Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG A very circuitous exercise Rare or unknown TV sets can present a difficult situation for .any serviceman. And in this case, the situation was further aggravated because the circuit diagram was inadequate and because the set employed unusual circuitry. The set in question was of French design, a Thomson model TS5152. Thomson sets appeared on the Australian market several years ago and I have encountered two or three of them in the past. The last one was a model TS5108 which had a tricky vertical scan fault. It was described in these notes in May 1989. At the time, I established that they were imported by Lemair Aust. Pty Ltd (23 Amax Avenue, Girraween, 2145) and that service and spare parts were being handled by Hills Telefix Service (10 Wigs Rd, Riverwood, 2210. Phone (02) 533 4855). As far as I can determine, this situation still applies. This information may assist anyone else who encounters these sets. Relatively speaking, I suppose that the faults in this latest set were not all that unusual. However, I could have been caught had I acted too hastily and the story is worth telling for that reason alone. But the other aspect is the one I hinted at earlier; the combination of an unusual circuit and lack of adequate data to go with it. And this circuit is unusual. So much so that I found much of it hard to follow. This is not a criticism of the set's design or its designers . If they can find a better way to do things , then good luck to them. The set is an excellent performer and, by all accounts, has a better than average reliability record. The fact that I have encountered so few of them would seem to support this. But a strange circuit, no matter how justified, can still present a problem at the service level. That's why a good circuit and other data are vital in such cases. But this is where the system fell down , because the circuit contained a number of errors -some trivial and some quite frustrating. Dead fuse 11-l~ se:.-r ~MF'L-01..(ED UN\JSUALCI 'RC.U1--f"1<'-f rv OF F1<E:NC.H DE:Sl G N,, , 40 SILICON CHIP As the set came in, the fault was quite straightforward; it was simply dead. One obvious reason for this was that the mains fuses were blown. These carry the part number FP0l and type number TlA, signifying a delay type 1A fuse. And this was the first circuit discrepancy; there are two such fuses in the set but only one shown on the circuit. This discrepancy is quite trivial of course and I thought no more about it. But it was a sign of things to come. Wµat was more important right then was to find the cause of the fuse failure . I naturally suspected that it was something in the power supply, most probably the regulator transistor. In fact, after studying the circuit, I wasn't too sure as to how it all functioned. Mains transformer UP0l has two secondaries, each feeding a bridge rectifier, DP0l and DP02 . The lower CP21 I1]'4,]S'- .ti.. BPO, _'"'-+l-'-E➔C lO_•fVY'~,--0-·_·+--+,◄...~,..~ '; ~ ...-.--fOJ '{[_:---+-...._---+---+',___._--..Uf'IO•,----;,-,------.---, >-~+=>=--.._-+-~-ro_,_•o+, L .J • I L FPOI TU OP l ~ ,......oo· I ) JNO A~•s •_, :)-., ~ UE • Ot~f"I S " :J" 1,._.-: S•{ "! !H ::>IY.· i A::)C: Fig.1: the power supply circuit in the Thomson model TS5152. Note the BC337 transistor (TP01) immediately to the right of the power supply board. Where was it? And where was the board on which it is shown? winding and DP0l, in conjunction with capacitor CP09 (lO00µF), produces a 23V rail, which goes up and to the left. It also connects to transistor TP01, a BC337 immediately to the right, and which appears to be a voltage regulator providing an auxiliary rail. The upper secondary winding and bridge DP02 produces what appears to be the main rail, at 108V. This connects to the horizontal output transformer primary and thence to the horizontal output transistor, TL02. down the drain - I eventually found where it should have been, but there was only unused copper pattern. I eventually worked out that TFlO and TFl 1 form part of the vertical circuit and, later again, found a "fine print" notation on the bottom of the circuit, under the heading "AUST ONLY", saying, in effect, to delete subboard PB 95 0800 and to add a diode, apparently in place of the BC33 7 and associated circuitry. At that point it became obvious that I had wandered up the garden path and I gave up trying to rationalise all The phantom sub-board this. But I did wake up to one interestHaving digested that much, I de- ing and useful point about the part cided that the BC337 voltage regula- numbering system. It appears that the tor transistor (TP0l) was a likely sus- first letter relates to the component pect. The only snag was, I couldn't . ("C" for a capacitor, "T" for a transisfind it. According to the circuit, it tor, etc), while the second letter rewas mounted on a sub-board, PB 95 lates to its circuit function. 0800, along with transistors TFlO and Thus, "P" indicates the power supTF11. ply; "L" indicates the line (horizonIn fact, there is no such sub-board. tal) system; "F'; indicates the frame (vertical) system, and so on. It can be After much searching, I found TFlO and TFl 1 mounted on the main board a help and is worth remembering. and, what was more, I realised that The next suspect was transistor they have no electrical association TP0l. TP0l? Hadn't I just given that with the BC337, as a closer examina- away? Yes, indeed, but there was a tion of the circuit will confirm. second TP0l, this time a BU807, imSo where was the BC337, TP0l? mediately to the right of the aforeAnswer: it didn't exist either. By trac- mentioned sub-board. So that was ing the copper pattern - more time another one up against the circuit draughtsman's duck house. It appears to perform some kind of regulator function but just how I was unable to work out. However, since it is a heavy duty device and is connected directly between the 108V rail and chassis, it was a natural suspect. Sure enough, it had broken down . If only I had checked it first. Anyway, that was it; a new one was fitted, the two fuses replaced and the set fired up. It gave a first class picture. End of story? Well, it should have been and I thought it was. But the COMING NEXT MONTH* • UHF TV Transmitter Transmits from your VCR to a second TV set in another part of the house without messy connecting wires. • Baby Monitor This useful project enables you to monitor the baby's room via an FM radio link so that you can attend to other chores. • lnfrared Door Minder It sounds a buzzer when someone walks through an invisible beam. * Subject to space availability. NOVEMBER 1991 41 lt" ,RANSPIREX> 111A, ~ se:.,"BE-l-01\lGS "'f'O H\S AG6.t:> GRANt>M01'""HE:'Ro ... ~ owner wasn't in a hurry so I let this one run on the bench for the next few days, putting it through a few switchoff, switch-on cycles in the process. All went well until I was about to ring the owner and tell him he could pick it up. Then I switched it on one morning and bingo! - it was as dead as the proverbial dodo. Not surprisingly, both fuses had gone again. I immediately suspected the replacement transistor, since it was a substitute type, although its ratings should have been more than adequate. But no, it checked out OK. De-gauss circuit I decided to live dangerously, fitted two more fuses and switched on. The set worked, which was a bit of a blow because it now meant that I had an intermittent to contend with. On the other hand, it immediately threw suspicion on another ofmy favourite fuse blowers: the de-gauss circuit and particularly the thermistor. I've dealt with faults of this kind many times before in these notes. In this case, the "thermistor" is actually a 3-terminal commercial package which is used in a number of sets. As nearly as I can determine, it is similar to a system I dealt with in some detail back in July 1990, in regard to an AWA Thorn 3500. This 42 SILICON CHIP used a positive temperature coefficient thermistor (PTC) and a voltage dependent resistor, to ensure minimum current through the de-gauss coils after the de-gauss function. However, this was the first time I had seen this package portrayed in any detail, as it is in this circuit. And while I accepted the drawing initially, I did a double take when I looked at it in greater detail. There's just no way that I can accept the idea of a PTC thermistor (which has low resistance when cold) being connected directly across the mains, as portrayed here. Not onlywould the de-gauss function not work but it would simply be another "little beaut fuse blower". So, unless I am much mistaken, this is another circuit blue. Anyway, this is a standard package which I carry in stock, so there was no trouble in replacing it. And that, I was confident, would settle it. Except that, a couple of days later, it failed again and, as before, at switch-on. This last point maintained my belief that it was still a de-gauss circuit problem. Not that there is much left in a typical de-gauss circuit. But there was in this case - a 68nF, 400V capacitor (CP0l) across the de-gauss coils. I pulled it out and checked it. It checked OK but I was not convinced. I was remembering some earlier experi- ences, involving some 0.lµF capacitors fitted across the mains in the old General GC181 sets. These were found to have a selfhealing function; not by design (there are such devices) but purely by accident. They would break down, blow a fuse and clear the breakdown all in one operation. So the serviceman was faced with a blown fuse and no explanation for it. A new fuse might last anything from a few hours to several months - and the consequences can be imagined. This was a much smaller capacitor, both electrically and physically, but could it have a similar fault? One way to answer the question would be to disconnect the de-gauss circuit and run the set for several days and see what happened. However, I had a similar capacitor on hand and it was a simple job to fit it. Granted, it was only similar. The one that came out was a quite tiny unit, probably a mylar type, whereas the best I could find was a disc ceramic, physically much larger and with rating of 3kV. There was plenty of room for the larger unit and I reasoned that the higher voltage rating wouldn't go amiss. All right, so 3kV is a bit of an overkill, but it was the closest I had and I wasn't going to waste a lot of effort just to satisfy a technical nicety. Anyway, for once, I hit the jackpot. The set ran for several days, through many on-off cycles, and was finally returned to the customer. That was several months ago and there has been no further trouble. Sorry, Mr Thomson; a nice set but no prizes for the circuit diagram. Tremendous trifles To change the subject, here are a few thoughts on what some may consider to be in the "tremendous trifles" category. And maybe they are but we should always remember that, for the customer, the reason a piece of equipment has failed is often of less importance than the simple fact that it has failed. Regardless of the cause, no matter how trivial, the end result is the same; the customer is temporarily deprived of the equipment - TV set, radio set, microwave oven, or whatever. On top of that, he has to arrange to have it serviced and transported, all of which can easily add up to a degree of frus- that he couldn't get anything out of it. TETIA TV TIP Sharp 9C140 (Kriesler 37-104 and similar Philips). Symptom : No sound or picture. Ticking sound from the chassis, like a slow hiccup which persists for some minutes after the power is turned off. Cure: R644, a 2.2Q 0.25W fusible resistor had gone open circuit. This resistor feeds the main B+ to the line output stage and also to the horizontal oscillator. The ticking arose from the chopper power supply, due to the total lack of a load on the supply. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician 's Institute ofAustralia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. tration and, possibly, some ill-will. Of course, sometimes it's the customer's fault and believe m e, the mistakes they can make are often way out. But that's their problem. What we have to do is to make sure that no act of carelessness on our part - trivial though it may seem - precipitates such a situation. Take my word for it; the customer doesn't regard our mistakes as trivial. So what brought this on? What disgraceful technical gaffe am I about to reveal? None really, but one u nusual fault did highlight a procedural weakness which could - and did - produce, in mild form, the kin d of situation I've described. It all started w h en a customer brought in a Sharp video recorder, a model VC-381. It had no specific fault but the owner felt that it was due for a general overhaul. And h e was right. It was a quite grotty, with a fair build-up of oxide on the heads and guides. These were duly cleaned, the rest of the mechanism lubricated and a faulty idler pulley replaced. It was then checked out on the bench and passed as OK all round. And that was more or less it. The owner duly collected the machine and I didn't expect to hear any more about it. Nor did I fo r a couple of days. Then he was on the phone complaining It appeared to be working mechani- cally, all the indicator lights were up and so on, but it wouldn't produce any picture. Such situations have occurred before and I have a few tricks which can help pinpoint anything the customer ~ ay have done to create it. The first thing to establish is whether the correct channel has been selected on the TV set to match the RF output channel of the recorder. This sounds simple enough but people do get confused. Also , it is quite possible for the output channel selector switch to be moved while the recorder is being handled. Another trap is the presence of a VHF /UHF selector switch on some TV sets and, if this is in the UHF position while the user is trying to bring in signals on, say, VHF channel 3, the result will be nothing. So my routine is to instruct the customer to try playing a pre-recorded tape , then try both channels and double check any other switches. This is a very simple and effective way of guiding the customer to get the TV set onto the correct channel. I went through this with the customer, suggested h e check it all out, and then ring me back and report. He was back on the phone again 10 minutes later. "Yeah, I got a picture but it's so blankety-blank snowy it's not worth watching". I double checked what he had done. It was just possible that he had managed to select a harmonic or something equally silly, but he was adamant that he had done everything correctly. So there was no option but to check out the machine again and, if it was OK, to suspect the TV set. A couple of days later, the machine was back on my bench and this time, attached to it, was the interconnecting lead to the TV set (about half the customers who bring in recorders leave this lead connected). Such accessories can be quite a nuisance. Once they become separated from the main device they are easily mislaid and, if they are not labelled, forgotten. The customer then takes his machine home, having also forgotten about it, and can't get everything together. The result is much confusion and ill-will. Anyway, I was about to disconnect this lead and fit one of my own when I realised that it was long enough to AUSTRALIAN MADE TEST EQUIPMENT YOU CAN TRUST SHORTED TURNS TEST R Built-in meter to check EHT transformers including split diode type, yokes and dri ve transformers. $78.00 + $3.00 p&p DEGAUSSING WAND Strong mag netic field , larger than usual coi l with multicore centre. Double insulated for yo ur safety. Also fitted with momentary on/off switch. 240V AC 2.2 amps. Just about as important as having a soldering iron in your toolbox! $75.00 + $10.00 p&p HI-VOLTAGE PROBE j Built-in meter reads positive or negative 0-50kV . For checking EHT and focus as well as any other hi-tension voltages. ;m $98.00 + $5.00 p&p SPECIAL PRICE REMOTE CONTROL TESTER (TO END OF NOV. 1991) Designed to test infrared or ultrasonic control un its. Wi th the extension lead you can also test infrared units which cannot be placed in front of the testing unit. Requ ires a 9V battery. Output is via a LED and piezo speaker. $70.00 + $4.00 p&p LASER DETECTOR PROBE A new addition to the remote control tester. Comparable with units costin g $500 or more . You can test the laser pick- up in compact disc players. $27 .00 + $2.00 p&p NEW PRODUCT TEST TUNER Save time and money with this teste r. Helps you determ ine if you have a tu ner problem, an AGC problem or a fau lt in the IF stages. $250.00 (Tax inc.) TUNER REPAIRS From $17 exchange+ p&p Cheque, Money Order, Bankcard or Mastercard 216 Canterbu ry Road, Revesby, NSW 2212. '"B"" (02) 774 1154 ,(" PAX 7\ (02) 77 4 1154 NOV EMBER 1991 43 SERVICEMAN'S LOG - CTD reach my test monitor. So, without further ado, I plugged it straight in, switched on and pushed in a test tape. And up came a picture immediately. But the customer was right; it was as sick as the proverbial dog, barely locking into colour. On an impulse I pulled out the lead and put a meter on it. And that was it; open circuit on the inner conductor. Just where the problem was I didn't bother to find out but it was an unusual fault, particularly as this lead normally suffers very little movement. It did make m~ realise, however, that I should check any such accessories in the future if they are supplied. No nothing And while on the subject of leads, here's another one. A customer brought in a General colour TV set, model GC187; a set which has been on the market for about eight years now. It was one of those abrupt confrontations. He simply pushed it across the counter with the brusque comment, "It's dead. Won't go. No picture, no sound, no nothing". Ignoring the double negative, I got the picture (no pun intended!) When I switched the set on it was as the customer had said; totally dead. I pulled the back off and made for a convenient HT rail check point. Still dead. Back tracking, I was soon into the mains input area, involving the mains switch on the front panel the mains fuse, and connections to the board, in that order. The fuse was OK, as was the switch, which left only one possible culprit; the mains cord itself. The multimeter confirmed this; one leg was open. This point settled, I looked along it for possible signs of damage. I found none. This was somewhat surprising. Open circuit cords are not all that rare but there is usually some obvious physical damage. Apart from typical domestic accidents and lack of care, I have seen cords that were chewed by ANTRIM dogs and which , in addition to the teeth marks , showed unmistakable signs of arcing. What happened to Pongo in each of these cases is not known. This cord was fitted with a 3-pin plug moulded to it, so there was no point in trying to repair it. Fortunately, I had salvaged several perfectly good cords from discarded sets against just such a need and one of these filled the bill nicely. When the customer came to collect the set, I explained what had happened. Suddenly, he was a lot more communicative. Shrugging his shoulders , he commented resignedly, "Oh, this is always happening". Curious , I pressed him for more details. And it transpired that the set belongs to his aged mother, who lives alone and has a fetish about removing plugs from power points, when the appliances are off. But the snag is that she always jerks them out by the cord; no one can persuade her to pull the plug itself. The result is a continuing saga of open circuit power cords. So that's another one to add to the list of power cord abuse. SC ATTENTION AUDIO SERVICE TECHNICIANS! TOROIDAL TRANSFORMERS The Audio Interface is Now Available! • • • • Now made in Australia Standard range 15VA to 2kVA Large ex-stock quantities available Competitive pricing available for OEMs & resellers • Simply the best transformers all round HARBUCH ELECTRONICS PTV LTD 90 George St, Hornsby, NSW 2077. Phone (02) 476 5854 Fax (02) 476 3231 44 SrLICON CHTP For years, you have had to endure an untidy mass of cables and connections to test audio equipment. The Audio Interface solves all that. This precision made instrument enables you to link all the audio test gear on your service bench to one control panel. No more temporary hookups - just one lead in, one lead out. You can do gain and power measurements up to 800 watts on stereo gear. Flick a switch or a knob and it's there. All this and more for an investment of $1495. Phone now for a free brochure. Kingsley Electronics, 17 Blackburn Drive, Cheltenham, Vic. (03) 583 4020.