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Ihe .Ieieuision' To1our Reteiuer 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.
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