve due to a falling off in the are adjusted to obtain the particular point on the emission. Such deterioration, however, results in characteristic at which the measurement is to be a chan,ge of,anode impedance and is made manifest made. A.C. at approximately 0.2 volt, 50 CIS, is by a change in the slope of the anode-current then applied to the grid of the valve under test, grid-volts characteristic. This quantity is termed and an equal voltage in opposite phase is applied the mutual conductance (g,) and expresses the to a calibrated slide-wire resistor, by means of change in anode current measured in milliamperes which a portion of this voltage is applied to the per volt change of grid potential, i.e., valve anode through a resistance of 100 ohms. A galvanometer circuit is connected, via a capacitor between the anode of the valve and the slider of the slide-wire, which is adjusted so as to obtain Clearly, g, will decrease as the anode impedance no a.c. in the measuring circuit. (r.) increases and therefore an accurate check of If the a.c. exciter voltage = En, the mutual conductance will indicate immediately g'n x En any deterioration in the performance of the valve. the a.c. anode current, I. = - 1,000 Instruction S.4 Section 12 then the voltage developed across the 100-ohm directly read as soon as the balanced condition obtains. gm x E, It will be obvious that a high degree of accuracy resistor = 1001. = - 10 can only be assured if the resistance of the slide- wire is made small compared with that of the anode If the condition of balance is obtained with a resistance so that it will have practically no effect portion x of the slide-wire included in the anode on the operating conditions of the valve. The circuit, the value of the balancing voltage applied actual value is unimportant so long as it is of the to the anode will be x x E,. correct order of magnitude. Similarly the actual value of the anode resistance is unimportant, but obviously if the value is increased the voltage applied to the slide-wire must also be increased in 'the same proportion in order to make the condition of balance obtainable. In order to make negligible a any variation of the voltage' across the slide-wire during the test, the impedance of the anode- balancing supply must be kept small relative to the resistance of the slide-wire. Similarly, since ANODE the test is normally carried out at zero bias, the BALANCING -Ep impedance of the a.c. grid-voltage supply also AC SUPPLY must be kept low in order to prevent fluctuation of the voltage due to grid current. Furthermore, to prevent the voltage of the h.t. supply from varying at 50 c/s, the fuiidamental frequency of the anode current, the supply circuit must be arranged to have a very low impedance at this frequency. Circuit Description The assembly consists of two panels, mounted either on a rack or on a trolley. The larger panel incorporates all the essential parts such as meters, slide-wire, galvanometer and operating keys. The GRID BIAS DC 1 smaller panel contains valveholders for accom- SUPPLY -vr modating the various typs of valves which may be required to be tested and switches for establish- Fig. 12. I. Method of Measuring Mutual Conductance ing the appropriate connections to thei,r sockets. employed in VT/4 The apparatus is suitable only for operation on 50-c/s controlled a.c. supply mains of 200-250 volts. For operation from a I 10-volt supply an additional auto-transformer is required. The standard model is not designed for operation on frequencies other Equating these two expressions we have than 50 c/s nor directly from d.c. mains, nor is provision made for the use of battery supplies. Where only a d.c. supply is available a motor alternator must be used. The standard apparatus is arranged for the whence g, = lox measurement of mutual conductance only at the fixed point on the valve characteristic correspond- That is to say, g, is directly proportional to the ing to 100 or 110 volts on the anode and zero grid length of the portion of the slide-wire included in bias. In special cases, however, e.g. for testing the anode circuit. The slide-wire scale can thus be variable-mu valves, it may be desirable for a small calibrated to enable the mutual conductance to be negative potential to be applied to the grid so that Instruction S.4 Section 12 measurements can be made at morc than one point anode connection is made via thc slide-wirc and a on the characteristic. For this purpose a special 100-ohm resistor. The anode current is read on a grid-bias supply unit with a very low impedance moving-coil milliammeter which actually requires has been developed which provides a continuously only 1 milliamp for full-scale deflection but is variable bias from zero to 10 volts negative. It normally shunted to read up to 100 milliamps. should be particularly noted that no other source The voltage is measured by means of an extra high of grid bias should be used for the purpose otlier- resistance voltmeter connected between the anode wise the readings obtained may be inaccurate. and cathode of the valve under test and passing The arrangement of the circuit of the mutual only 100 microamps at 100 volts. conductancc valve tester is shown in Fig. 15, and A separate potentiometer connected between that of the associated valve socket panel in cathode and h.t. positive and designated Adjzcst Fig. 16. Screen Volts controls the screen grid voltage Referring to Fig. 15, the filament-supply circuit supply, which is decoupled in the normal manner consists of a mains transformcr with a secondary by a capacitor connected between the slider and winding having taps at 2, 4, 6, 13, 26 and 40 volts the cathode lead. The voltage can be rncasured hy and capable of giving an output of 12 watts. In depressing a plunger key designatctl Press taRcud series with the primary winding of the transformer, Screer~ Volts, which transfers the voltmctcr from a variable resistancc, designated Adjzcsl Filame~rt the anode to the screened grid circuit. Volts is provided and is of such a value as to give Thc exciter and anode balancing voltagcs arc pro- nearly complete coverage betwcen the tappings. vided by a common mains transformcr. The A link, accessible when the back cover of the panel voltages in the two cases are equal but thc anodc is removed, is provided in scrics with the filament balancingswinding is connected so as to apply its supply. This can be withdrawn to permit the voltage across the slide-wirc in such a scnsc as to connection of an external variable resistance, on oppose thc voltage developed in the anodc circuit the rarc occasions when tl~cvaluc of filament due to thc excitcr voltage applied to thc grid. The voltagc required provvs to be unobtainablc with secondary impedance for both windings is approxi- the normal atljustmcnt provided. A rectifier-type matcly 0.1 ohm and the voltage available in cach voltmeter is used with separate leads which arc case approximately 0.2 volt. The exact valuc is taken back to the valveholder pins so that the unimportant and obviously any variation in the actual voltage applied across them can be read. mains voltage will cause both supply voltages to The voltmeter has two scales reading 0-6 and 0-40 vary equally. A link designatcd XY is provided in and the range is automatically changed by the tap the grid-cathode lead in series with the exciter switch, which is designated Set Filament Volts. An winding and is rcmoved for the connection of the ammeter is provided for measuring the filament grid bias unit when the lattcr is required. current and takes the form of a rectifier-type The slide-wire has a resistance of about 0.5 ohm millivoltmeter actuated by a current transformer and the full voltage of the transformcr would so that the load introduced into the filament correspond to a m~rtualconductance of 10 milli- circuit is negligible. The meter has two ranges amp/volts. A small resistance in series with thc reading 0-5A and 0-0.5A, the lower range being slide-wire, which is used for thc initial calibration, obtained by depressing a plunger key which reduces the maximum slide-wire reading to 0.5 increases the transformer ratio. milliamp/volts. However, a key designated G,,, x 2 For the h.t. a conventional mains-supply unit is provided by means of which a short-circuit is is used capable of delivering 100 volts d.c. at any placed across half of the anodc resistance and the load up to 60 milliamps. Very efficient smoothing range of measurement thus increased to 17 is provided and immunity from 50 c/s variation is millian~p/volts. A circular scale with a diameter of secured by the connection across the d.c. supply of about 5 inches and a slow-motion control is used an approximately tuned reactor and series capacitor for making the readings and permits of an xcuracy in order to make the impedance of the supply very bctter than 1 per cent over the whole range of low at this frequency. This also scrves to eliminate measurement from 0-17 milliamp/volts. hum from the galvanometer circuit which is The measuring device consists of a sensitive comected between the h.t. supply and the anode vibration galvanometer of the permanent-magnet of the valve. The supply is controlled by a poten- type sharply tuned to 50 c/s. The reading is pro- tiometer, drsignated Adjzrsl Anode T'olfs, and the vided by a mirror mounted at the centre of the Instruction S.4 Section 12 vibrating element which projects a spot of light oil of the plulsger key restores the grid voltage to a ground-glass screen. When the instrument is connection. passing a.c. the spot sweeps over the screen, tracing The anode current meter is also used for measur- out a beam of illumination extending an equal ing the insulation resistance between cathode and distance on either side of the centre line, the heater of indirectly-heated valves. The plunger key, amplitude of the ' spread ' being proportional to the designated Press for Cathode Insulation, when current. When the condition of balance is achieved depressed, connects the voltmeter as an ohmmeter a circular spot of light is obtained on the centre in series with the h.t. + supply lead and the line. The galvanometer may, however, be tuned to heater filament via the potentiometer designated any frequency between about 45-52 c/s. Therefore, Adjust Zero, the cathode being connected to h.t. where the frequency of the supply mains is not negative. The insulation resistance at 100 volts accurately controlled, or where a motor alternator can then be read directly on the auxiliary scale is used to provide an a.c. supply from d.c. mains, provided on the meter. the instrument can be tuned to the precise fre- In the Normal position of the 3-position switch, quency of the supply available and, provided the the anode circuit is arranged for the measurement supply frequency does not ' hunt ' by more than of mutual conductance. If the switch is moved to 1 c/s, the apparatus will work satisfactorily. the position marked Power Rectifiers Only a resistor The sensitivity of the apparatus at other frequen- is connected'in the anode circuit, such that normal cies is less than at 50 c/s due to the fact that the rectifying valves in good condition will pass at transformers in the power supply circuits and the least 50 milliamps of anode current per anode at tuned ax. shunt across the H.T. supply circuit 100 volts. In the position marked Diodes Only a have been designed to have a maximum efficiency much higher resistance is provided and the shunt at 50 CIS. The sensitivity of the instrument is, removed from the anode current meter. Diodes in however, much greater than that necessary to give good condition will pass at least 0.8 milliamp at the required accuracy of measurement and is 100 volts. accordingly reduced by placing a heavy shunt Referring to Fig. 16, a set of valveholders for' across it. The shunt has been made variable in 5-pin, 7-pin, 9-pin and octal based valves is pro- order to permit of calibration and compensation vided on the valveholder panel. Connectors are also where the galvanometer is tuned to a frequency provided for making the top-cap or side-terminal other than ?O c/s. connections required with some valves. The anode, With directly-heated valves the h.t. negative screened grid, control grid, and cathode or sup- return is to the centre point of the filament pressor connections from the main panel are and it is, therefore, necessary to determine brought down to the movable arms of a set of this accurately, otherwise hum will be introduced rotary switches, by means of which the appropriate from the filament into the anode circuit and will connections to the pins of the valveholders, and to result in a false reading of the mutual conductance. the top-cap or side-terminal, where fitted, can be A variable potentiometer, designated Adjust Zero, made. The filament or heater pins of each valve- is therefore provided across the filament supply holder are permanently wired, except for octal- with its slider connected to the cathode lead. To based valves, some of which have the heater determine the true electrical centre of the filament, connected between pins 2 and 7 and some between the conductance dial is set at zero, so that there will pins 2 and 8. An additional switch is therefore be no ax. applied to the anode from the balancing provided for making the appropriate connection winding, the h.t. is applied, and the plunger key, to this valveholder. It is only with valves having designated Press to Set Zero, is depressed. The 5-pin bases that the pin to which the cathode is latter operation isolates the grid from, the a.c. connected may vary; in all other valves the exciter winding and connects it via 50 ohms to the cathode is always connected to one particular pin. cathode. Any a.c. present in the anode circuit Except in the case of the 5-pin valveholders, under these conditions will be due to excitation therefore, the cathode connection is permanently from the filament circuit. The potentiometer, wired. The switch marked Cathode or Su$+ressor designated Adjust Zero, is therefore rotated until controls the cathode connection only for the there ceases to be any deflection on the galvano- 5-pin holder ; with other valveholders it controls meter, in which condition the true electrical centre only the suppressor connection. Very complete of the filament will have been found. The release screening of the control grid circuits is carried Instruction S.4 Section 12 out so as to prevent oscillation due to feedback. teiescope, the base of which is fitted with a This is extremely important in view of the high universal joint, should be adjusted so as to direct mutual conductance of many of the valves which the light on to the mirror. have to be tested. The low value of the grid- An adjustment is provided whereby the lens circuit impedance assists considerably in over- in the telescope can be moved longitudinally in the coming errors due to capacity currents and thus outer housing in order to focus the lamp. When enables a very high degree of screening to be the lamp is correctly focused a sharp image of the obtained fa'irly simply. The main panel and the filament will be obtained on the mirror of the valveholder panel are interconnected by terminal galvanometer. A well-defined spot should then be blocks so that any new valveholders that may be obtained on the screen. required in future can be added to the valve- If the spot does not fall on the screen, or is not holder panel without disturbing the main panel. central on the screen, the galvanometer itself must be adjusted. Adjustment in the vertical direction Setting-up the Apparatus for Use can be made by rotating the galvanometer body Insert the mains plug to correspond with in its tninnions, and adjustn)ent in the horizontal the voltage of the incoming supply and insert a direction by altering the angle of the mirrordy UU/5 or equivalent rectifier valve in position means of an adjusting screw at one end of it. at the back of the panel. Then operate the switch Whenever any adjustment is made the position of marked Mains. the telescope will, of course, also need to be re- Power is thereby applied to the primary wind- adjusted so as to obtain maximum illumination on ings of the h.t. exciter, and filament trans- the screen. formers. A voltage will be developed across the The sensitivity and tuning of the galvanometer output of the h.t, supply circuit, but since there should then be checked. The tuning should be is no valve in any of the valveholders and the h.t. such that the ' spread ' is a maximum for any switch is open, there will be no current in either particular setting of the sensitivity, and the sensi- anode or screened grid circuits. A voltage also tivity should be such that when the conductance appears in the filament circuit and the filament dial is set at 8.5 milliamp/volts the ' spread ' just voltmeter will read a value near that indicated by covers the full width of the screen. The tuning can the position of the selector switch. The indicator be adjusted by slightly turning a screw which will lamp associated with the mains switch and the be found on the body of the galvanometer at the lamp in the telescope of the galvanometer will both opposite end to that controlling the angle of light from the 6-volt winding of the exciter trans- the mirror, and the sensitivity is adjusted by the former. The lamps used are Osram &volt, 1.6 potentiometer located near the galvanometer watts, MEX Reference OS.7588. A.C. from the which controls the value of the shunt. anode-balancing winding of the exciter transformer Some of the early types of galvanometer have will be applied across the slide-wire and thus across been found to exhibit slight stretching of the the galvanometer input in series with the anode phosphor-bronze wire, and in some cases the resistance. The galvanometer should therefore stretch has sent the tuning range of the galvano- read and, if the conductance dial is set at zero, a meter beyond the normal range of the adjusting spot should appear at the centre of the ground- screw. If, therefore, it should be found impossible glass screen. with the adjusting screw to tune the galvanometer If no spot is formed it may be due either to correctly on either side of the resonant point, the failure of the lamp in the telescope or to displace- galvanometer complete with its magnet should be ment of the telescope so that the light is not removed from the apparatus, carefully packed, and directed on to the mirror. returned to Equipment Department for re-tuning. Remove the cover from the back of the panel, It is inadvisable to attempt to repair galvano- the mains switch first being placed in the Off meters on site, as special wire and special solder position and the mains plug withdrawn, in order for making off the end have to be used and the wire to obviate all possibility of accidental short- has to be heat-treated after assembly. circuits. Replace the mains plug and operate the mains switch. The conductance dial should Mutual Conductance Test remain set at zero. Assuming that the telescope Look up the valve to be tested in the RBC lamp is found to he alight, the position of the Valve Instructions Manual. The code number of Instruction S.4 Section 12 thc valvc will be found in the left-hand column of Triodes En 7 100 the appropriate table, and against it, in the third Tetrodes (othcr than KT column. appears the class of base with which it is types) En=-100, Eq--80 titted. 'I'he sixth nntl seventh columns show Pentotles an11 KT typr the manner in which the vario~~spins arc corn 'I'e t lodes En- 110, Ecc= 100, Ec~rp70 ~~ected. Put the Sel Filnnzeril I 'olls switch to the range Some of the larger valves take an anode current nearest the value .of filament voltage quoted for greater than 60 milliamps at 100 volts on anode the valve in column 1 of the table and insert or screen, consequently, since the rectifier will not thc1 valvc? in the appropriate socket. Close the supply 100 volts at currents greater than 60 milli- mains switch antl adjust the voltage applied amps, the conductance must be measured at a to the tilament accurately to the value listed, by lower voltage. The valves to which this applies means of thtr Aiijl~slFilanre~rl I'olts rheostat. Tht. are suitably indicatrd in the tables. tilarnrnt current can I)e read on he anmetcar For valves with directly-lieatetl filaments it provitletl. Ii the current is less tha~~0.5 amps is necessary before attempting to measurc the the lower range of the meter may he usetl by conductance, first to return the 11.t. negative cle}wessing the press-key provided on the panel to the centre point of the filament. This is done by by the side of the meter. The filammt voltage retaining the conductance dial at zero, operating selecting switch automaticallv changes the range the press-key marked Press lo Set Zero, and by of the voltmeter when 6 volts is exceeded. slowly rotating the control marked Alljmt Zero Hy means of the rotary switches on the valve- until a clear-cut spot is obtained on the gnlvano- holder panel the anode, screened gritl, control gritl meter screen. The press switch should then be antl cathode or suppressor grid connections to the released. Since the clectrical centre may vary various pins of the valveholder in use should be even for valves of the same type and make, this sct up in accordance with the connections listed adjustment must be made afresh for each valve for the particular class of valve base. Any switch with a directly-heated filament. not in use, e.g. the Screened Grid switch when Rotate the conductance dial until a clear- testing a triode, should be placed in the O[/position. cut spot of light is formed at the centre of the (The filament connections in the case of all the screen. The conductance can then be directly valveho1d;rs except that for octal-based valves, of read on the scale. If the ccnd~~ctanceis greater which they are completed via an auxiliary switch, than 8.5 milliamp/volts it will not be possible and the cathode connection in the case of all except to obtain a clear-cut spot unless the presz-key, the 5-pin valveholder, are permanently wired.) designated Gtn x 2, located alongside the conduc- Set the conductance dial at zero and then tance dial, is operated. In this case the mutual make the h.t. switch. The anode voltage conductance will be double thc figure indicated on and anode current can be read on the appropriate the scale when a clear-cut spot of light is formed. meters, and the screen volts on the anode-current Certain high-conductance pentodes or tetrodes meter by de1)ressing the press-key located along- are liable to oscillate when being tested in the side it. Individual potentiometers are provided for bridge. The purpose of the 0.0001-micro fared adjusting the two voltages but since the anode capacitors shown in Fig. 15 is to suppress this current depends upon the screen-grid voltage, antl oscillation. It has been found, however, that the adjustment of either alters the load on the these capacitors arc not always successful. They rectifier, the two adjustments cannot be made are therefore now omitted and instead distri- independently of one another. The correct settings buted capacitance is introducfd by cutting back are fairly quickly obtained by the process of trial the screening of the cable and connecting some and error, involving the checking and resetting of of thesc cables to the cathode. In practically the anode voltage alter each adjustment of the every case this has been found to be a complete screen-grid voltage and the subsequent rechecking cure: but it is not impossible that oscillation~may and readjustment of the latter. still occur in some isolated instances. In order to For all valves requiring an anode current of less detect whether a valve being tested is free from than 60 milliamps, the testing conditions for the oscillation, the control-grid terminal of the valve various types of vdves are as follows, Eg in every should be lightly touched with the finger. If the case being 7cl.o. light spot on the galvanomctcr screen does not Instruction S.4 Section 12 move, then no oscillation is present. If, however, a be switched off and the anode switch on the valve- movement of more than about 1 millimeter is holder panel, moved to make connection with detected, then it is possible that the valve is another of the anodes which should then be simi- oscillating. A cure can usually be effected by larly tested. For normal rectifiers the anode current connecting a 04001-microfarad capacitor from should be of the order of 50 milliamps per anode. either the anode, control-grid or screen-grid ter- On the ccmfiletion o/ the test restore the 3-posit to^ key minal direct to the cathode pin by very short leads. to the normal posiliol~. Should this not effect a cure the Valve Section should be notified. Testing Diodes Diodes are tested in the same manner as power Heater-cathode Insulation rectifiers except that the test is made with the 13y operating the press-key marked Press /or 3-position switch in the position designated Diodes Cathode Insulalioa, the anode voltmeter becomes Only. In this position the anode current meter is an ohmmeter conilectcd for reading the inslilatio~~ unshunted and reads 1 milliamp full scale. Most resistance at the anode voltage between filament diodes in good condition will show a current and cathode of indirectly-heated valves, the exceeding 0.8 inilliamps at 100 volts h.t. (The cathode being made negative with respect to diode elements of complex valves, e.g. double-diode the heatcr. The resistance is indicated on the lower triodes, may be similarly testcd by suitable con- scale of the voltmeter. Good valves should have a nection of the valveholder switches.) Upon !he resistance greater than 3 megohms. cmfilelion oJ !he lest relun~the 3-bosilimt ke,v lo With some valves, however the heater-cathode Normal. insulation mag be as low as 1 megohm without the performance of the valve being affected. Valves with Non-standard Bases No hard and fast rule can he laid down as to the Some valves, for example special diodes, are fitted lowest value of heater-cathode resistance which can with non-standard bases and certain other valves, he tolerated in practice. Valves should not, there- for instance a 2-volt double pentode, although fore, be rejected because they have a heater- fitted with a standard base, have non-standard cathode insulation resistance lower than 3 megohms connections. Should it become necessary to test unless some other factor influencing the per- any number of these valves, Valve Section should formance of the valve has been'noted, or unless a be communicatccl with and an adaptor will be number of other valves of similar type have provided. indicated a much highcr resistance. The heater-cathode insulation test will he non- Precautions operative with directly-heated valves. (I) Always see that the filamcnt voltmeter switch. is correctly set beiore inserting a valve or Testing Power Rectifiers switching on the mains. ' For testing power rectifiers, whether directly or (2) Sec that the switch marked Normal, Power indirectly-heated, all switches on the valveholder Reclijiers, and Diodes is in the correct position. panel should'be set at O//. The 3-position switch Nerer measurc condzrctance ~rnlessthe switch is should be operated to the position designated at Normal. Power Heclijiers .Only. The filamen t-vol tage switcll (3) Before making the h.t. switch, see that the should be set to the appropriate range and the correct connections are made by means of the valve inserted in the valveholder and the filament switches on the valveholder panel. voltage adjusted to the correct value by means of (4) Never move the valveholder switches unless the the Adjust Filame~it Yolls control. Each of the 11.t. switch is off. anodes is tested in turn. The anode switch on the (5) Never remove the back cover unless the mains valvcholder panel shonld be set to make connection plug is pulled out. with the anode to be tested, the h.t. should then (6) Do not interfere with the galvanometer more be switched on and the anodc volts adjusted to 100. than is absolutely necessary. Once it has been (For some high-voltage filament indirectly-heated set up it should require no adjustment unless rectifiers, it is also necessary to make the cathode the frequency of the supply is changed. connection.) The anode current will be indicated (7) Owing to its high sensitivity, the anode volt- on the anode current meter. The h.t. should then meter may sometimes ' stick ' slightly. If this Instruction S.4 Section 12 occurs it will be desirable to tap the meter ciples apply for pentodes. An a.c. voltage is gently when setting the anode volts or measur- applied to the anode from L1 and rectified by the ing the heater-cathode resistance. valve under test, the anode current Ia, averaged (8) At the commencement of the test some valves over one half-cycle, being indicated on the milli- may not give a clear-cut spot but generally this ammeter. (It should be understood that this will gradually inlprove if the valve is left in current b-ars no direct relation to the current circuit for a few minutes with the filament and which would be obtained by the application h.t. supplies switched on. If, however, after of a comparable d.c. voltage, and cannot be some minutes the spot still remains out of regarded as the Ea-Ia characteristic of the valve.) focus, primary emission from the grid should This rectified Ia is balanced off to zero bv a be suspected, but if everything else be normal second rectified a.c. voltage -Ea, applied in oppo- this will not as a rule be sufficiently serious to site sense from the coil L2, the potentiometer R1 warrant rejection of the valve. and rectifier MR 1. If, under these conditions, an a.c. voltage Eg is applied to the grid of the valve VALVE TESTER VT/5 in phase with the anode voltage, the mean anode a. General Description current will increase and a reading appear on the The valve tester VT/S is a portable instrument meter. This change in anode current (due to the operated from 200/250 volts a.c. supply and de- application of a voltage to the grid) is a measure- signed to test, by direct measurement, the mutual ment of the g, of the+ valve. The higher the conductance (t,)of most types of small valves in mutual conductance, the greater will be the read- current use. ing. This voltage is applied by depressing the I -- Fig. 12.2. Face Panel VT/5 The instrument incorporates new principles of switch S1. It will be seen that in the ' release ' g, measurement in that no d.c. potentials are position of S1, voltage is applied to the grid from applied to the electrodes of the valve under test. the coil L4, this voltage being in anti-phase to the This simplifies design by eliminating the necessity anode voltage, and that when the key is depressed for valve rectifiers and smoothing circuits. The an equal voltage of opposite phase is applied. degree of accuracy is slightly below that of the Let it be assumed that the first condition repre- VT/4 but the error does not exceed j: 3 per cent. sents an Eg of - .5 volt peak and the second an Eg of 4- .S volt peak. Then the change in anode g, Tests for Multi-electrode Valves current resulting from depressing and releasing the A simplified circuit showing the conditions when key would be equivalent to the change obtained testing a triotle is given in Fig. 12.3. Similar prin- by the application of 1 volt d.c. to the grid of a Instruction S.4 Section 12 valve normally having zero bias. B 7G Valves in relation to the R7G Filament switch. It now remains to measure this change in terms The switch labelled Normal, DH, DH, is provided ?f milliamps per volt ; this is done by applying a to enable directly-heated valves to be tested rectified a.c. voltage -Ea, to the anode from the with either leg of the filament earthed. (See circuit comprising L3, R2 and rectifier MR2. under Operation.) - Fig. 12.3. Schematic for g, Tests VT/5 This calibrating voltage, which is in opposite sense Anode Serpply to the anode voltage, is adjusted by the poten- The normal anode h.t, supply is taken from tiometer R2 until the anode current resulting from the 70.7-volts tapping L6-L8 of the transformer, the applied Eg is reduced to zero. The g,is then read off direct from the potentiometer scale. Emission Tests for Diodes axd Rectz)ers In this test, 100 volts r.m.s. is applied to the anode through the meter and a loading resistance as indicated in Fig. 12.4. A reading of more than 3.5 on the meter mdicates that the valve is func- tioning normally, although it should be understood that this does not represent the mode current - which would be obtained if the valve were tested by applying 100 volts d.c. Circuit Description (Fig. 17) Filament Szipfilies Fig. 12.4. Schematic for Diode Tests VT/5 The various tappings from the transformer filament winding are taken to the Filament Volts via the selector switch and the meter to the switch, from which the supply is wired direct to the traveller of the anodc electrodeswitch. (It should filament sockets of all valveholders except those be noted that 70.7 represents the r.m.s. voltage, for octal and B7G bases. The filament sockets of hence the peak voltage supply to the anodes is octal valves are wired via the Octal Filament switch. equal to 70.7 Y 2-- 100 volts.) The return lead This is necessary because, whereas most octal of the h.t. winding is taken through an overload valves have their filaments wired to .pins 2 and 7. circuit-breaker relay which, in the event of the a few types have them wired to pins 2 and 8, or anode supply being connected to an earthed 7 and 8, Comparable considerations apply to electrode, causes the main supply to be interrupted. Instruction S.4 Section 12 Screen-grid Supply 7'ke Grid Circuit This is taken from the transformer tapping Grid voltage is obtained from the transformer 1.7-L8 for 80 volts or L6-L8 for 100 volts. centre-tapped winding L9-Ll l. The application has already been explained. The actual voltage from the winding is 0.35 r.m.s., i.e. 0.5 volts peak Zero Adjustiq Circuit on either side of the centre tap, the circuit bring This circuit, which supplies the backing-off connected to the grid electrode switch GI via the voltage -E,, for setting the meter to zero, com- g,, switch and a jack which may be used tn provide prises the transformer winding L3-L4, the Adjust grid bias. Zero potentiometer, R6, and rectifier MR 1. Operation Calibratizg Circuit 1. 1M~d~4alConductar~ce 7'esls This includes the transformer winding L1-L3, (i) Before conilecting the instrument to the rectifier MI3 2 and potrntiometer KS,calibrated for mains supply, make sure that the green reading off the g,of the valve under test. There art. lead on the main Hexible cortl is properly four variations of this circuit, controlled hy thc earthed. Selector switch. (ii)>, Set the A.C. Mains switch to the correct g, Coarse. In this position of the switch stud for the local supply voltage. This step the circuit is completed to the anode is important since the instrument is not via section 3 of the selector switch, the provided with a variable potentiometer fnr meter being shunted on section 2 by the adjusting filament voltage. (The filament 2-22-ohm resistor R8. The purpose of this voltage is selected by the Filament Volls circuit is to permit in approximate adjust- switch, and will lie within the limits news- ment of the backing-off voltage -Ea (by sary for valve testing only when the mains means of R6) when lining up the meter. transformer is set on the right tapping.) gm Fine. In this position of the selector (iii) Set the Selector Switch to gm Coarse. switch the circuit is similar to (1) but with (iv) Set the Filame?rts Volls switch to thc right the meter shunt removed. This circuit value. permits a tine adjustment of the backing-off (v) Set the individual electrode switches to the voltage and, with the gm key depressed, appropriate pin numbers. Check thc Octnl for testing all valves other than diodes or,B7G Filament switch against the condition and rectifiers. If the normal g, of the required. valve is known to be above 10, the key (vi) Set the Screen Volts switch to the voltagc g,n x 2 must he pressed simultaneously required. .with the g, key. This shorts out half the (vii) Switch on the mains supply. The neon series resistance in the calibrating lamp will glow and a reading should appear circuit and, in effect, doubles the cali- on the meter after approximately 30 seconds. brating voltagc -Ea,. The actual gm will If there is no meter reading, operate the thus he twice that indicated on the scale. Press to Reset button. This restores the over- (iii) Rectqier. With this circuit, the anode load relay contacts (manually) to the supply voltage is taken from the tapping make position, thus completing the mains L5-L8 (100 volts r.m.s.) and a 1-kilohm supply circuit. If, on pressing this button loading resistor R9, is switched in series a loud buzzing is heard, release the button with the anode circuit. The meter shunt is immediately as this indicates a short-circuit also connected. Measurements are taken across the maill transformer winding, with the Adjust Zero potentiometer fully probably due to faulty setting nf the clockwise, and indicated readings should in electrode switches. all cases be more than 3.5 milliamps. (See (viii) Move the Adjt~st Zero control until the under Operation.) meter reads approximately zero. (iv) Diode. The same conditions apply as in (ix) Set the Selector switch to gm Fl)ie and (iii) except that the I-kilohm loading adjust for accurate zero if necessary. resistor R9 is replaced by 10 kilohms (XI Press the gm key and adjust the meter R10, and the meter shunt removed. reading to zero by means of the calibrating Instruction S.4 Section 12 g,,, dial. The reading of the dial will indicate 3. Emission Tests for Diodes and Power Reclifiers. the mutual conductance in milliamps per This- test gives the relative emission capabilities volt. In order to obtain an accuracy of of the cathodes and hence the d.c. impedance of within 2 to 3 per cent, repeat the operation the anode-cathode path. In each case, the test rapidly, alternately pressing and relesing consists of applying a.c. voltage to the anode the gm key until the zero adjustment and through a load and noting the feed shown on the the calibrated g,, dial adjustment give the meter. same meter reading. For valves whose conductances are over 10 (i) Carry out tests (i) to (viii) as indicated milliamp/volts, the key gm x 2 should be depressed under the heading 1. simultaneously with the gm key and balancing (ii) Set the Selector switch to Rectifier or Dio,ie, carried out as above. The gm will be twice the scale according to the.type being tested. reading. (iii) Set the Adjlcst Zero control fully anti- clockwise. A reading of 3.5 or above 2. Directly-heated Valoes indicates normal emission. The instructions given under 1 apply equally C to directly-heated valves, but an additional opera- Note :--It must be emphasised that this reading tion is necessary to eliminate errors due to un- has no ordinary relation to the current balanced filaments. This consists of taking a which will be drawn if 100 volts d.c. were reading on both D.H.positions of the special key, applied to the anode, as in the case of the and obtaining a mean gm from the two results. test panel VT/4. Instruction S.4 SECTION 13 MICROPHONE CABLE TESTER MCT/1 This unit has been designed to fxilitatc the primary circuit before making the secondary testing of studio and 0.B microphone leads 'for circuit and to break the secondarv circuit before continuity and noise breaking the primary circuit. The theory of the tests can be understood from The procedure for carrying out these tests at a the simplified circuit diagram Fig 13.1. The micro- studio centre and an 0.B point is as follows. For phone lead under test is connected in series with a the continuitv test the lead to be checked is 6-volt cell, a 6-volt lamp, a lever key and the disconnected irom the microphone terminals and low-resistance primary winding of a transformer is connected to the terminals marked Mic. Lead with a turns ratio of 1 : 11. The lamp lights to (MicrophoneEnd) on the unit. The other end of the indicate continuity and the circuit is so arranged microphone lead is unplugged from the wall socket that in one position of the key the continuity check and plugged into .the Wylex socket labelled Mic. is applied to the outer screen of the microphone Lead Plug below the terminals on the unit. At an lead, and in the other position of the key to the two O.B. point the end of the lead remote from the conductors of the lead connected in series. To microphone is connected to the terminals labelled test for noise the secondary winding of the Mic. Lead underneath the Wylex socket. The key V Fig. 13.1. Simplified Circuit Diagram of MCTI I transformer is connected to the input of a micro- is then operated to the up position to check screen phone channel and, with current in the primary continuity and to the down position to check circuit and the microphone channel faded up, the conductor continuity. lead is shaken and moved about whilst' the PPM To carrv out the noise test in a studio the micro- is watched. If there are any fractured strands in phone lead-is connected to the unit as already the screen or conductors of the microphone lead described and in addition the flexible lead labelled thc changes in resistance resulting from this To Studio Mic. Socket or Adaptw Socket which movement cause changes in the primary circuit, passes through the front panel of the unit and and produce e.m.f.'s in the secondary circuit which terminates in a Wylex plug is plugged into the are heard as crackles and are registered on the vacated microphone socket. At an O.B. point this PPM as kicks of the needle. 1'0 re vent loud Wylex plug is plugged into the Wylex socket noises when the primary circuit is made and broken, labelled Adaptor Socket on the right-hand side of the contacts of the key are arranged to make the the unit ; this socket is connected internally to a Instruction S.4 Section 13 second flexible lead labelled Local Amp. In Lead Microphone leads which pass the continuity test which passes through the front panel and has ends sometimes cause noise due to bad contacts near suitable for connecting to the input terminals of one end of the lead and a cure is usually possible an OBA/8. To test for noise in the screen circuit by cutting off, say, one foot of lead and re-making the key is operated to the up position and the lead the ends. If crackles are obtained giving deflections is shaken, particularly near its ends, whilst the up to 1 or 2 on the PPM (with ap amplifier gain of PPM is watched. The key should then be set to the 70 db) and if the fault cannot be traced to down position and the lead again shaken to check either end of the lead, the lead shol~ld bc for noise due to the conductors, rejected, ADAPTOR SOCKET AMP IN MIC LEAD PLUG LEAD Fig. 13.2. Complete Clrcuit Diogrom of MCT/I Instruction S.4 SECTION 14 AURAL SENSITIVITY NETWORKS ASNIB, ASN14, ASN/4P Introduction ivity of the ear to the frequency content of the Aural sensitivity networks ASNI3 and ASN/4 noise, and ideally, this particular discrepancy can are designed for making frequency-wcighted thus be eliminated by introducing into the measur- measurements of noise, and conform with the ing circuit a frequency-weighting network which C.C.1.F.' requirements for measurements on broad- modifies all peak-noise voltages in such a way that casting circuits, as based 'on appreciation tests for a given disturbing effect their peak separations carried out to determine thc disturbing cffect of from programme are all the same. noise in thc presencc of ml~sic. 'The ear is also sensitive to the repctition rntc of Both networks employ the same circuit, and arc the interfering noise, and again, ideally, this can used in conjunction with a T.P.M. or. P.P.M., but be taken into account by a suitahlc choice of time whereas the ASNI3 is in portable form the ASN/4 constant for thc measuring instrurncnt. is intended for rack-mounting. The ASN/4P J replaces the ASN/3. C.C.I. F. Weighling Characlerislic 'The weighting characteristic specified by the Noise Measurements C.C.I.F. is given below in tabular form and its General Comideralions general shape is indicated (necessarily to a lower The normal BBC practice hitherto regarding degree of accuracy) by Fig. 141.1. noise measurcments has been to measure the penk- noise to peak-programme separation and, providing this was better than 46 db, to rely on a listening test for the final assessmc-nt. The inadcquacy of this method has long bcen clear, since on the one hand induced 50-CIS hum or h.f. carrier break- through at 40-db $eparation pay occur without marring programme under normal listening con- ditions. whereas on the othcr hand such induced noises is 1.f. carrier, crosstall<, ringing, telcprinter signals and supervisory tones can be very objec- tionable at a separation of 16 db. Unfortunately, also, a subjective listening test is not a reliable means of estimating the disturbing effect of noises Fr.qusncy, cis on programme, as the assessment depends on the Fig. 14.1. C.C.I.F. Weighting Characteristic observer, the loudspeaker and the general listening ; conditions opinions are thus likely to differ both Freqftency Weighling Freqen c.y between BBC stations and between the RBC and kc/s db kc/s the Post Officc. 0.06 -32.2 4 The subjective influcncc co~ildbc rcduced by 0.1 -26.1 5 determining and tabulating the acceptable separa- 0.2 - 17.3 6 tion for each particular type of noise, but before 04 -- 8-8 7 such a table could be applied it would be necessary 0.8 - 1.9 8 to identifiy the noise, and even then the method 1 0 9 could only be used successf~~llyprovided that not 2 + 5.3 10 more than one kind of noise was-present at a time. The discrepancies between peak-noise measure- The C.C.I.F. recommended that a network with ments and the degree to which various noises mar the given characteristic should be used with a programme are due in part to the varying sensit- specified measuring instrument ; this had a time constant which resulted in indications approximat- * Comitt Consultatif International Ttltphonique : Paris, July 1949 : tome 4, page 193.' table 2. ing to an r.m.s. law. Obviously, however, it was Instruction S.4 Section 14 undesirable for the BBC to introduce this ad- The I-kc/s input and output impedances of the ditional type of measuring instrument unless network itself are both 600 ohms, but this is essential, and Designs Department therefore made increased to about 680 ohms in each instance by a large number of subjective tests to determine copper losses in the transformers. whether the standard T.P.M. when used with the Provision is made for replacing the weighting C.C.I.F. network gave consistent results for all the network by a 600/600-ohm loss-pad, which can be usual types of noise experienced on lines. The introduced by a key, and is the I-kc/s equivalent consistency obtained in these tests was of the same of the network between the apparatus sides of the order as that resulting from the use of an instru- transformers. The overall insertion loss between ment built to the C.C.I.P. specification, although the line sides of the transformers at 1 kc/s is about with neither instrument could exact correlation be 10 db in either position of the key. obtained. However, in measurements which The required frequency characteristic is achieved attempt to simulatc subjective opinions the by using two separate bridged-'r networks followcd :icceptance of somtb degree of compromise is I)y n low-pass filtcr. The shape of the curve is norrnally required. governed at low and middle frequencies by the bridged-T nrtworks iintl at high freqrie~~ciesby Circuit Description (Fig. 14.2) the filtcr. Cenernl Thecircuit takes the form of a constant-resistance Bridged-T Netxorks network designed for operation between 600-ohm Advantages of the bridged-T conformation are terminating impedances. The weighting network a constant input and outpi~timpedance (here600 Fig. 14.2 Networks ASN13, ASNI4. ASNI4P Drawings No. EB 7885 and 7944. EA 11 114 itself is unbalanced, but the input and output ohms) at all frequencies and a minimum basic connections are made via unity-ratio unbalance/ insertion loss using relatively few components. balance transformers. The ASN/3 is provided with The first bridged-T, incorporating C.1 and L1, is duplicate input and output jacks ; the ASN/4 is responsible for most of the fall-off in the bass. terminated on a tag-strip for permanent wiring. The second bridged-T has two resonant branches, Instruction S.4 Section .14 the series acceptor circuit C2 and L2, and the shunt which are attached four pillars supporting a top- rejector circuit C3 and L3 ; both branches are plate carrying the Line-IL$/A.S.A'. switch. The tuned to 5 kc/s, at which frequency the insertion base-plate forms a sub-panel occupying one quarter ldss of the complete network has its minimum value, of a standard 44-in. n~ounting-plateon a 22-ij~. the overall loss including transformers being then bay, and one such mounting-plate can thus be some I! to 3 db. The required shape of response equipped with up to four separate sub-pancls curve in the region of 5 kc/s is obtained by adjust- carrying sinall units of various types. The un- ing R7 and R8, it being necessary, however, to equipped 44411, by 22-in. plate ready drilled to maintain their prodiict at 6008 to avoid altcring the take fom sub-pancls and with end-slots for bav image impedances; increasing R7 and reducing fixing is designated a General Purl)ose hforrntin; K8 increases thc 5-kc,% insertion loss, and vice GIJM/2. versa. Operating Procedure LO~J-~~USSFiller See Communications Department Operational The low-pass filter coiisists of a 600-ohm proto- Instruction No. 205. type section with a cut-off frequency of 8.2 kc/s, *. preceded by a shunt-derived terminating half- General Data section with an m value of 0.7 and a frequency of ImPedmces infinite attenuation of 11.4 kc/s. Normal sourcr Z = - 600 L1 The terminating half-section, comprising L4, Input Z -- 680 L1 (I)alancrd) at 1 kcis C4 and 0-0227 pF of C5, improves the impedance \\.it11 600- R load. presented to the bridged-T networks, and intro- 011tln1t % - 6HO L1 (Ix~lancctl)at 1 lic,'s tluces considerable loss over a comparatively wit11 (3)O- i! S~IIKV. narrow band of frequencies immediately beyond J,oid % - 600 S? the pass-band of thc prototype ' section, thus providing the steeply falling response above 8 kc's I~rscrlioi~,Loss shown in Fig. 14.1. (The value for C4 of 0-0"'2 ficprtncy Iirscrlir~ir I 111serliotr marked on the circuit dia,gram, is somewhat kc/s Loss dl) kc/s Loss db below the calculated figure, but is built out on 0.06 43 5 2.4 test to tune with Ll at 11.4 kc/s. 50 c!s.) 0. I 36.9 6 2.6 The prototype section, which is a n-type struc- 0.2 28.1 7 3.5 tl~recomprising L5, C6 and ,0323 pF of C5, has , an attenuation characteristic which rises gradually beyond the cut-off frequency ; this section ensures that the insertion loss of the weighting network remains high up to the maximum frequencies at . which a T.P.M. is sensitive, and thus prevents in-. ~- correct weighting. Corrtpo~rertl'Ij~fies Capacitors ( f1 prr cent tolcrancc) : Mechanical Construction C1 C2, Muirhead Type 3'5A'l'. The ASK13 is constri~ctetl on a bent-up C- C3, XIuirhead Type 31.81. shaped chassis to which is attached a panel carry- (Cl -C3 in 33AT case, inverted mounting). ing input and output jacks and an earth terminal, C4-C6, L.E.M. Type 3220. together with a switch marked Line-up in the Inductors : see Fig. 14.2. normal positionand A S.N.in the operated positio~~. Jacks : P.O. No. 41 12B. The unit fits inside a wooden case provided with a Resistors (52 per cent tolerance) : Welwy~i hinged lid and a leather carrying strap. Overall Type SA3622. dimensions are 10 in. by 9 in. by 5 in. approx- Switch : N.S.F. Type TG.3 standard I-section imately. The total weight is 133 Ib. 4-p. 2-w. The ASM/4 is constructed on a base-plate to Transformers : see Fig. 14.2. G.H.0754 Instruction S.4 SECTION 15 PORTABLE AMPLIFIER DETECTOR PADi9 General Description In the position of SW C marked Norn~al,the The PAD19 is basically a portable version of the signal reaching the P/64P is attenuated .20 db AD/4, but has a slightly different circr~itand pro- by the potentla1 divider formed by R7, and R9 vides some additional facilities. The range of in parallel with the P/64P, while in the position level measr~rement extends from -- 75 db to marked - 20, R7 and R9 are taken out of circuit, -4- 10 db approximately, and tlie input impedance so increasing the segsitivity of the instrument by can he switched to either 30 kilohms or 600 ohms. 20 db and allowing the measurement of levels There is no provision for operating from batteries, down to - 75 db. but a built-in mains unit is included, and thc Between V2 and V3 is the .4djust Sensilivify instrument is self-calibrating from the 50-c/s control, R17, which is used in conjunction \~iLh supply. the calibrating circuit to stantiardise the gain of the instrument before taking level measurements. Circuit Description (Fig. 22) To minimise non-linearity, current negative General feedback is applied to V1, V2 and V3 via the The circuit comprises three pentode stages, cathode-circuit resistors R6, H15 and R24. A RC-coupled, with transfornier input and a 0/0.5- flat frequency response is obtained by suitable niA meter at the output. Level measurements adjustment of the combinations CS-R8, C9-R16 are carried out with the aid of calibrated poten- and C15-R26, which are effective in the ranges tiometers included in the grid circuits of the first 30 c/s-l kc/s, 1-10 kc/s and 313-35 kc/s respectively. and second stages, the potentiometers being Afeler Circuil adjusted to bring the meter reading to the region The anode of V3 is taken to the meter circuit of a fixed point on the scale. in parallel with the anode load and an Oulpd The connection to the input transformer, Lislen jack with 200-kilohm series resistor for use TRl, is via a jack (with alternative screw terminals) with headphones. The meter rectifier is a Westing- and a 3-position key. One position of the key house 1-mA instrument type. The meter itself connects the jack and terminals directly to THI is a 0.5-mA Sangamo Weston Model S.20 with a primary, where as the impedance ratio js 3 : 1, 0-dh calibration mark at 80 per cent of full-scale an input impedance of 30 kilohms is reflected from deflection, and additional markings corresponding the 10-kilohm potentiometer P/65P across the to f 1 db. Intermediate points on the scale secondary terminals. In the second key position, are also marked at i0.5db, allowing readings the 610-ohm resistor R32 is connected across TRI to be estimated with care to within 0.1 db. primary, reducing the input impedance to 600 ohms. The third position of the key connects a Calihaling Circuit, 50-c/s voltage at + 10 db to the input ; this This consists essentially of tlie potential divider voltage is derived from a calibrating circuit formed by R28 and the neon VS connected between which will be described later. one side of the h.t. winding and the centre-tap The P/65P has 5-db tappings with a maximum on mains transformer 'TR 2. The stabilised 50-c/s loss of 60 db to cover the range of input levels voltage across the neon is further divided by the from + 10 to - 50 db ; it has also an O// position: chain R29, R30, R31. The voltage developed The P/65P slider is taken to V1 grid via the anti- between the variable tapping on R30 and TR 2 parasitic resistor K 1. centre-tap is applied as a calibrating signal to The coupling from V 1 anode to V2 gi id is through TR 1 primary via the Cal. + 10 position of the capacitor C3, switch SW C and the 100-kilohm input key. potentiometer P/MP, which has 0.5-db tappings with a range of 5db. The P/64P acts as a fine Mains Ud control and also extends the coverage of the The 50-c/s mains supply enters the eqrlipment instrumrnt down to -- 55 db. via a 3-pin plug and lead and thc 2-pole O11/0]7 Instruction S.4 Section 15 switch SW A. A I-amp fuse is fitted and a neon 2. Before proceeding further, wait for at least ten indicator-lamp is provided to show when the supply minutes, to give time for the instrument to is on. The mains transformer ~rimarvcan be attain stability. switched to 210, 230 or 250-volt .tappings, which may be reduced by 10 volts in each instance by Hum Adjustment changing the internal strap. An h.t. supply at 3. Set the coarse calibrated control to O//, and the 275 volts is provided via the rectifier V4 and fine calibrated control to 0. Set the 20-db snoothing con~ponentsL1, C16, C 17. The 6.3-volt switch to -20. h-ater winding for V1-V3 is bridged by R33, which Fig. 15.1 PAD19 Face Panel Drawing No. ESK 1305 Mechanical Construction Calibralion at Base The instrument is constructed in a sheet- 5. Set the coarse control to + 10, leaving the aluminium case with overall dimensions of 68 in. fine control on 0. by 148 in. by 10j in. approximately. Its total 6. Set the 20-db switch to Nmmal, throw thr weight is 21 1b. input key to 600 iZ, and apply tone at a level The mains fuse and all controls except the of + 10 db to the input jack from a source of Calibvate adjustment are mounted on the face frequency around I kc/s. (The impedance of panel (Fig. 15.1), which is protected for transport the source itself is not important, provided by a 'detachable hinged lid fastened by trunk- that it delivers an accurate + 10 db into catches. A support bar is provided at each end 600 ohms.) of the face panel to allow the instrument to be 7. Vary the setting of the Adjust Sensitivit?~ inverted for maintenance without damage to the control until the meter pointer comes to rest controls. A 9-ft mains lead emerges through a opposite the 0-db mark on the scale. bushed hole in the panel, and can be held for NOTES:--(i) It is to be stressed that the validity transport by clips inside the lid. of the calibration depends entirely A carrying handle is provided at one end of the upon the accuracy of the + 10-db body. Rubber feet are fitted at the other end, level obtained from the tone source, and also on the base. and if the latter is not provided with an output meter some other means of Operating Procedure checking its output level must be Switching O,n. found. 1. Set the Mains Voltage Selecbv to the required (ii) Should a source of + 10-db level not position, and switch on the mains. The neon be available an accurate zero-level indicator-lamp should now light. source may be used, but *e coarse Instn~ctjonS.4 Section 15 control on the PAD/S should then be 14(c),it will be rlecessary to suhtrnct a further set to 0 instead ol -(- I0 during opera- 20 db from the figure so obtained. ti typical tion 5, and restored to + I0 prior calcuMtion might be as follows : to operation 8. (iii) Although a CALI1 rmit can con- veniently trc used lor checking the accuracy of the tone-source level against a P.P.Y., the 50-c/s output obtained Irom this unit should not be NOTE.-TO reduce risk of dan~ageto the measnr- used directly to calibrate the PADjS. ing circuit, the controls should always be handled 19. Throw the input key to Cai. + 10. and adjust in such a manncr as to cause the meter pointer to the Calibrate control inside the instrunlent approach the calibrated part of the scalc from until a meter reading of O is again obtained, btlnirr. Xote that the neon tube V5 (also inside the instrument) is liable to shake loose in transit, Valve Data , and as this can introduce seriohs calibratih i errors, it is important to check that the neon is screwed tightly home. Stage 1 , I 9. First carry out operations 1 and 5 as illrcady EF 50 14 0-5 6-3 0-3 given. Then throw the input key to CnI. t 10, 1 .I_------.--- and set the Adjrrsl St-nsilivie control for, a I Stage 2 meter reading 01 0. EF 50 1 1-6 0.5 I: 6-3 10-3 Mctrsuring Tone Lacis 10. First calibrate the instrunlent a? described under operation 9. 11. 'Set the input key to 60011 or 30 kI2 as required. 12. Apply the tone which it is desired to measure. 13. Adjust the two calibrated controls to bring Supplies the lneter pointer as nearly as possible to. Mains supply, 200-250 volts 50 c/s a.c. calibration mark O on the scale. H.T. supply, '275 volts 8.5 mA. 14. (a) If the meter reading is within :': 1 db L.T. supplics. 6-3 volts 0.9 amp, and 4-0 volts of thc calibrat.ion mark, proceed to opera- 1.4 amps LC. ~- tion 15. (b) If the meter rcads more than I db high, the tone Ievcl is more than + I1 db, and General Data is outside the range of the instrument. Impedancas (c) If the mctrr reads more than 1 db low, set the left-hand control temporarily to Input 2 - 600 11 or 30 kl? -30, throw the 20-db witch to -,20, and repeat operation 13. (d) If after operation 14(c) the meter reading is still more than I db low, the tone level is bclow -76 dh, and is outside the range Calibrated Poltnliomeldrs of the instrument. 15. The tone level ii-5 obtained from 14(a) will No, o/ Loss per be equal to the algebraic sum of the readings Type Residasce Slrrds Slud of the two caIibratcd controls and the meter. PI64 P 100 kll I I 0.5 db If the 20-db switch is thrown to -20, as in P/f%P 10 kR 14 5 db Instruction S.4 Section 15 Modifications to PAD19 (Fig. 23) was in parallel with the meter circuit is removed. Scope of Modifications A Metrosil disk is connected in V3 anode circuit to reduce long-term drift resulting from mains- The modifications to be described have been voltage variations ; the value of the smoothing carried out on all PAD19 instruments in use by capacitor, C13, is increased to 326 pF, thus Lines Department and by S.E.T.'s mobile main- reducing short-term level variations due to mains tenance teams. The amended circuit is shown surges, which are unaffected by the Metrosil in Fig. 23. disk, since this latter is temperature-operated and is The principal results of the changes are : consequently a slow-acting device. To provide (a) Some increase in the stability of meter readings adequate anode-bend current limiting, the valve in the presence of mains-voltage variations, is now triode-connected ; as a result, the maximum and improved protection of the meter against meter current on overload signals is only 1.5 overload signals by the use of anode-bend times that for normal full-scale deflection, ill current limiting. contrast to 5 times the normal full-scale currelit (b) Provision of facilities for using a PPh1/6 in before the change. conjunction with the PAD19 for such applica- The input-key connections'are rewired so as to S' tions as noise measurement. reduce the number of series break contacts in the A disadvantage resulting from (a) is a 10-db low-impedance path. This is to guard against reduction in the sensitivity of the PAD19 when possible high contact resistance which could used alone. It should also be mentioned that, introduce serious errors in measurement. except for special applications, the stability of the Since the increase in feedback and the triode standard unmodified instrument is normally per- connection of V3 together cause a drop in gain fectly adequate, as illustrated by the following of 10 db, the switched gain-control circuit between figures, which give average variations of PAD19 V1 and V2 is modified so as to produce a 10-db meter readings when the supply voltage is varied change instead of one of 20db. The two ranges over the range of 250/200 volts with the mains of the instrument thus become : switch set to 230 and I-kc/s tone at zero level (1) + 10 db to - 55 db. applied. (2) 0 db to - 65 db. Meter Reading Variations : db An additional I db in either direction is still Maim obtainable by using the I-db markings on the Volts Standard Modified meter scale. PA D/9 PA D/9 - 'r Use of PPMI6 250 I-- + 0.6 1 + 0.3 A jack is fitted between V2 and V3 to enable I + 0.3 + 0.15 the amplifier portion of the PAD19 together \;:: 1 O 0 with the calibrated controls to be used in con- 220 I -0.1 junction with a PPM/6, the facilities obtainable 210 i 0'3- 0.7 1 -0.3 with the combination being equivalent to those 200 - 1.1 - 0.35 provided by a TPM/3. Although the high- impedance input of the PPM/6 is used, the con- Stability Impro71e1~rrtsand Meter Protection nection affects the readings of the PAD19 internal Stability has been improved mainly by making meter, and to prevent mistakes auxiliary contacts the circuit a closer approach to that of the AD/4. are provided on the jack which when a plug is (See Section 1). Changes under this heading inserted automatically short-circuit the internal include an increase in the amount of current feed- meter. back applied to V1, due to the omission of the The signal applied to the PPM/6 is at zero level cathode-circuit by-pass capacitor, and re-arrange- when the calibrated controls are both at 0 db and ment of the meter circuit to bring the meter within the 10-db switch is set to maximum gain, thus a feedback loop between V3 and V2, the meter giving direct readings from the P.P.M. and the therefore measuring feedback current instead of attenuator dials. The switch is now labelled output voltage. The Adjust Sensitivity control is T.P.111. Nw~naland Amp. Det. - 10 db in this placed in 1'2 cathode circuit, and is thus also within position and Amp. Det. Normal in its other position. the feedback loop. The O~t+trtListen jack which G.H. 0555 Instruction S.4 SECTION 16 PORTABLE INTERMODULATION TESTER PIT/] Introduction simultaneously csisting tones, thc rcsults of inter- The PITI1 is a small prtablc instrument ~nwlulntio~htests sho~rldcorrelate well.with those designed to assist rapid routine checking Icr non- ol ordinary lrstening chccks on programme quality. linearity in amplifiers and related equipment. Previous BBC practice is assessing distortion General Description due to non-linearity has been to apply pure tone 'The front panel of the PI'T/1 carrying the con- to the input of the equipmcnt and to measure the trols is shown in Fig. 16,I, and n schematic diagram total harmonic distortion at the output. As illustrating the method of operation is given in the ear is not sensitive to harmonics unless they Fig. 16.2. 4 ]-kc/s oscillator is emhodied in the are fairly strong or include high-order ternrs (such instrument, and the output of this, mixed with as the seventh), it cannot be used xs thc detector' mains-derived 50-c/s tone at an 841) lower4eve1, and obiective measurements with filters and a is ied via coarse and fine attenuators and a 12-db meter are essential. Moreover, the degree ol loss-pad to the amplifier under test, the output objectionableness of harmonics increases with of which has been adjusted to standard line-up thcir order', and the nonnal technique of meawring level using a T.P,M. The amplified signal is total distortion does not take account of the returned to the tester, and thence to a Iislening relative magnitudes of the individual terms; circuit. By the operation of a key, the 12-db pad hence an objective nleasurement and the subjective can be removed from the feed to the amplifier effect may not correlate well. input, and a corresponding loss-pad inscrtrd When, howe ever, tones of more than onc fre- into the listening circuit, the level in which thus quency are applied simultaneously to a system, remains constant, while any non-linearity in the any non-linearity causes intennodulation2, and as amplifier introduces intermodulation products the resulting sum and difference tones need not which arc readily detectable. Since the amplifier be harmonically related to the original frequencies, output is initially adjusted to the normal line-up thc subjective rflect can be considerable. It thus level. the 12-db increase corresporlds to an output becomes pos5iblc to use the ear as thc detector of 4 db above normal programme peaks. distortion and to dispenx with filters. This The instrument is I)uilt into a sheet-metal case simplifies the apparatus considernbly, and makes with a carrying handle at tl~ctop and ventilating it possible to built a small portable internlodula- louvres at the sides. The overall dimensions are tion tester which can be placed adjaccnt.to the I I in. by 84 in. by 74 in. approximately. A per- equipment that is to be cheiked. manently-at tached mains cable emerges through a The YIT/I is not intcnded for making accurate rubber grommet at a corner of the front panel, measurements of distortion, but is designed- for and can he coiIed up for transport round cIcats clit-cking that distortion is not audible at 4 db fit tetl on one side. above the peak lcvels which programme will reach. This 4 db has hen fourrd to provide a Circuit Descriptio~r(FIB. 24) gmd margin of safety against errors of line-up and differences in judgmcnt on the part of different A complete circuit diagram of the PIT11 is operators (in practice surprisingly small), so that given ill Fig. 24. an equipment under test need not bc rejected from The I-kc]s tone is generated by VI, which is service unlcss distortion is clearly audible. Since connected as a cathode-couplt-d oscillator tuned programme normally contains large numbcrs of by C1 and L1 ; a thermistor, TH 1, which stabi- lises thc operating level, is connected across a cathode-circuit rcsistor, R4. The signal output is I. Shortur, D. E. I,. ' T'hc Influence of High-Order r'roducts in Xon-Linear Distortion.' Bk.slraic Engtnttr- takcn from the grid cnd of L 1 via C3 and R7 and in#, Vol. 22, Xo. 226 (April, l950), pp. 152-153. applicd to the dlixcd Torrc Finc control RI I, 2. Enginrt.ri>t# 7'nti>ti~gSrepplnnrrtf, A'". 3, isue 2. in parallcl with a mains-derived SO-cls tone applied page 18. via R8 and the pre-s~tSO-cis Adjrd control R9. Instruction S.4 Section 16 BBC 5MlXED UIXLD AMP >on boon TONE OUTPUT lPU UOHITOU /A I I - -- -- JI Fig. 16.1. PIT.1 Face Panel Drawing No EK. 8453 ~OOOC/~ 0-60rlh MIXED El--MIXED MIXED TONE 50.1. :pN",' AT T -- Sods-- -- I ADJUST / I \ Fig. 16.2. lntermodulat/on Test Schematic Diagram Instruction S.4 Section 16 A two-bank switch SW A with contacts acrus; The built-in power-supply unit incorporates a R9 and the screen-to-cathode circuit of V1 allows mains transformer TR 3 and two metal rectifiers either the 50-c/s or thc 1-kc/s tone to be used with resistance-capacitance smoothing. A mains alone if required. In the 1-kc/s position of the switch and fuses are provided, and a mains indica- switch R9 is short-circuited, thr~ssuppressing the tor-lamp is connected across the 1.t. winding of 50-c/s signal, while in the 50-c/s position the screen- TR 4, which also provides the 50-c/s test signal grid of V1 is connected directly to cathode, so to R9. preventing oscillation of the valve. Operating Procedure The slider of lill is connected to the grid of V2, which operates as a mised-tone amplifier in which Linin~-.UP the Tester distortion is kept low by voltage negative feedback Connect the Mixed Tone output jack to a applied to the cathode from the tertiary of trans- TPJI/3. As this has an input impedance of former TR I. Variation of 111 1 allows the output 10 kilohms, the Amp. Input Z switch on the level from V2 to be adj~rstedover a range of about tester should be set to High.. 25 clb. Turn the tone-selector switch to ' 1,000 cis ' and adjust the output level to a convenkt 'The secondary of Tli I is taken to the Miied L value by means of the Mixed Tone Fine control 'I'one Altenuator, the loss of which can be switched and the Mixed Tone Attenuator. to 0, 20, 40 or 60 db by means of SW R. The Now tun1 the tone-selector switch to ' 50 cis ' attenuator is followed by the 12-db loss-pad and adjust the pre-set control inside the cover R22-R24, which can be short-circuited by a key. until the level as measured on the T.P.M. is To obtain the full loss of 12 db from the pad for 8 db below that at 1 kc/s. all positions of S\Y H, it is necessary to guard Restore the tone-selector switch to 'the Mixed against the effects of longitudinal currents ; for position. this reason a repeating-coil TI12 is interpoied With the 12-db key set to Norr~rnl,chcck that between the attenuator and the pad. the tester is capable of delivering inisetl tone Since amplifiers under test may have various at zero level without audible distortion. input impedances, and both the 60-db attenuator and the 12-db pad are designed to operate into Testing an Amplijier 600 ohms, an impedance-matching circuit is 1. Plug the Mixed Tone and Amp. Output jacks provided, comprising 1129 and R30 controlled ,to the input and output respectively of the by SW C ; three values of amplifier input imped- amplifier under test. ance can be accommodated, 30ohms, 600 ohms ~u;nthe Amp. Inpd Z switch to the apprd- or High. With a 300-ohm amplifier, an additional priate setting for the amplifier. 300 ohms series resistance is provided by 1129 ; Set the tone-selector switch to Mixed. (The with a 600-ohm amplifier, a straight-through other positions of this switch are normally connection is made, while with a high-impedance used only when calibrating the tester.) amplifier a 600-ohm shunt resistance is provided Connect a loudspeaker amplifier or head- by 1130. The termination of the tester in each phones to the 600-ohms Monitor jack. (The instance is thus effectively 600 ohms. %-ohms jack is for use only with special low- The input to the amplifier under test is taken impedance headphones.) from the Mixed Tone jack, and the amplifier output With the 12-db key at Normal, adjust the mixed is returned to the Amp. Output jack, in parallel tone coarse and fine controls until the amdifier with which is a jack for a T.P.M. The wiring from output with gain controls at usual settings is at these jacks is taken via additional contacts of the line-up level measured on a peak-reading 12-db key which normally bridge a second 12-db meter plugged into the T.P.M. jack. pad, but bring it into circuit if the key is thrown. Adjust the Monitor Cain control for a com- The signal then passes through the Monitor Cain fortable listening level. control, RB, and via the 600-ohm Monitor jack Operate the key to the + 12-db position, and to a loudspeaker amplifier or headphones. A listen for a change in the quality of the 1-kc/s further Monitor jack, intended for use with 30-ohm tone. (The ability to detect slight distortion headphones, is fed via the impedance-step-down on headphones can be increased by lifting transformer TR 3. them slightly from the ear.) Instruction S.4 Section 16 Note that since the input to the amplifier is Mixd Tone Ozrtput Frequencies increased by 12 db (i.e., to 4 db above peak pro- 1 kc/s f5% (from oscillator) and 50 cls gramme level) the check for audible distortion is (from mains). made at a higher level than the amplifier normally NOTE.--50-CIStone level 8 db below 1-kc/s level. carries, so giving a 4-db margin in hand. The procedure outlined above should be used Mixed Tone Output Level for normal routine checks on amplifiers, studio At least zero level without audible distortion desks and similar 'equipment. when + 12 db key is at Normal. If it is necessary to determine the actual level at which audible distortion occurs. the input Combination Tones should be adjusted until the distortion is just With + 12-db key operated, and a mixed-tone perceptible with the 12-db key operated, and the output level of + 12 db, the level of each com- output level should then be measured at the T.P.M. bination tone (i.e., 950 c/s, 1,050 CIS, et~.),when jack. (When the output level is beyond the range measured on a harmonic bridge, should be at least of the meter, it should be measured with the key 50 db below the level of the 1-kc/s tone. a. restored to Normal, and 12 db then added to the figure obtained.) The use of the PIT11 as a Valve Data measuring device is not, however, recommended, Heater I Heater as there will be a certain amount of variation in the Valve 1 volts i Amps level at which different operators can detect ---- distortion, although the instrument does give Oscillator some idea of the safety margin available between V1 : CV 138 1 6.3 1 0.3 normal peak volume and obvious overloading I of the equipment undcr test. Amplifier V2 : CV 138 I 0.3 General Data Impednitce Mains Sufply Normal load Z = 300 TI, 600 fl or High. 200-250 volts, 50 c/s a.c. G.H. 0555 Instruction S.4 SECTION 17 A.C. TEST METERS ATM/1 & ATM/lP Introduction Circuit Description (Fig. 25) A.C. .Test Meter ATM/I was designed to replace Block Schematic (Fig. 17.2) Amplifier Detector AD14 and Test Programme The unit consists basically of two amplifiers in Meter Amplifier TPM/3 and to perform the func- cascade stabilised by negative feedback, each tions of both these units. Basically, therefore, it being preceded by a calibrated attenuator. The is a valve voltmeter which is calibrated to read second amplifier can be switched to either the a.c. vohge levels on a bridge-rectifier meter, and bridge-rectifier meter or the peak programme programme volume and noise levels on a pCak meter rectifying circuits and meter, depending programme meter. on the function required. In the TPM condition Yllll Fig. 17.1. A~M/I : Face Panel The following facilities which are not available the function switch also introduces a low-pass in the AD14 and TPM/3 have been embodied:- filter between the two amplifiers, and a variable (1) Sensitivity range extended to + 20 dB to 20-dB gain control in the feedback path of the first - 70 dB for both functions. amplifier. (2) Built-in power supply unit, making the unit readily adaptable for portable use. Input Circuit (3) Provision of aural monitoring for noise A balanced input is provided by the input measurements (TPM condition only). transformer LG/47SA which is loaded on its (4) Inclusion of bandpass filtering I0 c/s to secondary side by the first calibrated attenuator 20 kc/s in TPM condition. AT1 in parallel with R54 (5) Internal approximate calibration signal The attenuator is a stud-type potentiometer source (50 c/s) for check on operation in the PN/IOAl feeding the first amplifier and gives field. 70 dB attenuation in 10-dB steps, the actual cali- (6) Lengthened scale in AD cond,ition making bration markings being " + 20" to " - 50". Its the effective scale length about twice that resistance is 40 kQ and the 24 kQ resistor R54 is in the AD/4. connected in parallel with it to reduce the effective The unit is built on a 19 in. x 54 in. panel for resistance to 15 kQ. This reduction in impedance bay mounting and weighs 19 1b. It forms part level allows a simplification in the design of the of the equipment on A.C. Test Bay AC/55. For input transformer which has a turns ratio of portable use it is fitted in a standard carrying case, 1.88 : 1 to give an input impedance of approxi- CS2/2B, and is then coded ATM/lP. The layout mately 50 kQ of the front panel is shown in Fig. 17.1. As well as a high-impedance input jack, a 600Q Instruction S.4 Section 17 input jack is provided, wired via the inners of the maintain approximately the same source imped- high-impedance jack. Both jacks are partially ance, with the attenuator pad either in or out of screened from the adjacent AD meter, which is circuit, for the calibrated attenuator AT2 which at a relatively high a.c. potential, to prevent a feeds the second amplifier. This is necessary to low-amplitude high-frequency oscillation (about ensure negligible change in the sensitivity-frequency 150 kc/s) which occurred in the experimental characteristic when the " - 10 dB " switch is model under certain, high-sensitive conditions. operated. R20 has been kept as low as possible FUNCTION AD RECTIFIER SWITCH AND MTER 4 INPUT FIRST COUPLING SECOND 1 CIRCUIT AMPLIFIER cl~um AMPLIFIER Fig. 17.2. ATMII : Block Schematic First Amplifier to avoid unnecessary loss of sensitivity. Two EF86 valves are used in the first amplifier. The calibrated attenuator AT2 is a stud-type In the AD c.ondition of operation the gain is fixed, potentiometer PN/IIAI with a total resistance and 20 dB negative feedback is taken from the of 100 kR giving 10-dB attenuation in 0.5-dB second anode to the first cathode circuit. steps. It is calibrated to read " 0 " to "- 10 dB". In the TPM condition 20 dB variation in gain In the TPM condition a low-pass filter is con- is provided by the variable resistance R49 in the nected between the first amplifier and the 10-dB feedback path. The corresponding change in the fixed pad, and the complete response of the TPM amount of feedback is only 3.5 dB because the circuit gives an equivalent noise bandwidth of V2 anode circuit impedance increases as tbe resist- 22.6 kc/s (see later). (In the TPM/3 the response ance is increased, thus increasing the gain without extends to 100 kc/s with a 10-dB peak around feedback. 60 kc/s.) Coupling Circuits between First and Second Second Amplifier Amplifiers A CV 455 (6060) valve is used as a two-stage In both AD and TPM conditions the first amplilier with negative feedback from the second amplilier is followed by a 10-dB attenuator pad anode to the first cathode circuit. In the AD R21, R22. This pad is normally in circuit but condition the bridge-rectifier meter circuit is in a may be switched out by a push-button. It is negative current feedback path via C14 which primarily intended to give an extra 10-dB gain includes the AD sensitivity control R50. The for measuring noise in the TPM condition. If it latter gives a variation in negative feedback of is switched out in the AD condition small fluctua- f 2 dB and has a carbon track to give smooth tions in the meter reading may be noticeable when control. . transients occur in the mains supply. Tbese Additional negative feedback, giving a total of fluctuations are due to low-frequency components 18 dB, is provided by a separate d.c. path through and are prevented in the TPM condition by the R46. If this negative feedback were taken via inclusion of a high-pass filter with a cut-off fre- C14 the latter would have to be inconveniently quency of 10 c/s in the input circuit of the pro- large to prevent a peak in the response at a very gramme meter rectilier. low frequency. Between the first amplifier and the 10-dB fixed In the TPM condition the feedback has a fixed attenuator pad the resistor R20 is included to value of 17 dB. Ioshuction S.4 Section 17 AD Meter Circuit meter circuit and two in the circuit betwcen the The AD meter is fcd from four crystal diodes two amplifiers for swilching the TPM low-pass (CV 425) in a bridge connection. Thcso have bcen filter and the PPM sensitivity control. The metal found to be more efficient and less affected by backing ring of the switch rotor is earthed to temperature changcs in this application than prevent capacitive coupling which would give selenium and copper oxidc rectifiers. They have undesired negative feedback and affect the gain extremely low sclf-capacitance which is.of value at the higher frequcncies. in preventing the large stray capacitance of the meter itself and associated wiring from being TPM Aural Moni~oring coupled into the fecdback circuit. An unbalanced monitoring oulput is provided The meter used is a shaped-pole type with 0-I-dB in thc TPM condition by the inclusion of a 680Q scale markings from - 0.5 dB to + 0-5 dB and rcsistor (R48) in the PPM rectifier input circuit. with further markings at - I dB and -+ l dB. The signal dcvelopcd across it is brought to a A current of 500 pA corresponds lo 0 dB on the listcn jack on the front panel. Short-circuiting scale. The shaped pole-picccs are designed to R48 has a negligible elTect on the PPM rea9g. expand the scale ncar the middle of the range, The monitoring circr~it is primarily intendcd for u and givc a scale which is effectively about twice as headphone checking of the character of the signal long as that of the AD/4 meler. being measured. It docs not provide high-quality Thc capacitor C16 shunting thc mctcr terminals monitoring because of the varying shunting effects prcvents a visible vibration of the meter pointcr of the diodes (V4). Distortion on steady tone is when the apparatus is used for low-frequency of the ordcr of 0.5 per cenl, measuremenls, and it also serves to limit surges through the meter when the apparatus is switched Calibrating Volrage Circuir on. When no accurately-known calibrating voltage The AD meter has a dead-beat response, and is available a rough chcck on the sensitivity of the stiction, measured as lhc change in inpul Icvel ATM/l is provided by switching a zero level requircd to give a visiblc change in thc mcter 50 c/s signal (derived from a potentiometer across reading at mid-scalc, is less than 0.02 dB. the 6-3-volt winding of the mains transformer) on lo the input lerminals of the unit. Thc connec- PPM Circuir tion is made via the inner contacts of the input In the TPM condition the output of the second jack JKA. The 60042 input impedance and amplifier is connected to the transformer T2 resistor R43 form the potentiometer. (AGG/I I SH). The capaci!or C12 is connected in series with the primary of this transformer to Power Supply form a high-pass filter with a cut-off frequency A conlactcookd type bridge rectifier MRI at about 10 cjs to reducc interference from trans- provides the h.t. supply. This component is ients in the mains supply as already mentioned. considerably undcrrun, the current delivered being L The rest of the PPM circuit, which includes the 22 mA whcreas the manufacturer's rating is 40 mA. full-wave rcctitier V4 and the variable-mu pentodc valvc V5 for obtaining the logarithmic law required, is almost identical with that used in Monitoring Con~rols The following controls are provided:- Amplifier MNA/3. A peak programme meter with the standard I. + 20 dBe to - 50 dB in 10-dB steps-cali- movement and scale is included in the unit and brated variable attenuator ATI. there is provision for connecting an' external PPM 2. 0 ro -- 10 dB in 0.5 dB sleps-calibrated in serics with it. variable attenuator An. 3. - 10 dB push-button to give 10 dB increase Appnrnrus Funcrion Swirch in gain. The changeove~from AD to TPM condition is 4, CAL push-button to inject 50 c/s at zero accomplished by operation of the switch SWA, leveI approx. a 3-pole, 2-way lever type, which has ono pole at 5. TPM LAW preset screwdriver-operated thc anode of V3(b) for switching to the appropriate potentiometer. Instruction S.4 Section 17 6. TPM SENS preset screwdriver-operated does not change by more than -j= 0.3 dB potentiometer. from its value at 1 kc/s, as the frequency 7. TPM ZERO potentiometer with knob. is varied between 30 c/s and 15 kc/s. 8. AD-TPM changeover lever-type switch. 9. AD SENS potentiometer with knob. Input Impedance 10. MAINS 2-pole, 2-throw toggle switch. At 30 c/s: 34 kQ ,, 50 c/s: 39 kR ,, 1 kc/s: 46 kR Performance ,, 10 kc/s: 48 kR Power Supplies Mains Supply: 200 V-i50 V 50 c/s TPM Response Time, Recovery Time and Law Consumption : 23 VA These are similar to the TPM/3 and MNA/3 and the same tests apply. Sensitivity (a) AD CONDITION Valve Data Input levels for " zero " reading on meter: Anode Screen Cathode b Minimum: -60 dB (normal) Valve Current Current Voltage - - 70 dB (with " - 10 dB " Vl (EF86) 0.7 mA 0.14 mA 1.3 V button pulled-out) V2 (EF86) 1.3 mA 0.25 mA 1.5 V Maximum: 20 dB + - 1.1 Range of AD SENS control: From t 2.0 to V3A (46060) 1.5 mA v - - 2.0 dB. V3B (46060) 5.0 mA 1.4 V v4 (CV140) - - - Note 1: The extra 10 dB sensitivity with the " - 10 dB " button pulled out is primarily intended for noise measure- v5 (CV454) 5.5 mA 2.2 mA 1.5 V ments in the TPM condition. If it is used in the AD - - condition small fluctuations in meter readings caused V6 (CV449) 5.0 mA by transients in the mains supply may be noticeable. Note 2: A change in mains supply voltage of & 5 per cent Equivalent Noise Bandwidth causes * 0'1 dB change. in sensitivity, and one of 3 10 per cent: causes * 0.2 dB change in sensitivity. The measurement of noise requires that the sensitivity/frequency characteristic of the measur- (b) TPM CONDITION ing apparatus be accurately known. For noise Minimum and maximum input levels for a having equal energy per octave (" white " noise) reading of" 4 " on the meter are - 70 dB the measured noise is proportional to an integral and + 20 dB respectively. hmg2 df where g is the sensitivity of the 'Range of SENS control : From + 10 dB to measuring system at a particular frequency f. - 10dB. For the ATM/I in the TPM condition the value of this integral is best found by constructing a Sensitivity-~requenc~Characteristic graph of re/ative smsitivity2 against frequency With test tone from 6004 impedance source. from the known relative sensitivity/frequency - (a) AD CONDITION characteristic, and taking the area under the curve For any setting of the controls, the sensitivity by counting squares. Dividing this by the mid- does not change with frequency in the band value of relative sensitivity" which could range 20 c/s to 20 kc/s by more than -j= 0.05 be made unity, gives a quotient which is the dB relative to its value at I kc/s. bandwidth of the hypothetical system having a rectangular-shaped gainlfrequency characteristic (b) TPM CONDITION and which would have an equivalent response to The frequency response is such that the noise. This is termed the " Equivalent Noise equivalent noise bandwidth is 22.6 kc/s. Band-width" of the system and should not be (See later.) confused with thi " Bandwidth." For any setting of the controls, the sensitivity WG.0758. Instruction S.4 SECTION 18 ROUTINE LINE TESTERS RLTO AND RLT/lP Introduction decade resistor as in the Bridge-Megger. A low- Routine Line Testers RLT/I and RLTllP are voltage source is provided for bridge measure- designed to carry out all the normal d.~.tests on ments to reduce the probabihty of a line fault. lines. The RLT/I is for mounting on a 19-in. such as a high-resistance joint, being influenced bay and is used on A.C. Test Bay AC/55, and by the testing source. contains its own internal mains unit. The RLT/I P The galvanometer used k- of thz shaped-pole Fig. 18.1. RLT/ I : Face Panel -. -. .. ,.l .I> Fig. 18.2. RLT/ I P: Face Panel is the portable version of RLT/I and is powered centre-zero type giving high-sensitivity at mid- from self-contained batteries; it replaces the scale with great overload capacity and a con- Bridge- Megger. venient scale when .it is used as an ohmmeter The performance of the two units is similar with a high-voltage source for measuring insulation and both have facilities for checking the insulation resistance. resistance, the loop resistance and the out-of- balance of lines, and for checking that there is no General Description standing d.c. voltage on the line. Each unit is constructed on a 19 in. v, 5 .&in.panel, Quick operation is provided by using a single the RLT/I having a standard 6-in. rear dust cover, multi-position rotary switch to cover all test con- :lnd the RLT/IP being mounted in an aluminium ditions, and by using a continuously-variable alloy carrying case. The layout of the two panels measuring resistor instead of the usual 3 or 4 is shown in Figs. 18.1 and 18.2. Instruction S.4 Section 18 Calibrated Dials On the RLT/IP two pairs of spring terminals The calibrated dials of the two continuously- are provided so that two lines can be terminated; variable resistors marked VARLEY and LOOP either line can then be connected to the test respectively are mounted behind the panel to save circuit b f' a short patching cord plugged from the panel space and to protect them from dust and associated parallel jack to the Test-In jack, the handling. Their markings are visible through insertion of the plug in the latter jack clo,sing a perspex covered cut-outs in the panel. Each dial pair of protective contacts in the bridge battery is mounted on a detail having a thin-walled sleeve supply circuit. with longitudinal saw-cuts surrounding the resistor Direct access to the testing circuit in the RLT/lP spindle and fitting inside the knob. When the is provided by spring terminals in parallel with knob is removed the dial can be rotated on the the Test-In jack, but a plug must be inserted in spindle which is provided with a screwdriver slot the Test-In jack to complete the battery circuit. to enable it to be adjusted in the absence of the knob. When the knob is replaced and the two- Circuit Description (Figs. 26 & 27) set screws tightened, the whole assembly is locked Insulation Resistance to the spindle. With the main rotary switch SWA in positions 1, 2 or 3 the voltage source in series with a limiting Multi-position Switch resistance of 82 kQ is applied between the A wire The multi-position rotary switch which arranges and earth, the B wire and earth, or between the the circuit of the unit to suit the particular test A and B wires through a universal meter shunt to be carried out is situated on the right of the consisting of a 3.3-kQ fixed resistor (R11) in panel. The dial is engraved to show the function series with a variable 2-kQ resistor (R21 in RLT/I performed for each position of the switch. and R20 in RLT/IP) which is used to adjust the Galvanometer instrument zero. The centre-zero galvanometer is provided with With switch SWB in the High Insulation position two scales; the top scale is calibrated in megohms, the meter is connected directly across this shunt the left-hand quadrant reading 1.5 to 60 megohms as shown in Fig. 18.3(a) which is a simplified from left to right and the right-hand quadrant diagram of the circuit for measuring insulation reading 0 - 2 megohms from right to left. The resistance to earth. The pointer of the meter indicates the insulation resistance directly on the lower scale is marked - 50, - 25, + 25, + 50 volts. When the galvanometer is used as a null indicator left hand quadrant of the meter scale. in bridge measurements the sensitivity can be With switch SWB in the Low Insulation position increased by pressing the button marked INC. SENS an additional shunt consisting of R12 (3.3 kQ) which short-circuits a 12-kQ resistor which is and R13 (268Q) in series is connected in parallel normally in series with the meter under these with the previous shunt, and the meter in series conditiohs. with R14 (2.2 kQ) is connected across R13. (See The coil of the meter has a resistance of 1 kll Fig. 18.3(b).) The connections to the meter are and the current taken for full scale deflection is reversed in this case thereby causing the pointer f 55 PA. to read over the right-hand quadrant of the scale. In the RLT/lP a micro-switch, operated on In both cases the arrangement is such that the closing the lid, short-circuits the galvanometer shunt resistance across the meter terminals is for safety during transport. high (5,300 or 2,500Q). This ensures a lively movement of the galvanometer needle so that Battery Charging Socket short breakdowns due to sparking will be notice- In the RLT/lP a socket, accessible through the able, and a reading can be obtained quickly on a side of the case, allows the usual O.B. charging normal line. equipment to be connected. The internal resistance of the whole circuit is also kept fairly low (about 84 kQ) in both cases Line Termination resulting in a rapid charging time, 'even for a line On the RLT/l two tags are provided for line of considerable length. This low resistance may termination and these are intended to be wired result in a meter current of 1 mA in the High to the tip and ring of a suitable jack. Insulation condition if there is. a direct earth on Instruction S.4 Section 18 the line, but the meter can withstand pulses of of internal resistance are in any case negligible over 2mA without ill effects. compared with the lowest significant line insulation The meter scale is calibrated to read resistance resistance of 1:s MQ. when the slider of the variable resistor marked No provision has been made to correct for INSTRUMENTZERO is to the extreme left in the variation in internal resistance of the source diagrams (i.e. the whole of the resistance is in which is negligible in the RLT/l and is also series with the external resistance to be measured) negligible in the RLT/lP if proper battery main- and the source e.m.f. is 100 volts. The zero mark tenance checks are carried out since the normal internal resistances of the 135-V battery is about 300Q which is only 0.35% of the minimum circuit resistances. Line Discharge Circuir Position 4 of the main switch connects the A and B wires of the line to earth via R15 and R16 (3.3 kfl) of Figs. 26(a) and 27(a). Normally thdine will retain a charge from the insulation test, and moving the switch through the discharge position to subsequent positions for further tests gives ample time for complete discharge and thus prevents spurious meter deflections. Foreign Barrery In positions 5, 6 and 7 of the main switch the galvanometer in series with a I-MQ resistor (R9 of Figs. 26(b) and 27(b)) is used as a voltmete'r A OR 0 WIRE t \ connected to line in accordance with the engraved designations. The meter will be only lightly damped owing to the high series resistance, and the indications will, therefore, be as rapid as practic- able. Only two voltages are marked in each quadrant, 50 and 20 volts, which are in the regions Fig. 18.3. Simplified Measuring Circuits of the foreign battery voltages most likely to be (a) high-insulation resistance to Earth encountered. (b) low-insulation resistance to Earth Loop Resis!ancr which is at the right-hand end of the right-hand In positions 8, 9 and 10 of the main switch the quadrant correspo-nds to a galvanometer current circuit of the unit is arranged as a Wheatstone of 55 pA and a short-circuit line current of 1.19 mA. Bridge for measuring loop resistance, the simplified Negligible error occurs in the resistance scale circuit for unity ratio being shown in Fig. 18.4. if the source e.m.f. lies between 100 and 160 volts The positive side of the low-voltage source is provided the deflection for a short-circuit is connected to either P' or P" for the other two adjusted to the zero mark by means of the Insrru- ratios >: 10 or + 10 respectively. men! Zero resistor which allows the multiplying A 1-kbl resistor (R7 of Figs. 26(d) and 27(d)) is power of the universal shunt to be increased up to inserted in series with the galvanometer in the 1.6 times its minimum value. inequality ratio positions to protect the galvano- The values of the resistors in the padded galva- meter. nometer circuit for the Low Insularion condition Thc resistor R6 (12 kbl) is normally in series have been chosen to give negligible error for low with the galvanometer but can be short-circuited values of resistance by making the input resistance by operating the key marked INC. SENS to give of the padded galvanometer circuit almost equal full sensitivity; the meter will be slightly sluggish to R11. In the High Insula!ion condition variations in this condition. Instruction S.4 SecHon 18 readings. A reading is first taken in the normal position and then one in the reverse position and each is given its correct sign (readings in the red portion of the scale are positive). The reverse reading should then be subrracred alge- braically from the normal reading, and the result, divided by 2, will give the true unbalance apart from second order errors. This takes accouht of errors in the ratio arms and in the location of the scale of the measuring resistor in relation to the wiper position. These errors may be important if unbalance is to be measured to fine limits on a long line. The lamp adjacent to the scale of the measuring resistor marked VARLEYis illuminated when the switch is in either Position 11 or 12. Earrh-ro-Earlh Tesls Positions 13 and 14 of the main switch s.imply give a unity-ratio Wheatstone Bridge conn,ection similar to that shown in Fig. 18.4 except that the Fig. 18.4. Simplified Circuit of Unity Ratio Bridge for A or B wire of the line is connected to point q loop resistance measurement and point r is earthed. The wire under test is In order to check that a true balance has been earthed at the distant end and the bridge measures obtained (which is only necessary in special cases) the resistance of the A or B wire plus that between the Check Null key is depressed; this disconnects the two earth connections. Earth potential the galvanometer from the bridge circuit and differences which may be appreciable in comparison connects it across R8 (15 kR) which provides with the source voltage are to be expected on enough damping to bring the pointer to rest quickly but still allows a little overswing to show that the movement is free. A lamp near the calibrated dial of the loop measuring resistor is automatically illuminated from the low-voltage source. Resis~ance Unbalance Positions I1 and 12 of the main switch give the bridge circuit for the normal Varley Loop measure- ment except that a 2542 fixed resistor R5 in Figs. 26(d) and-27(d) is included in one leg so that for a perfectly balanced line the bridge will balance when the resistance of the 50-R variable resistor marked VARLEYis 25 Q. This gives zero unbalance in the middle of the resistor scale with positive and negative unbalances on either side of this zero. Hence it is never necessary to reverse the bridge to obtain a balance. The simplified circuit is shown in Fig. 18.5. The Varley Reverse position (No. 12), which reverses the A and B wires and is only used in special cases, is provided to enable the bridge to Fig. 18.5. Simplified Varley-Loop Bridge Circuit for be checked or to give very accurate unbalance measurement of resistance unbalance Instruction S.4 Section 18 long lines and a steady reading will not always Resistance Unbalance be obtainable on this test. On a loop of 2,000-51 resistance or less the maximum error on a bridge reading should not Powa Supply exceed 0-2552. If the mean of a normal and In the RLT/I a simple self-contained mains reversed measurement is taken the error should unit provides a low-voltage d.c. supply at 4.5 V not exceed 0'152. These values apply to un- nominal for bridge measurements and a high balances between 0 and 552: voltage d.c. supply at 150 V nominal for insulation For higher unbalances, the error should not resistance measurements. A neon stabiliser valve exceed f 0.5R. The limiting factors here are the is connected across the high-voltage supply. resistance per turn of the 50a variable resistor, With the main rotary switch in any of the posi- which is approximately 0'1 62, and the accuracy of tions for bridge measurements and the appro- the ratio arms, the latter factor becoming the priate lamp alight, the voltage across the capacitor more important on long lines. C2 (Fig. 26(a)) measured on an Avometer should be about 5.9 V. Calibration Chgks a' With the main rotary switch in position 3 If the dials of the Loop and Varley variable (INS.A/B) the voltage across the neon stabiliser w resistors are removed at any time, it is necessary valve measured on an Avometer should be to ensure that they are replaced correctly otherwise I50 v f 5%. the calibration will be lost. The recommended In the RLT/IP a 4.5-volt battery (3 Siemens method of checking the accuracy of the dial " S " cells) is used for the low-voltage source and locations is as follows :- three 45-volt batteries (Ever-Ready Type B102) in series for the high-voltage source. Measured Loop Resistancc~ on an Avometer the voltage across the 4.5-V Connect a resistor of 1,000 ohms f I ohm to battery should b: 4.5 V $1 10 % and that across the Test In jack or terminals. In the RLT/IP, the high-voltage battery should be 135 V f 10%. the resistor R17 is provided for this purpose and The 135-V battery should be capable of delivering is connected to the bridge input by strapping the a current of not less than 0.25 A when the I A Test In spring terminals to thy Line 2 terminals. range of an Avometer is connected momentarily For the rack mounted RLT/I, a high-grade resist- across its terminals. ance box should be used. The main rotary switch SWA is set to position 9 (Loop Xl), and the Accuracy bridge should of course balance when the Loop Insulation Resisrance dial is set to read 1,000 ohms. Should it be neces- Specified as f 10% of scale reading and likely sary to adjust the dial, first temporarily mark to be within f 3%. the approximate position of the knob relative to the dial, then remove the knob. Set the resistor Foreign Bartery by means of the screw-driver slot in the spindle Specified as f IOPL at calibrated points and V so as to balance the bridge, and then set the dial likely to be within -j=1 volt. (which is now free on the spindle) to read 1,000 ohms accurately. Replace the knob in approxi- Loop Resisranc~ mately its original position relative to the dial The maximum errors specified are:- and tighten the grub screws. The whole assembly is Ratio -+ 10 f 1 % of reading or :+ 0.2561 now lozked. Check that the end stops are working ,, x I &I%,, , ,, :tZa5i2 correctly-tkre should be full movement covering ,, x 10 f 1 % ,, ,, ,. f 2.X) the calibrated part of the dial, and rotation should The larger of the two values applies and scale be restricted at each end by the end stops, not by readings below 100 are excluded. The limit of the wiper of the variable resistor. accuracy at low readings is chiefly in the linearity of the variable resistor winding which is stated by Resisrance U~~balunci~ the makers to be f 0.2% of th? full winding On tht RLT/IP, short-circuit and earth the resistance. The engraving also introduc~ssome Test In spring terminals. On the RLT/I, plug error. the Tesr In jack to an earthed loop by a known Instruction S.4 Section 18 good cord. Set the rotary switch SWA to Position of rotary switch SWA. Since this merely reverses 11 (Varley Normal). It will be seen from Fig. the connections to the earthed loop under test, 18.5 that the circuit is now that of a unity ratio it should not change the balance unless there is bridge, comparing the 50-ohm Varley resistor appreciable contact resistance, or unless the earthed with R5, 25 ohms. The Varley resistor should, loop is faulty. If it is necessary to adjust the dial, therefore, give a balance when adjusted to the see the relevant remarks under Loop Resistance in midscale value, calibrated as Zero unbalance. the preceding section. Check in the Varley Reverse, or No. 12 position WG.0758. Instruction S.4 SECTION 19 STANDARD LEVEL PANEL SLP/3 General Description input impedance of the bridge equal to 600 ohms Standard Level Panel SLP/3 is used on A.C. as accurately as possible. Test Bay AC/55 to provide facilities for the accurate The 10-dB attenuator pad is constructed from measurement of tone level at +10 dB or +20 dB. Muirhead Type-A70 resistors which have values It is basically a lamp resistance bridge which is within 50.1% of those given in Fig. 29. adjusted on test before the unit is issued to balance when an inpur level of + 10 dB is applied direct or Performance when an input level of +20 dB is applied via an Each individual SLP/3 is issued with calibration internal 10-dB attenuator. information showing its actual performance. *The The only controls provided are U-links on the specified performance and allowable limits for all front of the panel to connect the attenuator in or models are as follows :- out of circuit. The unit is assembled on a 19" x 34" panel and (a) As a Voltage Indicator ( 10-dB Position) the input and output are wired to jacks on the bay Frequency Balance Volts (dB) jackfield. The A.C. Test Meter ATM/I which is loo0 c/s +10 50.1 mounted on the bay, is used to indicate when the 30 c/s to 8 kc/s +10 50.1 bridge is balanced, i.e., the output is zero. 15 kc/s +10 50.5 20 kc/s +lo f0-75 Circuit Description As shown in Fig. 29 the input tone is fed through (b) As a Calibrating Devicz for a 600-ohm Sending U-links either dirccr or through a 10-dB attenuator Circuit (10-dB Position) to a lamp resisrance bridge via a repeating coil CL4137-33 which has been tested for winding and Source Voltage in Series capacity balance to close limits; The two halves with 600 ohms Relative of the secondary winding of the repeating coil form , to 4.9 volts at Bridge two arms of the bridge, the other two arms consist- Frequency Balance ing of the lamp LPR and the resistance RI. The lo00 c/s f0.1 dB output of the bridge is connecrec! via the trans- 50 c/s f0-2 dB former LL/27SA to the output tags. 90 c/s to 10 kc/s f0.1 dB Bridge balance (i.e. zcro output) is obtained when 30 c/s to 20 kc/s f0-5 dB the resistance of the lamp is equal to R1, and since (c) Input Impedance (10-dB Position) the resistance of thc lamp dependb upon the current Frequency Modulus Ohms Argument through it balancc will only occur at one particular 50 c/s 600 f 20 =P 8" value of applied voltage which is dctermined by loo c/s 7 the value of R I. On test the value of R I is chosen 250 c/s 600 5 12 > 8" so that balance is obtained when the voltage lo kc/s] applied to the repeating coil is 2.45 volts, i.e. .a lo00 c/s 600 5 3 >'1.5" level of + 10 dB. The lamp used is a P.O. No. 2, 6V, which has (d) Sensitivity been aged and selacted on test for resisrance and The output level at balance should be not greater stability. The resistance of RI is in the neighbour- than -65 dB and for a change of 0.1 dB in input hood of 40 ohms and the precise value required is level from the balance level the output level should determined on test to within 0.01 ohm and is be between -35 dB and -45 dB. provided by a wire-wound non-inductive resistor shunted by a stabilised carbon rtsistor Erie type (e) 20-dB Input Position 108. With the U-links in the 20-dB and 10-dB positions The resistor R2 is made up on test from Erie the corresponding input levels for bridge balance Type108 stabilised carbon resistors to make the should differ by 10 5 0-02 dB. Instruction 5.4 SECTION 20 AX. TEST BAY AC/55 General Description A.C. Tcst.Bay AC/55 is a 19-inch bay of equip- ment for the audio-frequency tcsting of sound apparatus and the audio-frequency and direct- current testing of lines. At small centres where the full facilities are not required the bay is supplied partially equipped as necessary to suit the requirc- ments of the particular station. A fully-equipped bay comprises the following (see Fig. 20.1):- Type 12 Day Framework Equaliser Panel ET/I I General Purpose Mounting GPM/3 fitted with Aural Sensitivity Network ASN/4 and High-pass Filter FHP13A Amplifier Test Panel ATPI1 lnlermodulation Test Oscillator 1TO/I Routine Line Tester RLTI I Tone Source TS/IO A.C. Tester Meter ATM/l Atlenuaror Panel AT130 Jackfield JF/I 16 Telephone Panel TP/ 12 Desk nBA/lO5 Relay and Repealingcoil Panel RRC/ I Standard Level Panel SIdP/3 Mains Distribution Panel MDP/S, Connwtion Strip Mounting CSM/2 A block schematic showing how the various testing facilities are obtained via the bay jackficld is ,given in Fig. 30. Description of Apparatus Equoliser Pone1 ET/1 I Pancl ET/II is provided for convenience in testing equalisers which are mounted on equaliser chassis CHI18 or CH/34. Two equalisers on chassis CH/18 nnd one on chassis CHI34 cnn be rnountcd on the panel and their inputs and'outputs brought to jacks on the bay jackficld. Aural Sensitiviiy flefwork ASN/4 This network is used for making frequcncy- weighted measurements of noise nnd is described in Section 14. It is fitted on the General Purpose Fig. 20.1. A.C. Test Bay AC/55 : Layour Mounting GPM/3 together with the High-pass Drawinf NO. DB 3236 Filler FHPl3A. Input and output jacks are providcd on the jackfield. Instruction S.4 Section 20 High-pass Filter FHP/3A Attenuator Panel AT130 The FHP/3A, also known as Harmonic Routine This panel contains two-pairs of 600-ohm Tester,' has a similar circuit to that of the FHP/3 variable attenuators each pair covering 0-60 dB in described in Section 5 but is mechanically 0-5-dB steps. The input and output of each pair constructed for mounting on the GPM/3. It is are connected to jacks on the jackfield. The panel used in measuring total harmonic content at the also contains a key switch for operating the relay two fundamental frequencies of 100 c/s and 1 kc/s. on Relay and Repeating-coil Panel RRC/I. Its input and output terminate on jacks on the jackfield. Jackjeld JF/ 1 16 This is fitted with jacks giving access to the various units mounted on the bay. It also includes Amplijer Test Panel ATPI 1 jacks for bay tie-lines and miscellaneous jacks Panel ATP/I is provided to simplify the testing providing additional facilities for setting up test of plug-in amplifiers used with Type-B studio circuits. Some of these jacks are connected to such equipment. It will accommodate two amplifiers of commonly used apparatus as fixed attenuator pads this type and provides power supplies for both and 600-ohm' resistors which are mounted on the r together with switching and metering facilities. back of the jackfield. Fig. 30 shows a block schematic. The panel contains input and output jacks for both - positions and for an attenuator pad for use when Telephone Panel TP/ 1 2 testing C/9 amplifiers. A programme meter is Panel TP/12 contains the necessary apparatus provided for testing MNA/3 amplifiers. The for a single channel telephone with ringing and circuit is shown in Fig. 32. lamp indicator facilities. Fig. 33 shows the circuit. lntermodulation Test Oscillator IT011 Relay and Repeating-coil Panel RRCII This oscillator provides accurate zero-level tone The RRC/I provides facilities for the insertion at 900 c/s from a low-impedance source for lining-up of repeating-coils between balanced and unbalanced purpose and mixed 50-c/s amd 900-c/s tones for circuits when measurements are being carried out, intermodulation tests. It is an improved, rack- and also for quick comparison between two test mounted version of the Portable Intermodulation circuits. As shown in Fig. 34 it contains two separ- Tester PIT/] described in Section 16. The ate repeating-coils which are connected to jacks on 900-c/s oscillator is of the Wien-bridge type similar the bay jackfield, and a change-over test circuit also to that used in the Tone Source TS/lO. Jacks are connected to jacks. The latter circuit has a specially provided on the unit itself and there are also balanced repeating-coil which can be switched to the parallel jacks on the bay jackfield. The circuit input of either of two test circuits by means of a diagram is shown in Fig. 31. relay to give an accurately balanced 600-ohm source impedance. Simultaneous operation of a second relay changes over the outputs of the two Routine Line Tester RLT/l test circuits to whatever measuring apparatus is The RLT/l is intended for carrying out all being used, usually the ATM/ I. The key switch con- normal d.c. tests on lines and is described in Section trolling the relays is mounted for convenience on the 18. Jacks for line connections are fitted on the Attenuator Panel AT130 which is largely used in bay jackfield. measurements for which the change-over circuit is used, e.g. in comparing the gain or loss of two Tone Source TS/ 10 circuits at different frequencies. This is described in Section 8. A parallel output jack is provided on the bay jackfield. Standard Level Panel SLPI3 This panel provides facilities for the accurate measurement of tone level at +I0 dB or +20 dB. A.C. Test Meter ATMI I It is described in Section 19. The ATM/I can be used as either an amplifier detector or a test progra.mme meter amplifier and is Mains Distribution Panel MDP/5 described in Section 17. A 600-ohm input jack The MDP/5 distributes the mains supply to the and a high-impedance input jack are provided on various mains-operated units via fused connectors. the jackfield. Five of its six outlets are used, leaving one spare. 20.2 W.G. 5/60 Instruction S.4 SECTION 21 FIXED FREQUENCY OSCILLATORS : OS2 SERIES OSCILLATOR OS2/4 General Description resistance network. This second therinistor helps The OS2/4 is a 9004s RC oscillator of the to correct variations in output level caused by the Wien-bridge type, with accurate zero-level output effect of ambient temperature changes on THI, and from a low-impedance source for level setting. It the two thermistors are therefore mounted close supersedes the OS/10 (described in Section 7), together so that they have the same ambient except where there is a need for the full facilities of temperature. the Intermodulation Test Oscillator ITO/I, which The level at the output terminals is adjusted by is used on A.C. Test Bay AC/55 and is described RV I to be zero into 600 ohms and the oscillator is in Section 20. designed so that this level is maintained conshint The OS2/4 is constructed on a plug-in chassis, of to within f0.2 dB for all likely changes in supply the type used for amplifiers such as the GPA/4A, voltage and ambient temperature. As the output which are employed with Type-B Studio Equip- impedance is less than 1 ohm, a change in load ment. (See Instruction S.3 Section 21.) impedance from that of a high-impedance amplifier- detector to 100 ohms causes a drop in output level Circuit Description (Fig. 35) of not more than 0.1 dB. The first stage V1 is an RC oscillator of the The adjustment provided by RVl enables the Wien-bridge type, described in Appendix'A and output level to be varied by at least f1 dB about used in other BBC tone sources. It is followed by a zero level into W ohms. conventional amplifying stage V2 to give zero output level. General Data A double-triode valve is used for the oscillator Power Supply Voltages stage instead of the two, separate valves shown in H.T. supply, 290 V f15 V. the arrangement described in Appendix A, and Valve heaters, 6.3 V f0'15 V the basic circuit is virtually the same as that of the PTS/16 described in Section 9; .it is also Valve Currents identical with the oscillator stage of the Inter- Cathode currents may bechecked by measurement modulation Test Oscillator ITO/l. (See Section 20.) of the voltage across cathode resistors with a high Two arms of the bridge are formed by the resistance voltmeter (Avometer No. 8) as follows: frequency-determining networks, R1, C1, and R2, V 1 a 2.4 V across R4 C2, and the two resistance arms are formed by Vl b 3.7 V across R6 R4 and the thermistor THl. As in the PTS/I6 and V2 17.5 V across R15 the TS/lO, the thermistor replaces the lamp All should be within f20% resistance shown in Appendix A, and the two resistance arms have been interchanged because Thermistor THI the thermistor has a negative resistance/tempera- 900-c/s current: About 2mA read on an ture coefficient whereas that of a lamp is positive. Oscillation is produced by positive feedback average meter (Avo). from the anode of the second half of valve V1 to the grid of the first half, via the series reactance Output Frequency. arm CI, RI. The amplitude of the oscillation is 900 c/s f4%. limited by negative feedback via the thermistor TH1. The values of C I, R1, and C2, R2 are chosen Output Impedance: to give an oscillation frequency of 900 c/s f4 per Less than I ohm at 900 c/s. cent. The output from the bridge is fed to the amplify- Harmonic Content: ing stage V2 via a second thermistor TH2 and a 1% approximately at zero level output. Instruction S.4 Section 21 Stability &lo% to the power-supplier normally used should The output level is stable to within &0.2 dB result in an output level change d not greater than for all likely changes in power supply.and ambient *0.1 dB. temperature. Variation of mains input voltage by W.G. 2/63 SECTION 22 STANDARD LEVEL METER ME1611 General the'output voltage is 2.45 x (4.62/14-62), or The Standard Level Meter ME1611 is intended 0.775 volt; this is the 0-dB reference level, because for lining up progiamme meters and amplifier- the square of 0.775 volt, divided by 600'ohms detectors, and provides a standard sending level equals 1 mW. of either zero or + 10 dB at a frequency of 50 c/s When the switch is set to the +lo-dB position, with reference to I mW in 600 ohms. The unit may the output line is connected across both output be supplied from either a 4-volt or 6.3-volt 50-c/s resistors, and the full output voltage of 2.45 volts a.c. source, and is contained on a 19-inch panel is obtainable at the output terminals; this repre- suitable for rack mounting. (See Fig. 22.1 .) sents a level of + 10 dB, since 20 log,, (2.45/0-775) The ME1611 is similar to the CALI1 (Section 4) = 10. and CALIIA, except that both these latter are on In the +lo-dB position, the voltage level w8l w 226-inch panels and the CALI1 is intended for be correct for any load impedance. operation from the mains. In the Zero position the output impedance is METER STANDARD LEVEL ME1611 I I Fig. 22.1. Standard Level Meter ME1611 : Front Panel Circuit Description (Fig. 22.2) about 3 ohms, which is sufficiently low to make The 4-volt or 6-3-volt a.c. supply is connected to the error when feeding into a 300-ohm impedance terminals 1 and 2 (Fig. 22.2) and is reduced to the only 0. I dB compared with the voltage level into required sending level by the potential divider an open circuit. For load impedances above 300 e network R1 to R5. If a 4-volt supply is used, the 13.8-ohm series dropping resistor ~2 must be short-circuited. The + 10 dB output level is monitored by means of a moving-coil rectifier-type a.c. voltmeter which is connected in parallel with resistors R3 and 84. This meter is uncalibrated except for a zero mark and a mid-scale reference mark which indicates an r.m.s.voltage of 2.45 volts.Thepointer may be accurately set to this reference mark by adjustment of the 0-7 ohm Adj. Level resistor R5. Resistors R3 and R4 are card-wound components Notc Car~ultshown 11 for J 6.W heater SueDly of 10 ohms and 4.62 ohms respectively. For a dV hcoter suoply rhor+-c~rw~tR? 5.11142 When the + 10 dBIZero selector-switch is set to Zero, the voltage developed across R4 only is Flg. 22.2. Standard Level Meter MEI6lI: Circuit connected to the output line. Under this condition Drowing No. EA 9409: Itsue I Instruction S.4 Section 22 ohms, -the output level error is less than 0-1 dB 2. Vary the setting of AG'~.Level control R5 and may be ignored. until the meter indicates the mid-scale refer- ence point. Operation 3. The output sending level is now either 1. Connect terminals 1 and 2 to a 4-volt or 3- 10 dB or zero, according to the position 6.3-volt ax. supply, taking care to short- of the selector-switch. circuit R2 if a 4-volt supply is used. C.W.P.M.(X) 8/65 VALVE BASES BRITISH 7-PIN 87 13 V.M. NOTES:- I UNITS WiTH SERIAL NUMBER 136 & UP MAY BE USED ON AC OR DC LT SUPPLY 3 FFAhE 2 R20 ADJUSTED ON TEST FOR FLAT FREQUENCY * RESPONSE PANEL AMPLIFIER DETECTOR AD14 FIG 2 S 4 DIVIDE !--?GALVO PILLAR + -0- TERMINALS BATTERY A A GALW BATTERY TERMINAL BLOCK WHEATSTONE BRIDGE BG/I ISSUE AMENDMENT INPUT I L3 L4 4 4 - A- REP COIL I:I HARMONIC ROUTINE TESTER FHP/~ 17 4v R26 METER 0- I.5V. K < 020 Y NOTES:- Y X * FREQUENCY TO BE ADJUSTED ON TEST TO 900 CIS. I. WHEN LT SUPPLY IS A.C. TERMINALS 13,17 8 18 ARE TO 1- l l - SI mAt2 BE STRAPPED. R24 TO BE SHORTED BY ADJUSTING SLIDER. 2. RIam MAY BE ADJUSTED ON TEST FOR CORRECT FREQUENCY 9-19- FIL. VOLTS DC OSCILLATOR OS/9 7475 ( MULLARD NEON &HI- WTO BE ADJUSTED 10 ClVE 900 CIS NOTE I.ADJUST TO ClVE Odb WHEN LOADED WITH 60n LTERMINALS 3,5,6, ONLY STRAPPED WHEN USING K. HEATER SUPPLY OSCl LLATOR OS/IO FEED METER 0-I.5mA I TAG No. REF. ' 1 -[I--- 5rm~ NOTE :-I. WHEN LT SUPPLY^ IS AC~ERMINALS 13 17 E 10 ARE TO BE 2-12 - AmA+2 STRAPPED R24 TO BE SHORTED BY ADJUSTING SLIDER METER 4 - 4 - S2mAf2 2. RESISTANCE R3 MAY BE ADJUSTED ON TEST FOR SWITCH 5-15 - A2mA CORRECT MEAN FREQUENCY 7- 17 - TOTAL mAK2 3. POT. R2 TO BE ADJUSTED TO GIVE 900CIS 9 - 19 - FIL VOLTS D.C. 4. POT. R13 TO BE ADJUSTED TO GIVE Odb WITH 600n LOAD OSCILLATOR OSllOA I SEE NOTE I 15 @ + 1 %? RELAY 24 V ! R37 ADDED TO - COMPONENT TABLE METER RELAY REWIRED R23 WAS ADJUST ZERO 6 R25 WAS ADJUST LAW - R38 ADDED SEE NOTE 3 I II CALIBRATING SWITCH Rli ADJUST f SENSITIVITY I I A I I,. NOTES :- I. THIS RESISTANCE IS ONLY FITTED WHEN USING X)V SUPPLY 2. WHEN LT SUPPLY IS AC,TERMINALS 11 5 46 ARE 10 BE STRAPPED. R31 TO BE SHORTED BY ADJ. SLIDER 3 R33 TO BE ADJUSTED ON *FLICK* TEST IF NECESSARY METER SWITCH TAG Nos. REFERENCE 1-11 -VI SCREEN mA 5-15-VI ANODE mAx 2 9-19 - FIL. VOLTS DC PPMI~ PEAK PROGRAMME METER AMPLlFl ER I FIG.8 2nd ISSUE 2K)v --- a - 1 METER 1 1 I R33 ADDED AND GENERAL REVISON INPUT l -60011 INPUT 2 -HIGH PORTABLE PEAK PROGRAMME METER AMPLIFIER PPM /G I I FIG 9 'OH L I C8 A/6DR5/6 ~2 08 I n o R SJSC COIL L3 A8 , IOH L4 MI2, I LF/4 4 TI F2 , ALleSE T2 , K15 , a-200~ I AC MAINS NOTE:-VALUE OF CI -IS DETERMINED ON TEST VALVE BASE FREE END MAINS I 12-FREQUENCY OSCILLATOR PTS/~ FIG 10 TEST IN THERMO - I t0Si PADS CALIBRATE COUPLE I G ALVO I SHUNT T + GALVO TO SHUNT I 500 FOR 24V 26.78 SUPPLY ?SO FOR 50V SUPPLY 1 wGALVO PANEL NOTES SWrrCHlNG AT 0 @@ & @ BY 2 3 -POSITION INTERLOCKED KEYS 1 L SWITCHING AT THESE POINTS PERFORMED BY MEASURING CCT 4 2 MEANS OF KEYS EA~TH F&ME OR + SOV TRANSMISSION MEASURING SET TM /I FIG I I s4 3 '*ISSUE R16 WAS AT JUNCTION OF CBS Fill R34 6 G35 ADDE TERMINAL 5 CENTRE TAP OF 1 IT2 ADDED 1 C10 REMOVED KEY No228 REWIRED T2 WAS LGG/lOlS PLACED BY JACK NOTE I AMD R21 WAS ADJUST ZERO 8 R24 WAS INPUT CCT I 1 MODIFIED I HEATERS C7 @la I R31 i 'id - 6- 20 R28 t 'h' 'I' SERIAL RELAY WORKING Nos TYPE VOLTAGE TPM/3 BELOW 207 L34.43 24 -- . , -..-- I I .-- I './ I bll - Slmk TPM/3 207 ONWARDS 31102 24 OR 50 RIO 5000 ., I ,, R35 40MQ 1 METER TPM/3A RD2500 50 0-1.4rnA 2 & 12 - AlmA 1 RI I I 50000 ,, 1 I ,. 1 TI. . 1 1:. 3.- 16 I LG~SA-. . - . I I 1 METERING'CIRCUIT 4 B 14 - S2mA R 8s 1:2 LCCllSH -- NOTE I R34 TO BE ADJUSTED ON FLICK 12 3000 .I 1 0.25 1 T2 / 5 B 15 - A2mA 1 R 13 33.3.. 1 R36 5000nl LHNAP 7 B 17 - TOTALmA X 5 TEST IF NECESSARY 9 8 19 - FIL VOLTS DC TEST PROGRAMME METER AMPLIFIER TPM/~8 TPM/~A This drawlng Is the property of rhe British Broaduulng Corporation and may not be reproduced ar drxloscd to -.f a third party Inany form wlchour the wrltten permission VI of the Corporrtlon. P -.P L -0 ABCDEFGHJK LUW OPORS TUVW I l-4,ACIP CIO + SO - TONE SOURCE TS/5 AMPLIFIER COMP LOC 1 VALUE TYPE I CoMP LOC VALUE TYPE CI B5 1 50 rF I C13 03 -001nF C2 €5 1 ,005 = I C14 P3 -- LA I I TS15 OSCILLATOR AND DETECTOR 1 FIG 13 s4 I" W X Y I SEND gRi5 $' $ RII L F.O. 1 - LI,L2BL3 COUPLED L4,L5BL6 COUPLED POSITION 1. DC VOLTS (01400) 2 DETECTOR FEED (012mA) I 3 AC VOLTS (0120) LII LI2 MAINS A 1 7 -- MAINS UNIT PANEL -X- THESE COMPONENTS ARE ONLY FITTED WHEN NECESSARY TO GIVE CORRECT FREQUENCY CHARACTERISTIC VALUES ADJUSTED DURING TEST. TONE SOURCE TS/~ FIG 14 s4 NOTE: R43844 ADJUSTED ON TEST SO THAT R43 IN PARALLEL WITH R44 RECTIFIER a METER 600n t 2O/o LL + 2Q db INTO THIS 60Qn rqRl74 PRODUCE A METER READING OF 0.90 --.-.-.-.-.-.-.-*---- r BUFFER I.2mA -17 . VARIABLE TONE SOURCE TS/~ A B C D E F G -H J K L M N 0 P Q R S U VALUE 54 -- MAINS SWITCH 8V MAINS BV F4.T. PILOT LAMP PILOT LAMP 250 8. cs 4. 2ndFIG ISSUE IS C6 Y9 t00. 2000 .. PRESS TO READ PRESS FOR SCREEN VQLTS CATUDDE 'NSULATDNS o-12ovVoo-o~n ADJUST SCREEN VOLTS j R21 R22 R19 SCREEN GRID ANODE @ I RIB I CATHODE - .- 4 SUPPRESSOR PRESS TO SET ZERO. DIRECTLY HEATED VALVES - GRID - .- I I I A--7 ADJUST ZERO I HEATERS I I LINK I FOR EXTERNATSERIES I I RESISTANCE IF REQUIRED ; ; TO HEATER ! TERMINAi-5 Oh1 I VARIABLE GRlD BIAS UNIT *VALVE HOLDERS I (SUPPLIED ONLY 1F REQUIRED) NOTE:- I WHEN TINSLEY GALVANOMETER FITTED R9 IS TO PREVENTVOLTAGE DROP 7-WATT FIXED RESISTOR VALUE 5n 25% IN HEATEP WIRING CAUSING VOLTMETER ERWRS 1 VALVE TEST-- PANEL VT/4 This drawing is the property of the British Broad- casting Corporation and may not be reproduced or disclosed to a third party jn any form wlthout the written permlssion of the Corporation. r. TC. ? 8 - BOU~IUGOC bLeLElN5. 5.7.- 51-El TL(C)V(IUAL. T.C. - VALVE TOP CAP. VT/4 VALVEHOLDER PANEL ( 'is drawing is the property of the British Broadcasting ( Corporation and may not be reproduced or disclosed to a third party in any form without the written perrnisslon of the Corporation. R14- RIL VALVE TESTER VT/S No4 BANK No3 BANK RANGE SWITCH I liI l NO2 BANK I BATTERY O 01 I I MAINS L 00-25OV A C BANKS Nos ?. & 4 CARRY 600nATT A HT VOLTSX2 D VI -lOrnA L,, BANKS NOS I a 2 B V34OrnA E ADJ MAINS ow CARRY 752). ATT NOTE:- I SWITCHES SHOWN AS SEEN FROM FRONT. Nol BANK IS NEAREST PANEL C V2+lOrnA F TONE LEVEL BK 2 C22 ADJUST FOR BALANCE TO EARTH ON 75A OUTPUT (APP a0006pF) \, C23 !- - ,, ' 600A (*. (APP ,0004pF) 3 CI 8 2 GANGED COMP I LOCATION 1 VALUE I TYPE 1 COMP 1 LOCATION 1 VALUE I TYPE I COMP I LOCATION I VALUE I TYPE n COMP I LOCATION I VALUE I TYPE CI I 81 I MAX 533 rrF [ PLESSEY TYPE E I RI I Al I 0.33~nl2WTYPEA3635+1%! R27 I P7 I 220 nlaw ERIE TYPE~I~~IO~R53 I I V5 317 AlfW TYPcA3622 Z IC/c C2 I B5 I e, 533.8 1 . .. I R2 I Al I 3.3 I ...... 1 R28 I PB I lo r, IWERlETYPE100~5•‹/pU R54 I X5 I 317 m 1 ...... C3 I G5 I 16 rF I BEC 500V Wkq I R3 I A2 I 4.7 ,...... 1 R29 I UI I 16 .* 1 ...... R55 I V8 I 337 " 1 .. .. ., ,, PORTABLE TONE SOURCE P TS/13 CIRCUIT EUE AMENDMENT FIG 19 2nd ISSUE RESISTORS RIA- R6A AND RIIA ~~~~~~sOFRES 1 RIO.RI9.24 8 25 1 REVISED S2-POSN I -VI ,: :I:$- FEEDS ,. 4-v4 " 5 - 6-MEAS OUTPUT / VALUE TO BE DETERMINED ON TEST TI TURNS RATIO 8:l lr------.I, i OUTPUT TONE SOURCE TSI9 5 2/GW H/ IOI €A 7157 ISSUE 3 1 b-75 ,'O'IN 600 2 20-600,+d IN COO 3 Zo-300,+IZ IN 300 4 EARTH WOP, MFTER SWiTCI-4 PoblTtON I TONE LEVEL 2 L T VOcTS,VI ,VZ 3 L T VOLTS '/3 * 1ON-OFF 4 H T. VOLTS (X 100) THE FOLWWNG S Vt ,VL rn~.(~d COMPONENTS ARE TO 6 V3mA.(~25) BE AONSTEO OIJ TEST RANGE SW\tCH RS ( APPROX 1 00 a) PO5\TlON RE~(APPROX 3 kn) 1 25 - 250 C/S cu (APPPOX o OI,F) z 250 - 2500 C~S 3 2500 - 25090 C/S EAPTH 'COMP LOG COMP LOC COMP LOC COMP LOG COMP LOC COMP LOCI COMP LOC C I A2 C I1 E 4 R 3 A 3 R 15 01 R 29 F 2 $36 H 11 R45 K 3 C 2 A2 C I2 D 4 R 4 8 4 R 17 C4 R 27 -14 R 37 GI! Q46 J 3 C 3 02 C 19 0 3 R 5 A A R 10 04 R291 J5 1 R38 U I I R47 J 3 C A A 3 c 14 E 3 R B C '3 R PO t3 R 30 3 5 I m '39 GI 1 RAe L3 , C 3 A3 C 15 E 4 R 7 c 2 R 21 E4 R3l b! 5 I P do I4 I R A9 K 3 C 0 6 3 C 10 F 4 R 10 C 1 F122 E4 R 32 G 5 R4i Hi, I2 50 E 3 C 8 C5 RII 0 3 R 23 e I R33 f l CZ 42 HI, C 9 C3 rZ I A 4 - R 13 C I B24 C I R 94 CP R43 64 TI D 2 C 10 C3 R 2 04 R 14 C I R 25 F I R 35 a I R44 T2 F 2 I PORTABLE TONE SOURCE PTS / 10 COMPONENT TABLE: FIG. 21 ~omp. LOC. 1 TYpe TYpe Tolerance 8 per cent AT1 U9 I Painton 172019.1 MRI Westinghouse I mA AT2 Painton 172025 / (7 1 RIA 87 Welwyn SA3635 12 } E9 4-Gang C60.04/1 RIB B7 Erie 9 f5 C2 C9 I T.C.C. Ceramic SCH l A7 .. 8 C3 C5 1 Eddystone 582 87 Welwyn SA3634 & 2 C4 ,, 582 87 , Erie 9 C5 I C: ; T.C.C. Ceramic SCHI R3A C7 ( Welwyn SA3634 C6 E7 i Mullard E7850 I 1' R3B C7 15 C7 CIO Muirhead 33AT 1 .5-. i R4A 7 ,3635 CIO ! ,. .. 1 RIB 1 D7 Erie9 E: 1 Dl0 1 ,, 1 R4C D7 1 ,,8 CIO Dl0 i ,, R5A E7 1 Welwyn SA3634 1 *'i CII El0 I .. 1 R5B 1 ti 1 Erie9 rt 5 CI2 FIO E7 I Welwyn SA3634 : *2 C13 ' GI0 ! 1 E7 Erie 9 37 5 C14 r GI0 , , ! R7 H 6 ., ,. rfi 10 C15 G7 I 10-- R8 / H4 ,, ,, C16 I H5 ' B.E.C. CE15129 'IR9 1G5 ,) PI C17 I J6 2 T.C.C. SM1007 --- 5 RIO / 110 .. ., C18 ' K7 , Muirhead 33AT -.- 10 ,I RI I j L5 , Painton 7W P401 1 ::I C19 K5 ' B.E.C. CE15129 I/ RI2 ' K4 ! Erie8 ; -C 10 C20 K6 Muirhead 39AT I, I , R13 / LIO ; ,, 9 I C2I I M7 .. 33AT R14 : 18 ' Welwyn SA3623 C22 ; N6 ' T.C.C. SM 1007 . -I'; Rl5 I M7 1 ,, SA3622 ..+ 5 C23 ] NIO ., SM2N Morganite MNAP I C24 : N9 1 ., CE30N I I lOl250 26000 C25 4 P7 ,, CE34P MI0 ! WelwynSA3622 C26 1 46 Muirhead 39AT N5 Erie9 I I C27 , R5 I T.C.C. CP46S I R19 M5 I ,. ,, ! I( C28 SIO , ,, SM3N ! ,, R20 ! NIO ! Welwyn SA3622 i-2 C29 I SIO ., ! R2I 010 Erie9 + 10 ! " " C30 i R4 ,, Ceramic SCD4 ' 20 1 R22 1 P4 .. 8 C31 I ~5 ,, ,, ,, I i R23 1 010 ,. ,, c32 i U4 ,, CE25P : ~8 Welwyn ~~3634 ; 52 C33 ; U4 : B.E.C.CE15129 Q7 1 ,, SA3622 C34 I T4 ! ,, I R9 I ! &I C35 1 J3 I T.C.C. CE32A R27 Q5 1 ,+5 I C36 C7 i Mullard E7850 : 1 R28 C37 B5 T.C.C.7OISMB ' C38 F9 R30 I NI ; Erie9 1 i10 C39 87 Mullard E7850 R3 1 C40 F7 ,, ,, Erie 9 LI P5 R/I L2 T3 CH. I TR I QlO AL/33R LP I 2 I X4 MI50 FIG 21 SW I - WSN I - RANGE I 20-40 C/5 VALUES SUBJECT TO AOJUS' MENT ON TEST METER SWITCH .5:3 R 30 * 2 - RANGE 2 40-400ClS WIN6 T91MMlW5 OR ALTERN&IVE COMPONENTS 3 - RAffiE3. 400-4000C/5 81 4 - RANGE4. 4000- 20.000 CIS swz - WS'N.I - INCR. I - 4 SWJ - WS'N. I -MEAS OUTPUT * 2 - INCR 2 - 2 2- 3 - INCR. 3 0 " 3-VI,V3 " 4 - INCR. 4 + 2 * 5 - INCR. 5 + 4 1 FREQUENCY I RANGE ! SWITCH SWI BRITISH OCTAL I +PIN GLASS MAZDA OC'I'AI. B9G M.0 AT I 0-60dbBY IOdb 0 d ld FREQUENCY CONTROL FREQUENCY INCREMENT CIA~CIB(GANGED) SWITCH OUTPUT - LEVEL ATTENUATORS swe -- , (600 SL BALANCED) OSCILLATOR Vl K Vi? AMPLIFIER : V3 &V4 NOTE : CIA & CIB EICU COMPRISE TWO 532~F PARALLEL SECTIONS OF THE 4- GANG VPRIABLE CAPACITOR CI PORTABLE TONE SOURCE PTS/IZ INSTRUCTION S.4 COMPONENT TABLE : FIG. 22 I I : Tolerance j Tolerance Comp. Lor. Comp. TYpe 1 I Per cent I Per cent C I C7 T.C.C. 82 1M ,. 9 C2 C3 . B.E.C. CE511!15 ,, 8 C3 D4 T.C.C. 82 1M ,, 9 C4 D7 , CE32A ,, 8 C5 I ES ,, CP37S ., 9 C6 : F7 ,, 82 1M Eri: 9 C7 , G3 B.E.C. CE51 1/15 9 9 C8 H4 T.C.C. 82 IM Morganite MNAR C9 H4 , CM20N 25450 26000 CIO G7 , CE32A I Erie 9 CI I JS ,, 82 IM 9 9 C12 K7 ,, CE32A ,, 9 C13' K3 B.E.C. CE5llj15 ,, 9 C14 K4 T.C.C. 821M ,, 8 CIS M3 ,, CM2ON .. 9 C16 07 B.E.C. CESI I ;IS ,. 9 C17 P7 , CESlli15 i .. 9 -C13 comprises two 16-p,F capacitors in parallel .. 9 I I I .. 9 .. 9 3, 9 Morganite LHNAR 20350 18000 Erie 9 ,, 108 Colvern CLR. 1 106/8S Erie 9 This drawing is the property of the British Broadcasting Corporation and may not be reproduced or disclosed to a third party in any form without the written permission of the Corporation. R2 RII stoh Idz,,R7 Fl 'I0' VALVE BASES ADJUST ON TEST FOR FLAT FREQUENCY RESPONSE 610 4) (I JKI 000 -E .,.IY - ?+IN GLASS BRITISH OCTAL B9G MAZDA OCTAL M 0 PORTABLE AMPLIFIER DETECTOR PAD/^: CI RCUIT INSTRUCTION S.4 I COMPONENT TABLE : FIG. 23 I i Tolerance Tolerance Comp. TYpe Comp. Per cent C I T.C.C. 82 1M R9 C2 B.E.C. CE5 1 1 / 15 -1 per cent R I0 C3 T.C.C. 82 1M RI I C4 ,, CP37S RI2 C5 , 82IM R13 7, 9 C6 B.E.C. CE511j15 I R I4 Rel~anceTN WW C7 T.C.C. 82 1M I R15 Erie 9 C8 .. CE32A R16 ., 9 C9 ,, CM2ON I R17 Erie 9 CIOZ B.E.C. CE5l l I5 i 1 R18 #. 9 CI I T.C.C. 82IM 1 R19 ., 9 C12 R2O 2W C13 B.E.C. CE5llil5 (16~~)l R2I Erie 8 + Hunt KB422KZ ! R22 ., 8 010 PF) R23 C 14 B.E.C. CE51 1 ! 15 R24 o 16-pF capacitors in parallel R25 ,, 9 R26 ,, 9 R,i l A 1 R27 M~rganitoLHNAR I 20350 18000 Erie 9 I i20 R28 Erie 9 ,, 8 120 R23 ,, 9 ,. 9 R30 Covern CLR 1 106!8S .. 8 R3 1 Welwyn A3623 .. 9 R32 Erie 9 3, 9 1 ,, 108 j 12 TR I .. 108 1 *2 --TR2 u drawing is the property of the British Broadcasting q -poration and may not be reproduced or disclosed to ( 52 a third party In any form without the written permission gs of the Corporation. a*w 5 I 1 CAW10 0 v 600 (NOTE 2) 10 - NOTES I ADJUST ON TEST FOR FLAT FREQUENCY RESPONSE 2 ADJUST FOR 6OOn INPUT (APPRO% IS^) 0' 3 ADJUST RIS FOR APPRO% MI0 -SETTING Of R14 9-PIN GLASS BRKISH OCTAL B9G MAZDA OCTAL LI 0 PORTABLE AMPLIFIER DETECTOR ~~019:MODIFIED FOR USE WITH P PM wN V, P COMPONENT TABLE : FIG. 24 j Tolerance Cornp. Loc. TYpe TYpe 1 Per cent ---I U.I.C. SM 1007 I .t5 T.C.C. CE306 R14 , 15 ,, CP33S R15 , N2 !: 5 I ' , CP33S R16 02 ,, 9 I I5 ,, CP37S R17 P2 ,, 9 i.5 ,, CP37S R18 P2 ,, 9 ,, 346 R19 M 2 .. 9 B.E.C. CE820 R20 ,. 9 ,, 45025 R2 1 i 15 R22 E :: 1 i5 LD/ 1/45 R23 R3 . ,. 9 I :kS R24 53 ,, 9 11.5 Westinghouse 16HT24 R25 ,, 9 I .t5 16HT24 ,, 9 3. 9 R I B4 Erie 108 If_ 5 R28 45 Reliance TW/I (linear) I R2 C3 ,, 9 i. 10 Erie 9 15 R3 C3 r. 9 f10 ., 108 R4 C4 ,, 9 f10 R3 1 F8 ,, 9 R5 D I vs 9 &I0 R32 H 4 ,, 9 110 R6 El ,, 9 rfr lo R33 j K3 ,. 9 I :t10 I R7 F2 9. 9 * 10 P,8 F4 ,, 9 110 TH I D4 S.T.C. A54 12i100 1 R9 F4 Colvern CLR 1106175 1 RIO G4 Erie 9 TR I L2 AL/13RB RI I H4 TR 2 R2 LLiI06SA I 10510 26800 LLj l5SA Erie 9 . 7 M.172, i - I. -~:::-. - - I -. . - .. . .. -- -- . s 4 FIG 24 . - T MIXED TONE AMPLIFIER I , VALVE BASE BUTTON MILIIATURE 7 -PINGUS5 B 7Li ;.71 MAINS UNIT PORTABLE INTERMODULATION TESTER P IT/ I : CIRCUIT INSTRUCTION S.4 COMPONENT TABLE: FIG. 25 Tolerance Tolerance Comp. Loc. TYpe per cent. Cornp. Loc. TYpe per cent. AT I C4 PN/IOAI AT2 L4 PNII IAl C I E6 T.C.C. CE32A/PVC C2 F2 Plessey CE8 181 1 C4 €4 T.C.C. CP37NjPVC C5 F5 Muirhead 39-AT C6 G6 T.C.C. CE32AIPVC C7 H5 Muirhead 39-AT C8 G4 Mulrhead 39-AT C9 18 Muirhead 39-AT CIO K8 T.C.C. CSM20N CI 1 N4 T.C.C. CP37N/PVC C12 P7 Hunt BSO3K C13 M6 T.C.C. CE32A/PVC C14 N3 Dubilier B211 rect. metal case CIS R6 Hunt BSOOK C15A 56 Hunt B807 C16 P9 T.C.C. SCE79BlPVC C17 LI I Plessey CE9 1 1 / 1 CL 1-4 N9-I0 CV425 FSI. 2 AIO Belling Lee L562/250 Miniature cartridge R42 U I Erie 9 JKA A6 S.T.C. 41 12C (Pt. con- R43 D9 tans) ,, 109 R44 F9 8 JKB A l S.T.C. 41 128 .. R45 FIO 8 JKC A7 S.T.C. 41 128 ., R46 N 5 ,, 108 R47 N6 ,, 9 L I J7 B.B.C. F/32A R48 P8 ,, 9 L2 LIO B.B.C. S/19G R49 G7 Morganite LH, linear, M 1 59 Spec. ED. 1473 *" spindle, slotted. M2 T3 Spec. ED.1415 (Sq. case) R50 M7 Morganite LH, linear, Ig" spindle R3 A7 R5 1 T6 Colvern CLRI 13211 55, R5 D6 linear, f" spindle. R6 D8 slotted R7 E7 R52 U3 Colvern CLR3001/21, R8 €6 linear, IF spindle R9 El R53 HI0 Painton MVI RIO FI R54 c4 Erie 109 RI I F I R 12 J I SWA H4 K4 03 N.S.F. TG3 CP, 2-W R13 G 6 SWB K3 K7 Painton 31021 1 P.B. R14 F6 SWC A8 Painton 501520 P.B. R15 F5 SWD B10 N.S.F. 8373 10145 R16 17 Toggle, D.P.C.O. I I 54 58/W~/001 SECTION 17 D D3374 ISSUE 8 LISTEN INPUT INPUT 4E$pI,, This pdr is connected only when the R 32 IOK 4; L---ia/ CAL. ?---?6.W.C CV425 - L MAINS N 1 i 3 1 E 0 I PORTABLE USE. * ADJUST ON TEST I A.C.TEST METER ATM/ I I INSTRUCTION S.4 COMPONENT TABLE: FIG, 26 Tolerance Tolerance Comp. TYpe per cent. Comp. TYpe per cent. T.C.C. SCE76PE/PVC - 20+ 50 RIO Painton P302 T.C.C. CE32A/PVC RI I Erie 108 Belling & Lee L562/1. IA miniature R12 R13 R14 P.O. No. 284 .. 9, R15 Erie 9 ,9 R16 P.O. No. 2, 6V R17 I, I. ,I ,. ., R18 Painton P301 Westalite 16RD-2-2-8-1 R19 Painton CV25S S.T.C. B18-I-IW R2O Painton CVlSS Wirewound R2 I Painton CV2P Erie 9 SWA Painton, Winkler, AS/2P/14/4B I, I, SWB B.B.C. EPA7606 Erie 108 SWC N.S.F. 837318145, Toggle, D.P.C.O. SECTION 10 FIG 26 I L.J INS. ZERO L a TO"TEST-IN" - 7.6 Rll R I0 - FGJ, e JACK RING c - b is, I em rR1l (c) INSULATION RESISTANCE CIRCUIT (a) MANS UNIT LAMPS & LINE. I(b)I GALVANOMETER DISCHARGE CIRCUIT CIRCUIT - -- - - ,R3S ADJUSTED TO MAU€WMWB+R+ i (dl LOOP RESISTANCE & VARLEY ROTARY SWITCH :::$$}XI CIRCUIT .SWA SPEC.ED~RLVI. ROUTINE LINE TESTER RLT/I CIRCUIT INSTRUCTION S.4 COMPONENT TABLE: FIG. 27 Tolerance Tolerance Comp. TYpe per cent. Comp. TYpe per cent. B I Siemens Size S (3 off) RlOa Painton P302 B2 Ever Ready BlO2 (3 off) RlOb Erie 109 RI I Erie 108 JKA S.T.C. 41 148 R12 JKB S.T.C. 41 128 R13 JKC S.T.C. 41 12C R 14 ,. 3, P.O. No. 284 K A R15 Erie 9 KB I> I, R16 9, .. LPI P.O. No. 2 6V R17 Wirewsund L P2 ,, ,, R18 Painton CV25S R I-R5 Wirewound R19 ,, CV15S R6 Erie 9 R20 .. CV2P R7 ,, .9 R8 I, I, SWA Painton, Winkler, AS 2P 14 48 R9 Erie 108 SWB B.B.C. EPA7606 RIO Painton P302 SWC Bulgin, microswitch, S500 ISSUE 2 (MAY 1966) I I I ZK I Ym 3 5K 1'(LINE) 1NS.W I 1 I (a 1 LINE TERMINATIONS BATTERIES I (b) GALVANOMETER I (c) INSULATION RESISTANCE CIRCUIT LAMPS & LINE DISCHARGE CIRCUIT I CIRCUIT I ,R38 ADJUSTED TO MAKE R3WRJB1R4 1 ~l(i1.q(A WINE) FIG I,~(BWIQE) FIG 2, FIG 2,h (SALVO -) 9 (GALVO+) t FIG I, f1 (EARTH) FIG IL(Q 5~t) FIG I,F (4.5~) A/E 4 4 2- WE lffi 3- n/e3 4- DISCHARGE. 8- 410 I LOOP RESISTANCE & VARLEY RPTARY SWITCH 14-13- WE Cl RCUlT SWA SPEC.ED/RLT/I ROUTINE LlNE TESTER (POR~BLE)RLT/ IP : CIRCUIT INSTRUCTION S.4 COMPONENT TABLE: FIG. 28 ! Cornp. LOC. 1 TYpe Tolerance TYpe Tolerance per cent per cent Wingrove and Rogers Erie 9 ,i 10 C 60-541 1 Morganite LH, linear, Wingrove and Rogers I in. Sp. slotted, with C31-I 111 81.0075 in. washer and locating lug -+A 20 Wingrove and Rogers Morganite spindle re- C31-l l/l 8/.0075 in. duced to 13/16 in. T.C.C. CSM2ON G8 Erie 9 - 1 0 T.C.C. CSM2ON F4 Painton P406 + 1 T.C.C. CP47S/PVC F4 Painton P301 $5 T.C.C. SCE70B/PVC F5 Erie 16 - 10 F6 Erie 16 $ I0 CIO E5 1 Plessey CL809/I F8 Erie 9 + 10 Q5 Painton 73 & I Q5 Painton 73 Plessey CE8 1811 Q3 Painton 73 Hunt B810 Q3 Painton 73 T.C.C. CSM2ON Q4 Painton 73 ' *I T.C.C. CP37N/PVC R5 Painton 73 i *I T.C.C. CE25AAR 05 Painton 73 Hunt B501/P R4 Painton 73 Hunt 6815 Q4 Painton 73 t I R5 Painton P406 Plessey CEQl 111 H 6 Erie 16 T.C.C. CSM2ON (Fitted H4 Painton 73 on test) H4 Erie 9 -L 10 H7 Erie 9 BBC S/17H J8 Erie 9 BBC S/17H J6 Erie 9 1 + 10 L4 Painton P406 ! .--. 1 MRI N2 I Westinghouse I rnA L4 Painton P406 ; I K5 Erie 16 I , 10 R I Painton 74 K5 Erie 16 .- 10 R2 Pai nton 74 K7 Erie 109 j .. 2 R3 Painton 75 L7 Erie 109 '8 4- 2 R4 Painton 75 K8 Painton 73 -C I R5a Painton 76 L8 Painton 73 R5b Painton 76 L6 Erie 9 R6a Painton 76 N3 Painton 73 R6b Painton 76 P3 Painton 73 , I I ' R7 Erie 109 (Fitted on test) P3 Erie 109 (Fitted on test) R8 Erie 109 (Fitted on test) P3 Painton 73 R9 Erie 109 (Fitted on test) P5 Painton 73 RIO Erie 109 (Fitted on test) LIO Erie 8 RI I Erie 109 (Fitted on test) L I0 Erie 8 L R12 Erie 109 (Fitted on test) MI Painton 73 - I R 13 Erie 9 M4 Erie 9 R14 Erie 16 MI0 Painton MVI 1': 5 R 15 Erie 9 I R16 Painton 73 R17 Erie 9 BBC AAL/ZO-2P Ass.E, I R18 Erie 9 Class Z I R19 Erie 109 KIO BBC M187, murnetal R20 Erie 9 can (modified) 58/~~/004 EC9010 ISSUE 2 ISSUE 4 (MAY 66) VALVE BASES N3TES:- 1 * R7 - R12 S R57 & Ct9 FITTED 3HTEST 2 STRAP 5H3ht4 DOTTED TO BE A03EO WHEN UNIT 15 MA3E Button Niniature Nova1 PORTAXE. 7-pin Glass B9A B 76 TONE SOURCE TShO & TSllOP : CIRCUIT I ( rhis drawing is the property of the British Broadcasting Corporation and may not be reproduced or disclosed to a third party in any form without the written permission of thn Corporation. NOTE: RI I RZ WIRED AND ADJUSTED ON TEST LPR b TRI TO BE SELECTED ON TEST. STANDARD LEVEL PANEL sLP/3 : CIRCUIT 601WG1120/AF s 4 EC 10585 FIG. 30 SECTION 10 ISSUE 2 ISSUE 1 (MAY 1966) TAG BLOCK B TAG BLOCK A TRkq ( TEST 2 u TEST 1 KEY KA (ATIIO) , 'z TIP 1 '"I RLB I 6 ~(RRCII) 2.22 1515 1514 8 X T L R STRAPPED -1 171 1 as. s4 + 1414 Z A.C.TEST BAY ~~155:BLOCK SCHEMATIC INSTRUCTION S.4 COMPONENT TABLE: FIG. 31 Cornp. Tolerance Cornp. Tolerance per cent per cent B I S.T.C. Brimistor C23 R24 Erie 109 R25 Erie 109 CI T.C.C. CSM2ON R26 Erie 109 C2 T.C.C. CSM2ON R27 Erie 109 C3 T.C.C. SCE70B/PVC R28 Erie 109 C4 T.C.C. CP32N/PVC R29 Painton P406 C5 Hunt BE15 R30 Erie 109 C6 Plessey CE808/ I R3 1 Erie 109 C7 T.C.C. CP37N/PVC R32 Erie 109 C8 T.C.C. SCE70B/PVC R33 Erie 109 C9 T.C.C. CP37N/PVC R34 Erie 108 CIO T.C.C. CP37N/PVC R35 Erie 109 CI I Plessey CE91 1 /I R36 Erie 109 R37 Erie 109 LI R38 Erie 109 L2 R39 R40 Erie 109 RI Erie 108 R4 1 Erie 9 R2 Erie 108 R42 Erie 9 R3 Erie 16 R43 Erie 16 R4 Erie 109 R44 Erie 109 R5 Erie 9 R45 Erie 109 R6 Erie 9 R46 Painton P406 R7 Erie 9 R8 Erie 109 SWA N.S.F. 8373/B145 R9 SWB N.S.F. DM (S) RIO Erie 109 SWC N.S.F. TL2 (S) RI I Erie 9 SWD N.S.F. TL2 (S) ,RI2 Erie 9 SWE N.S.F. TL2 (S) R 13 Erie 9 SWF Painton 31021 1 R14 Erie 109 SWG N.S.F. TG3 (S) R15 Erie 109 R16 Erie 9 TH I R17 Erie 109 TH2 R18 Erie 109 R19 Erie 109 TR I R2O Erie 109 TR2 R2 1 Erie 109 TR3 R22 Erie 109 TR4 R23 Erie 109 MAINS VALVE BASES NOTE :- STRAP SHOWN DOTTED TO BE ADDED WHEN LNIT IS MADE PORTABLE. Button Miniature Novol 7-pin Glass B9A 076 , INTERMODULATION TEST OSCILLATOR IT011 : CIRCUIT DIAGRAM ( 1 This draw~ngis the property of the British Broad- casting Corporation and may not be reproduced or disclosed to a third party in any form without the written permission of the Corporauon. VALVE BASE Button Miniature -- AMPLIFIER TEST PANEL ATPI I: GiRCUIT DIAGRAM TRI C410211 LINEcFj 1I 2(SEE NOTE I) RELAY 0ATT SEE NOTE I ) *b EARTH - 13 WHITE - MII LPI NOTES: I CIRCUIT SHOWN FOR 50V RELAY SATTERY 2 IF 14V RELAY BATTERY ISUSED RIISNOT REOUIRED R3 IS TO BE 250A AND POLARIW OF MRl TO BE REVERSED TELEPHONE PANEL TPIIZ: CIRCUIT DIAGRAM s 4 FIG. 34 TRI TR I LLlb3RB LLlbJRB r------I r-----7 REPEAT1 NG COlL--3 I COlL l REPEATING COlL 1 LlNE LlNE APP l7 1 12 9 4' L-- - TEST I TEST I OUT 1'" t C/O OUT t TEST 1 TEST2 IN OUT -RLB n 4 DESIGNATIONS REFER TO JACKS ON AC155 BAY JACKFIELD CIb AND Cls IF NECESSARY TO BE CONNECTED AS REQUIRED RO. 3001 * KEY +TO BE INSERTED ON TEST SWITCH RELAY BATT RELAY & REPEATING COlL PANEL RRC/ I :CIRCUIT DIAGRAM s4 FIG 35 SECTION 21 ISSUE P (MAY 1966) VALVE BASES OSCILLATOR OS2/4 : CIRCUIT