78 .IEIEUISION' IHE TO1OURRETEIUER IW ffil Tne purpo'se of this feature, started last month. is lished in this feature will be paid for a our normal to oo-ordinate hints and tips from readers who have rates. Contributors' addresseswill not be publis,hed. successfully completed the Terevrsror.r Colour Pleasedo not send in parts of receiversunless Receiver and to pass them on for the benefit of requestedto do so. We regret that it is not possible othErs. We have already been in touch with a to answertelephone enquiries regarding this-feature number of suocessful constructors as a result of or the set itself. which it has been possible to present readers with a The Fault Finding Advisory Service(form in the considerable amount of useful material. We invite August, September and October issues) will be others to contribute notes on their findings, com- conlinueduntil furthernotice. ments or suggestionsthat may be useful to others. Two constructorsoutline below their own Dower Clear sketches and monochrome photographs of sets supply circuits: both seem to be verv satisfictorv or parts of sets are welcome. All contributions pub- and have their own obviousadvantaees,

DearSir, cable fqr the link designated V, from D16 cathode to C43: earth the ends a,t the earthv end of R63 and POWERSUPPLIES to L6 can earth respectively. In order to get the set working without further loss Coaxial cable earthed at both ends should also be of time I decided to forgo the advantagesof using used for the 80V pulse feed to I D on the decoder a mains isolating transfo,rmer and having thA from tag 9 on the line output transformer. Other- chassisearthed, taking the chassisto the neutrtl side wise unstable sync can result due to radiation. of the mains supply instead. This is safe so long as you ensure tha,t (1) all exposed metalwork goes to BEAM LIMITING mains earth, i.e. disconnect the braiding connections I found that the beam limiting to the front panel, and (2) the c.r.t. protection band circuit tends to spoil the effectivenessof the black-level is connected to chassisvia an RC leakage path, not clamping sysiem by endeavouring to maintain directly (use a 3.3MO lW resistor and 0.0018r^F4kV a constant average beam current. Accordingly I tubular ceramic capacitor in parallel). The circuit I have modified the circuit as shown in Fig. 2 (left-hand used is shown in Fig. l. The resistor connected side). Tr702 is now normally off. When excessive from one side of the c.r.t. heaters to the red gun beam current flows the voltage at 4Hl9Q cathode is to ensure that the safe c.r.t. heater- rises until 0.6V is developed at Tr702 base. It then turns cathodevoltage is not exceeded;it also removes hum on, holding -fhe the c.r.t. grids at 23V or so thus limiting injection. slightly increased complexity of the the beam current to a safe 20V supply is well worthwhile. Note that the audio value. board supply (6L) will rise above 30V if the 20V load is reduced. so 6L should be connected last-if FLYBACKBLANKING you are a purist ! Incidentally I feel that the line The arrangement I used for field flyback blanking output valve screen feed would resistor be better is shown on the right-hand side of Fig. 2. The fly- rated at lOW: this is not essential but is advisable back pulse appearing at D305 cathode is inverted by in the interestsof reliabilitv and stabilitv. the BCl07 and used to cut off the c.r.t. erids. I also found that improved line flyback blinking was WIRING obtained using the circuitry shown. (l) I recommend that the signal connections to the RGB board, from 2E to 3C. lE to 3E and lF to 3D, DECODER be made with v.h.f. coaxial cable. This avoids loss The referenceoscillator refused to lock with L2lCl3 of h.f. signal components and spurious pick-up. in circuit. No trouble was experiencedafter remov- Earthing at both ends has not causedany hum loops. ing them. I found that the preceding d.c. amplifier If any setting up is done without the decoder fitted, Tr3 likes to oscillateas well: a 470pF capacitor from i.e. with 3D and 3E disconnected,these two points collector to chassisstops this. should be taken to earth via 1OpF electrolvtics. Low ident output was cured by changing L4 (2) On the decoder board t have found tiat to pre- core, adding an 0'068pF capacitor across the coil vent subcarrier pickup it is best to use v.h.f. coaxial and changing T16 to a high-gain BCl49 transistor. *{f

't! Slow blow RS heai€. transforrca CRT heat.rs

Degausing ls0k {w t Red gun athoda

R508 0.17n 5JL .5w

Oouglai mutti-tap 60V tA li.e. 60VA) transtormer

l2V suppty: connect RSll/RSt2 to junction R509/R510

H+Erp6ed matalwdk, NOI chasis r75tl Fig' 1: Power supply circuitused by L' cook' with stabilised20v tine.36v a.c.is obtainedacrcssthe24vand60vtaps on the secondary winding of the Douglas _iii-ip tnnsfomet. in getring ...^Difficul11 rhe.bistablero trigger reliably by changing-C{O not _recommendits removal. The trouble y.1:..y,:d to 0.O0o8r,F'inO'R5Jto appears to dr\11 also an d,u.eto.spreads in rhe valu!'Jr'irr" extra 0..00697rFcapacitor may !e .iii"i,kewise needed across _ be rriggeringgit.rj,v;;;'. C39. It is f,;;;;;,'iirat r_s :l:^Pi.l^,bt:, l;'ti duero may be faulty, .possible spreadsrn the value, which is quite so these last,amendments-ui. only criticai, oi LS. tentatively suggesred rhough r-h;;'i;;; i|._ ,o worl(. Dear Sir, L. Cook Edit,orial BOARD LAYOUT ^cotntnent; We have heard from readers other As of trouble with the n"t*"it r_)lti:'uu, mentionedIast month irnproved performance oo c.a.n Pe obtainedby moun-tingthl three,'ignui'oourO, side-by-sideon a iheer hi.Jb;;;l-imo"k.''.ut_our. for access "f to the -trreprinteO_siOe of li-,-ei"u.a.i'ut tt. left-handside of cibtnet.-id;-";;;;ilent is shown in Fig. 3. Fig. 5 showsif,. .."ji'i"*ru il which the RGB Lrhe t(yback drivl co_ntrols"are-m"ouii.iiro,t,u,"" pulses the from signals,with rhe RCB board ;;;;d iowaros LOPT tag 9 the-ce.ntre of the chassisto tt. f.fi'"iii,.-...]r., .un be-fed via shorr connections io- itre-cit.' t'aJe',circuitpanet). . In .connecrionwirh the modified-p"rt bi;;ii;; From described "t". last mon,th lt st ouf.l ."0" clear tj.at the line flyba,ckUt".tl"d p"li!."o. ui! as in Fig.4 tut.n, on pagi 403 of the J;li;;;;;,-irom crrt via C70.1ana R70Dto thejuncrion-'niDlnios.

ADJUSTM ENT IM PROVEMENTS I found .have. that horizontal centring can be Fig. 2: Beam limiting achievedmuch more sati sf and flyback blanking citcujt usedby potentiomerer lc^toriiy^ii'"'..f; tr"_iupp.o L. is usedfor R354.t;;;.;t^il; Cook. tap to i"n,r" PL30l: the flying lead ro SKij0ill'-i. tt.n 80

Fig. 3 (left): Ailangement of the three signal boards side- by-side adopted by E. Erven. This arrangement can be used only with 22in. or larger c.r.t.s. lf a single regulated 20V line is used, add the link shown in broken line between lA and 24 and use only one 20V connection to the power supply. Note that this diagram shows the interconnections between the thrce boards only.

not necessary. The potentiometer should be a wire- wound type rated at 2W (available from Forgestono Components, complete with mounting details). On the decoder board it was found that the B-Y quadrature adjustment was more easily made if the value of C37 is reduced. I have used a 6.8pF capaci- tor in this positi,on.

REGULATED POWER SUPPLY CIRCUIT I have now built up on a printed panel (Figs.6/7) and fully testedthe p,owersupply circuit shown in outline form in Fig. 6 last month. The final circuit is shown in Fig. 4. The componentsused are those supplied in Pack 18 plus a number of extra ones. The adjustmentsnecessary (R5 and Rl2) are easyto

ct5a I6 .007 to0l T"'I

Rr6 3.3|

C] r8 'ooJ, To FSI 1A c24 ---L T'

R? t0 r0 2N3053

ac8 'F 1650 \t033 tu I'6s0

c12'oT To' r2k< I 2N3053 R20 It1 tk 8C183LA

TO' T' Fig.4: Power supply circuit designedfor the receiverby E. Erven,with stabitised40V and 20V outputs.R14 can be made up from the 30C)and | 5{2 resistors (R504 and R509) supplied in the original component pack. 81

The stabilisercircuits have been devisedso thail.if the outputsare inadvertentlyshort-oircuited the fuses will blow withou the transistorsgoing. The ratings of the various mains (ransformer secondarywindings are as follows: 0-250V 1A, b q O-42V 14, 0-30V 500mA, 6.3V lA. Measurementson the circuit give the following +'l performancefigures. The output from the 40V regu- iator is 40V with no load falling to 39.9Vat an out- msrh r"..}5 put of 1A into a resistiveload. 2.5V p-p ripple at =l-l\_-/ \r_-/ \__/ the input is reducedto 50mV p-p at the output. The regula or draws 12mA, the short-circuit current is fl 5'6.4 and the 'output can be adjustedfrom 36-44V by meansof R5. With no load the output from the 20V regulatoris 20V, falling to approximately19.9V Fig. 5: The small board used by E. Erven to mount the wi'th an output of 500mA into a resistive load. RGB drive controls: drill t in. holes in the positions marked 400mV p-p ripple at the input is reducedto 10mV X to securc the board to two of the RGB output trcnsistol p-p at the output. The regulator draws 12mA, the heatsinks-ensurc that the boad is insulated frcm the short-circuitcurren is 2.2A and the output can be heatsinks.Shown half scale (viewed frcm the coppet sttip adjustedfrom 18'5-22Vby meansof Rl2. The h.t. side). suppliesremain stable to within + lO/" of the speci- fied figuresat normal averagebrightness levels: at carry out: the potentiometers are simply adjusted excessivebrightness levels the voltagefalls to -10% to give the stated outputs. or worse.The regulatedsupplies will hold their out-

6N/P

-

D

Tt-s6-l

Fig.6 (left): Layout of E. Erven'spower supply, viewed from the component side. C1, C7/8, Cl4 and Cl5 are mounted off the board. Fig.7 (right): Pilnted panel pattetn (copper side),shown half scale. 82

puts within 0'5'/. for a varia"tionof the nominal mainsvoltage of +15%. E. Erven Editorial comment:The power supply circuit devised by E. Erven is illustratedin the accompanyingpho- tograph. We have found it to provide wholly satisfactoryresults. We understandthat the compo- nents required can be obtained from Forgestone Components.

CROSSHATCH GENERATOR It was originally intendedto publish details of a crosshatch generator devised speciflcally for the colour receiver.After cos.tingthis out however we have found that the advantagelies with using the design by C. R. Bradley B.Sc. published in the September1972 issue (during the series on the colour receiver).This has been tried out with the colour receiverand operatesperfectly satisfactorily. Linepulses can be pickedup by wrappingthe genera- tor's insulatedinput lead around C327 (timebase board): feed the output signalfrom the genera- tor to the baseof Tr109 on the i.f. panel.Note the crosshatch generator m,odification given in the January 1973 issue (page 129)-adding a 200pF E. Erven'spower supply unit. Fout large can electrolytics are capacitorfrom Trl baseto chassis. sepantely mounted using suitable clips. The generator(assuming purchase of a Bi-Pre-

components list (Fig. a) Resistors R7 10 O 10W R14 47 A' 15W R21 6.8kO R1 6.8kO R8 5.6kO R15 50 c) 10W R22 12kA R2 8.2kO R9 12kO R16 3.3kO 1W R23 10c) R3 2000 2W R10 4700. R17 1.8kO 15W R24 2204 0.3A R4 27kO R11 8.2kO R18 1.5k O R25 560c) RS 2.2kO |W pot R12 2.2k4 |W pot R19 47OA R6 8.2kO R13 10kO R20 1krl .See All {W unlessotherwise indicated. Fig.4 caption.

Capacitors C1 3300;rF 63V C7 1650pF\ ^.,, C13 0.1pF 50V C19 50pF 350V c2 250pF 50V C8 1650pFJ """ C14 400pF 350V C20 0.1pF 400V C3 0.01pF 50V C9 0.01pF 50V C15 400uF 350V C21 100pF 35V C4 0.01pF 50V C10 0.01pF 50V C16 0.1pF 400V C22 100pF 15V C5 0.1pF 50V C11 0.01pF 50V C17 100pF 215V C23 22OOttF 35V CO 1pF 50V C12 10pF 25V C1B 0.1pF 400V C24 0.1pF 900V Semiconductors D1 2A bridge D9 12V 1W zener(1 N3022) T16 BC107 D2 l0VlWzener(1N3020) D10 30VlWzener(1N3031) Tr7 BC183LA D3 8Y133/1N4007 D11 8Y127 TrB 2N3053 D4 BY133/1 N4007 Tr1 2N3053 VA1 VA1033 D5 10V1W zener(1 N3020) Tr2 8C107 VA2 VA1104 D6 14 bridge Tr3 2N3055 VA3 VA1026 D7 BY133/1 N4007 Tr4 2N3055 D8 BY133/1 N4007 Tr5 2N3053 D3, D4, D7 and D8 mustbe the sametype.

Miscellaneous FS1 14 T1 Mainstransf ormer (see text) FS2 500mA Heatsinkassembly type FS3 100 mA A1057D/T03;transistor mounting FS4 34 kits;printed board; capacitor All anti-surge. mountingclips. 83

Pack kit) can be mounted in the receiver as follows. TIMEBASEFUSE Discard the original box. Mount the rotary switch in Note that the 500mAfuse FS301 on the normal position but for the slide switch break off the timebase board must be an anti-surgetype (otherwiseyou the mounting side plates and bend back the three may get repeated fuse blowing). contacts opposite the slide switch lever itself. Mount these three contacts through the holes in the board closest to the edge and wire up the other three con- CRT FIRSTANODE SWITCHES tacts with connecting wire. Discard the original Several readershave coiiplained about a crack from VR2/53 and use instead an RS 4.7k1) lgrnm pelsn- these switches when they are operated. One of the tiometer with s.p.s.t.switch. Glue this on the corner modifications given in E. Erven's letter last month, of the board where the original component was to decoupling the c.r.t. first anodes with 0'01pF capaci- be mounted, using Araldite: the switch contacts tors, has beon found to overcome this. should hang over the edge of the board. Wire the potentiometer and switch contacts to the printed cir- cuit tracks with connocting wire. Mount all the POWERSUPPLY other components in the usual way. The generator In the event of it being necessary to replace the can then be positioned in the convergence drawer zener diodes D506, D507 or D508 following the behind board three. We expect to be able to pro- power supply modification suggestions made last vide details of the convergence drawer top panel month we recommend the use of 7 or l0W types. shortly. Partial loading of the low-voltage d.c. outputs is not recommended otherwise these will rise exces- DEGAUSSING sively giving false operating conditions. C.R.T. demagnetising was not mentioned in the setting-up instructions. Several constructors with MISCELLANEOUS whom we have talked have reported that they found The component reference number R605 was un- it unnecessary. It is possible however that some fortunately used twice in the project. There should c.r.t.s may have become excessively magnetised be no confusion however since one "R605" is the before use, making purity difficult to obtain. Details 470!) lW carbon resistor connected in series with for the construction of a suitable degaussing/demag- the black lead from the e.h.t. multiplier unit while netising coil were given in the June 1972 issue, the other one is the VA8650 positive temperature during the series on trhe colour receiver, on pages coefficientthermistor in the degaussingcircuit. 368/9, together with instructions on its use. A number of readers have asked why a ferrite loudspeaker was specified. The author states that SETTING-UPPROCEDURE this type of loudspeakerhas a low radial field com- First a correction: the instruction to restore the pared to its fairly large axial field: the low radial field power supply connection to 5P should have been minimises its effect on the picture purityiconver- given under the heading "Timebase Checks" along ' with the instruction to restore the connection to 48, It is possiblehowever that some slight picture dis- not just prior to making the picture geometry adjust- colouration may still be experienced if the loud- ments. speakeris close enough to the tube: its siting is an Secondly mention of the coil (L501) on the tuner/ important factor therefore. i.f. preamplifier board was omitted. The tuning is not very critical: adjust for maximum output, after tuning in the varactor control panel on one push- IIEWFBOM MULLABD button. This is done during the "initial stages". We shall be publishing next month a detailed pro- An improved version of the Mullard luminance and cedure for aligning the decoder using just a multi- signal processing i.c. is being intro- meter and a simple diode probe. duced: the new version is designated TBA560C. It offers the following advantages over the earlier MODULEINTERCONNECTIONS version: full short-circuit protection: increasedauto- matic chrominance control (a.c.c.) range; reduced The fact that there are two earth pads marked 8D crosstalk between the chrominance and luminance on the tuner board was overlooked in the module channels: improved stability with vari- interconnectionlist given in the July issue.As men- ations in picture content and contrast control setting. tioned in the alignment instructions (May) the i.f. When a C version is used to replace the original tt preamplifier output (8K) be to the should connected T8A560 certain minor component changes in the point (2B) the board via coaxial cable input on i.f. peripheral circuitry are necessary. (no than with longer l0in.) the cable screen earthed For applications-includingline timebasecircuitry end at both ends, i.e. at 8D adjacent to 8K at one -where a fast diode "snap off" characteristic can is inrportant. and 2J adjacent to 2B at the other. This introduceunwanted interference Mullard have intro- the 8D pad to the main 2J to 6C Connect other duced four new "soft recovery" diodes, types BY206, point (not 6C). on the i.f. board direct to 8Y207, BY2l0-400 and BY2l0-600. The soft recovery characteristic is obtained by using a p diffusion BOARD POSITIONS into an n substrate.This results in a more gradual A number of readers have pointed out that the line reduction of the stored charge when the device cuts output transformer and e.h.t. tripler cannot be off. The BY206 is a direct and improved replace- mounted between the boards on the right-hand side ment for the BAl45 and 8A148 which are found of the cabinet as suggestedin the June issue: they quite widely in line timcbasecircuits and are now should be mounted behind the RCB board. being phasedout by Mullard. 113 IHE,IEI,EUISIOII'tOlOUR RETEIUER

IN answer to many requests we are this month notes on their findings, also any comments and sug- providing a set of instructions for setting up the gestions. These will be carefully considered and decoder in the absence of elaborate test gear. Only points likely to be of help will continue to appear r a multimeter and simple diode probe (details given) in this feature. All contributions published will be are required. Details are also provided paid * of ways in for at our normal rates. Contributors' addresses t which you may be able to obtain improved per- will not be published. [' formance from the line oscillator and the i.f. strio. ?: We regret that it is not possible for us to answer Constructors of the receiver are urged to consider t telephone enquiries regarding this feature or the carefully the points raised this month and in the two set itself. The Fault Finding Advisory Service will con- previous Forum articles: they contain advice on all tinue-apply on the forms provided in August, problems reported by readers to date. the September or October issues. The purpose of this feature is to co-ordinate hints ti and comments from readers who have successfully Those wishing to build the power supply circuit completed the project and to pass them on for the devisedby E. Erven (see last month) should note the I benefit of others. We invite constructors to send us dimensionswhich are approximately5x-5X l0in.

I t F DECODERALIGNMENT WITH A MULTIMETER t Testing, fault finding and alignment of the various use of an oscilloscopemay however be essential in I I modr-rlesin the colour receiver can be :rchieved the rare caseof a very obscure fault. A great deal of t reaclilyenough if a large selection of test equipment trouble can be saved by simple ohmmeter tests of is at the constnlctor's clisposal.Many constructors the components, especially semiconductors, before t are not in this fortunate position however. Since the fitting them to the board. t i.f. strip can be put in the hands of the Alignment The other main proviso in providing the following f. Servicewe are left with the ptoblem of aligning the instructions is that the rest of the receiver is fully I restof the sct trsingonly basicequipment. The time- working and tested.If setting up is to be done in this I, basc ancl display section can be set r"rpquite easily way it is rather pointlesseven connecting the decoder provicleclyou have a crosshatch generator and an board into the set until a fully satisfactory mono- t picture grey t. e.h.t. mcter. Both these items can be made very chrome has been obtained, with good l cheaply and articles have appeared previously on scale, purity, geometry, convergence and resolution. I their construction(see for example June 1972,pages A strong -freeaerial signal is also a must. 367-8.for an e.h.t. meter and Seotember 1972 for First double-check that you have all the external F the crosshatchgenerator). connections to the decoder board correct. Ensure We are thus left with the problem of decoder the receiver is tuned correctly so that the sub- 'fhis that alignment. can be achieved using only a multi- carrier is being received. This can be checked by meter and the transmittedtest card: it is not claimed temporarily disconnectingCl37 on the i.f. board: a that the precision of adjustment is as good as can be crawling line pattern should then appear on the areas +{ obtained by more sophisticated methods, but the of the picture which are coloured. a displayed picture will neverthelessbe of very good You will need to carry out the alignment at a time quality becausealignment is carried out by carefully when the test card is being transmitted on BBC-2. observing the display. Fortunately the trade test films have now been discontinued so that the test card is available for muclr longer periods on weekdays. lnitial Conditions It is assumeclto start with that the decoder is in working order, i.e. that there are no faulty com- Presetting ponents or rviring errors. The vast maiority of home- One of the main difficulties with this method of built clecoderfaLrlts will be due to incorrect Dosition- alignment is that the adiustmentsmay initially be ing of components,errors in module interconnection, so far out that no output at all is obtained. To wrongly phased transformer windings and so on. minimise this possibility the various adjustments Such faults can be traced by careful examinationof shoulcl be set initially as follows: the board itself and of the displayed picture. The I.F. Board'. Rl49 a.c.c. bias midway; chroma notch 114 filter Ll14 core fully out of former; chroma output probe from connection 2 of T2 to chassis and adjust coil Ll2l core level with top of former (do not alter L3 for maximum reading. Now recheck step (9). L121 however if the i.f. strip has been aligned by the Alignment Service). Phasing the Signals Decoder Boord: Ll, L3, L4, L6, Ll0, Ll3 core level with top of former; L5, L7, Ll4, Lls core fully out Stage two is to line up the chroma amplifier and of former; potentiometers Rll, R75 and trimmers bring the chroma and reference signals fed to each C14, C38 midway. demodulator into phase with each other. We also set Front Panel: Saturation control midway. up the delay line circuit to give optimum cancellation of phase errors.

(ll) Setting up the Reference Oscillator Adjust L6 for maximum saturation, ignoring any Hanover bars or patterning. The first stage is to make the reference oscillator lock correctly to the incoming burst. Remember that (12) A coarsepattern which wriggles with the sound the line output stage must be operating and rvill be visible over the whole picture. This pattern, * provide synchronised in order to the correct burst predominantly blue in colour, is the 6MHz signal gating pulse. Proceed as follows: after passing through the chroma demodulators. Advance the a.c.c. bias or saturation controls if (1) Check with a multimeter that the *20V and necessary to make the pattern easily visible. Screw -8V present. supplies are the core into L7 until the pattern is notched out. t (2) Connect the negative lead of the multimeter to (13) This is a convenient point at which to establish the a.c.c. output terminal lC, positive lead to chassis. the correct operation of the ident circuit, and here we can make use of the fact that the ident output is (3) Screw the core into L5 until the first peak in the also used to operate the colour killer. a.c.c. voltage is reached. Remove the short-circuit across D8. This will probably cause the colour to drop out, but adjust- (4) peak Further up the reading by adjusting Ll and ment of L4 will bring it back again. Reducing the the i.f. strip chroma output (unless coil Ll2l the i.f. input signal by means of the a.c.c. bias preset on has through the Alignment strip been Service). the i.f. board will enable a more accurate settine of Recheck the setting of L5. The reading should now -10V. L4 to be made. be in the region of The ident should now be correct, that is reds should be red and greens green rather than vice (5) Disable the colour kilter by connecting a shorting versa. If not there are two possibilities.Try interrupt- link across D8. This should result in some sort of ing the aerial signal several times. lf the ident can (no colour doubt completely incoherent) on the eventually be made correct by this method then the probably screen. If not, then the adjustment of L3 ident signal itself is adequate.Turning L4 a couple of is so far out that no reference signal is being turns one way or the other should put matters right. give obtzrined. Roughly set L3 to some sort of out- Alternatively if the ident is wrong euer_ytime reverse put. the connectionsto orrc winding of T4. (6) (6'8k0) Connect a shorting link across Rl9 to (14) We must now bring the reference signal and disable the reference oscillator. chroma signal applied to each demodulator into phase with each other. As is well known, the purpose (7) Connect the multimeler from Tr3 collector to of the chrominance delay line is to remove the (a.p.c. chassis.Adjust Rll bias) to give a reading of effects of phase errors. For the present however we +6V. wish to show up the errors, so we take the delay line out of use by connecting a shorting link across (8) Remove the link across Rl9. Adjustment of C14 R77 (100!D. should now enable a lock to be obtained so that the There are several adjustments which affect both colour is not divided into broad horizontal bands but the I{-Y and the B-Y phasing. In the B-Y is steady although it will still have considerable channel however there is another variable, the 1 patterning on it. quadrature trimmer C38. For the moment therefore n we concentrate on the red picture by switching off (9) Having obtained a lock, finally set Cl4 to again the green and blue guns. Observe that on areas of F give a *6V meter reading. If no lock can be the picture where a large R-Y signal is present- (10) obtained go on to step and then try again. such as the cyan, green, magenta and red colour bars (the middle four on the screen), the red dress (,|0) We now need to adjust L3 for maximum output on the test card, etc.-there is a coarse flanover from the oscillator. For this we need a diode probe- blind pattern which consists of two dark picture a suitable circuit is shown in Fie. l. Connect the lines followed by two brighter lines, repeated over the whole area in question. This pattern appears to ToT2 pin 2 drift up the screen.The pattern is due to the refer-

Vott m et ?r. ence signal fed to the R-Y demodulator being out rilga as of phase with the R-Y component of the chroma rcqurrcq signal. To put this right we shift the phase of the lo chatsis l-?-Zl reference signal, first by adjusting Ll3. This has only a small range and if the bars cannot be nulled Frg. l: Diode probe for setting L3. out a small adjustment of Ll should do the trick. 115

There are nearly always some slight Hanover bars of iust the colour-differencesignals will be seen. The with simple PAL operation. which is what we are in cross-colourwill be observedas a pattern correspond- effect using here, especially on transitions between ing to where the vertical ljnes would be on the one colour and another. It is often found that the luminance image. adjustment giving a null on some parts of the picture is not quite right for others. Do not worry about this (19) Some cross-colour is inevitable of course but -the delay line *'ill take care of the errors. Just adjustmentof the i.f. strip chroma output coil Ll2l aim for least bars over the whole oicture. This should enable a definite null in the cross-colour to incidentally demonstrates why hue errors are so be found. If this coil does not give enough range common on the NTSC system and why simple PAL without excessiveloss of gain L6 may be adjusted, operation is seldom used. but this should be done only as a last resort since The actual phase error present depends a lot on any tampering with L6 means that the phase adiust- component tolerances and wiring capacitance. There ment of Ll must be done aeain. may be the odd case where adjustment of Ll does not provide enough range to cancel the errors with- out causing undue loss of ga'in in the burst channel, Correct Saturatio n Setting the latter effect showing itself as a reduction of the a.c.c. output and hence an increase in saturation, or (20) Finally, restore the luminance signal and set the even difficulty u'ith the colour lock. contrast control for a normal picture. Switch off the ln these casesL6 must be used to shift the phase green and blue guns and reduce the brightness so c of the chroma signal. This is preferable to too much that picture highlights are iust visible. Set the detuningof Ll. user saturation control midwa1,. Set the a.c.c. bias preset Rt49 (i.f. panel) so that the two pairs of red (15) When you are quite satisfiedthat the red signal bars seen at the top of the test card are of equal is correct, switch on the blue gun only and set the brightness. This establishesthe correct saturation. quadrature trimmer C38 to minimise the Hanover bars. Grey Scale Delay Line Circuit Norv on switching on all three guns and resetting (16) Unfortunately we nou need to make the phase the brightness control y'ou should have a high- adjustment wrong againl This is becausewe need to quality colour picture. If 1'ou don't have a reference set up the delay line for optimum cancellation of white source the RCB drive controls R401-R403may phase errors and for this we need some errorsl be set up carefully for a good skin tone on the test (having Switch on all three guns and detune Ll so that card first checked the c.r.t. first anode "back- fairly bad Hanover bars are visible. Turn up the ground".adjustments R435-R.137as these have a pro- saturation if necessaryto make them clearer. Now found cflect on the colour fidelity). The background remove the short-circuit across It77 (which will controls are used to remove any colouring in dark causean increasein saturationof ahout 2: l). Care- gre-vareas. They can be set up on a test card witb the fully set R7-5 and [-10 until brightne.sscontrol se,tfor a n,ormal picture. Turn the -[hen all Hanover bars are eliminated. briefly restore the short-circuit controls to maximum. note which are the dominant across R77 to enable I-l to be reset to its correct colours. then reduce the settings as appropriate to position. balancethe beamsfor correct dark grey reproduction. So there it is-setting up the colour circuits can (17) Turn off the colour and set the i.f. strio sub- be clone without an oscilloscope, wobbulator etc. carrier notch filter Lll4 for mininrum line oattern it takes longer this rvay, but the resultsare in no way on colouredareas (e.g. the colour bars,).Turn up the inferior. colour again and set Ll4 (red picture) and Ll5 (blue picture) for minimum patterning. SIGNALBOARDS Overall Chrominance Response The impression given by the way in which we drew Fig. 3 last month could be misleading. The boards So far we have set the 4'43lvlHz tuned circuits in are rnounted side-by-sidewith the tuner board at the the chroma amplifier for maximum output at that t'ront of the cabinet. the i.f. board in the centre and frequenc:y. The overall bandpass response however the decoder board at the raur of the cabinet. is the combined response of the i.f. amplifier and the chroma channel. It is often forsotten that this overall responsemust bc slmmetri&l and centred Dear Sir, exactly on 4'43MHz'this for optimum cross-colour performance. is just one of the reasonswhy i.f. alignment is so critical. MODIFIED LINE OSCILLATOR Fortunaiely there is an adjustmentwhich tilts the Students of the sinewaveline oscillator u'ill know responsewithout any side effects (except for a small that there is a number of ways of arranging this change in overall bandwidth). This is the chroma circuit. In the design published for the TelevrsroN output coil on the i.f. strip, Ll2l. We do not recom- Colour I{eceiver the quadrature feedback in the mend altering the tuning of this however if the i.f. reactance stage is from the anode of V30l triode via strip has been aligned by the Alignment Service. C32l to its grid. This particular Mullard circuit was never adopted by any' of the setmakers and after (18) Disconnect the luminance input to the RGB experimenting with other variations of the circuit to output board. On advancing the brightness a display obtain the best line locking I have found that the 116 more usual conflguration, with the quadrature feed- back to the cathode of the triode reactance valve, gives much improved results-it is well worth making the change I'd say. The circuit is shown in Fig. 2. Fortunately the total number of components used is the same as before though some values are changed and the layout must of course be rearranged (Fig. 3 shows the changes needed). The signal developed across R34l is fed via C32l to V30la cathode to establish the quadrature conditions in this valve: the feedback via C320 is to reduce the damp- ing of the tuned circuit. B. J. Hufton.

Editorial note: We have tried this one out and although it involves some modification to the board ]!} agree that it gives a worthwhile improvement.

Fig. 2: Modified line oscillator arcuit. L30l is the standard Dear Sir, line ascillator coil available frcm Foryestone Components. { Component values are shown only where they have been LINEOSCILLATOR LOCKING changed. Component ratings: R355 lW 5%: C320 40V: I have read with interest E. Erven's comments on C32l 400V; C322 5o/o400V; C323 400V. Forgestone Com- the line oscillator in the November issue. The reason ponents tell us that they can provide the components why the oscillator will not lock in satisfactorily requircd to caily out this modification. when the tuning of the tuned circuit is adjusted by adding an 0'0l,uF capacitor across it as previously suggested is as follows. C322 in the original circuit serves two functions. First it forms part of the capacitance tuning the coil L301, and in this respect can be regarded as having its upper end virtually earthed to a.c. by the low resistance of R341. Secondly however it also forms part of a two-stage phase-shift network along with C321, R34l and for phase from R338. The condition a "12 shift such a network is shown in Fig. 4. The component values given in the original circuit satisfy this relationship. By adding an 0'01sF capacitor to tune the tuned circuit correctly however the phase quadrature feedback in the reactance stage is disturbed so that the lock-in arrangement may no r- -t!!t-s- -e longer operate satisfactorily. Indeed by introducing a resistive component into the variable reactance the Q of the system could be lowered sufficiently to stop oscillation altogether. This could explain why the circuit then operates better with a PCF80 since this has a higher mutual conductance than a PCF802. Timebaseboard alterations required to caffy out the I suggest that adding a capacitor of about 0'0025uF Fig. 3: modifications shown in Fig. 2. across the whole of L30l should correct the tuning error without upsetting the phase-shift network. H. Pursey. cr 2.Rt.R2.ct.ct - t phase for f rclation bet*ln input end outpqt * Editorial nore: Replacing L301 with a standard line mil oscillator coil for this type of circuit and leaving the tuning capacitance as originally overcomes. the Fig. 4: The coupling from the anode to the gid ol the * problem. Amending the circuit as suggested in the PCF802 reactancetriode consistsof a two-section phase- letter above provides improved performance. shift network-see lettet from H. Pursey.

on how good the performance of the strip is to start Dear Sir, with (some variation is inevitable with home- constructed modules). The following points are well I F STRIP worth observing however by anyone having difficulty I have been experimenting with a number of i.f. with the i.f. board. strips built to the original published design. As a Note first that it is vital to keep component lead result the suggestions below are made. These lengths to the absolute minimum except where certainly produce a worthwhile improvement in the otherwise specified (i.e. the TAA350 i.c.). performance of some i.f. strips though with others As mentloned in previous articles the critical the improvement is only marginal-it rather depends components are the capacitors associated with the 't17

*.1;t:,,..-:' i*,.:,"

!i::

(a)

r-- t I --=-r-'- z ------t rf ----'-----l l'l .__ll T- t8 l"

-I-r t ri '-L .l"l'trt J,, I ri U lL r" I F ;q l-__21___J I-i-J-i _L------J | | .,'---r i i' F---r t t l-l5r-l {c}

Fig.5: (a) The performance of the i.f . strip can be improved by adding coppet screening on the print side as indicated here. The soldered connections between the screens and the hoard must be in the positions marked above in heavy line. l Transfer Cl l6 and Cl l7 to the ptint side of the board-ensure that they are mica types and mount them with the shortest possible I leads. The dimensions of the two screens are shown in (b) and (c).

upper section of the cascode stage-Cll6. Cl 17, from the original circuit) can be restored to l'2kO to Cll8. Cl20 and Cl2l. The-,-'mL.st be mica gain. 'I'r1' t1'pes, increasethe i.f. Cl 17 as well. ntounting Cl l6 and Cl 17 on the E. R. Hilt. print side of the board, as close to the board as possible,then place copper shielding over the print side of the board as shown in Fig. -5. Unetched MAINS TRANSFORMER printed circuit board sheet bolted to the board can Electlokit tell us that they have written to all pur- be usedif you have diffhcultl'obtainingcopper sheet. chasersof Pack l8 offering to supply a replacement Whichever you u:ie however the so'ldering points mains transformer. Details can be obtained from shown in Fig. -5 are critical and must be observed. Electrokit at their new address8 Cullen Wav. London There must be no other connection points. Once NWl0. The price of Pack l8R with the new trans- this has been done the value of Rl30 (if different former (type 0748) is f16'95.