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• A baffling exercise It was the Count of Monte Cristo who, prompted by his creator Alexander Dumas, made the profound observation that, "Only he who has known the greatest sorrow can know the greatest joy." After battling with a particularly stubborn video recorder recently, I felt the same way. Fortunately, my first story did not cause all that much frustration. It concerns a Toshiba colour TV set; a model C-1416, 33cm set of some five years vintage. The job started out as a fairly routine assignment but caused some head scratching at one stage. It started off with a phone call from a new customer who, after describing the make and model of the set, complained simply that it had stopped. I tried a few discreet questions to get some idea of what I might be up against, but quickly realised that this wasn't getting anywhere; the set had stopped and that, as far as the customer was concerned, was all there was to it. All I could do was suggest that he bring it into the shop. So it was that he turned up at the shop a couple of days later and sat the Toshiba on the counter. I plugged it in while he was there and confirmed that, at least as far as he was concerned, the set was totally dead; no picture, no raster, no sound. But it wasn't totally dead to my ears, because I could hear the power supply hiccuping away merrily, suggesting an overload which it didn't like. I am not very familiar with this particular set but there were a couple of points in my favour; I had a service manual and, when I came to work on it, I found it a lot easier to get at than many designs I could mention. The circuit was well laid out and easy to follow, and appeared to be a fairly conventional arrangement. As I said, it all looked fairly straightforward. Likely suspects With symptoms like this the two areas I first suspect are the power supply and the horizontal output stage. The power supply was fairly typical; a switchmode arrangement running from a bridge rectifier connected directly to the mains. Much the same applied to the horizontal output stage; a horizontal driver transistor, Q402 (2SC2482), driving __ PIN 1, LOT T461 .JLf\_ ......,. / 890VP-P C464 560pf C463 .0022 2kV Fig.1: horizontal output stage of the Toshiba model C-1416 TV set. Note the protective resistor and diode built into the 2SD896 transistor. 50 SILICON CHIP the horizontal output stage (Q404) via a transformer (T401). The horizontal output stage was a 2SD869, a transistor with built-in protection (ie, a resistor from the emitter to base and a diode from collector to base). The main HT rail was shown as 114V, which was applied to pin 3 of the horizontal output transformer, then from pin 1 to the collector of the aforementioned 2SO869. That much digested I decided to measure the HT rail, which can often provide a clue as to the likely culprit. It turned out to be well down, around 50V, and I mentally filed this figure for future reference. At this stage it was a toss-up whether to move to the right of the circuit, towards the horizontal stage, or to the left towards the power supply. Fully aware that Murphy would be lurking around the corner, to make sure that which ever way I went would be the wrong way, I took a punt on the output stage. Well it seemed that Murphy must have slipped out for a cup of Irish coffee, because that was the right decision. In greater detail I simply disconnected the output transistor. These transistors are not the easiest to test, since they tend to show a low resistance from collector to base, regardless of polarity, due to the protective diode. Similarly they show a low value resistance, ranging from about 400 to 900 according to type, between base and emitter. Granted, one can allow for these characteristics but it is usually easier to simply fit a new transistor. But, before fitting a new one, I switched the set on with the original one removed. I wasn't quite sure what kind of a HT reading I would get, but I hoped it would come up to something near normal, if the transistor had been faulty. On the other hand, there was a risk that it might go high and trigger any over-voltage protective circuitry. Once again, it seemed that I had done the right thing because the HT rail came up almost spot on the circuit value. It seemed that it was my lucky day; I should have the job knocked over in short order. All I had to do was fit a new transistor and we should be up and running. It must have been around this time that Murphy finished his coffee and came back on duty. I switched the set on, full of confidence, only to find that it was in exactly the same condition as before; power supply hiccuping and low HT rail. Somewhat taken aback, I reached for the CRO leads and made a quick check of the signal from the driver stage, through the driver transformer, and to the base of the output stage. As nearly as I could tell, allowing for the reduced rail voltage, this section appeared to be functioning. At that point the only logical thing I could think to do was to take the new transistor out, check the HT rail and power supply behaviour again, and try to decide what to do next. And that operation produced surprise number two the rail voltage had risen, but not all the way; it was now sitting at around 75V. What the heck was going on? Smoke signals I double checked the connections to the transistor, confirmed that they were correct, and began looking for any other silly mistake I might have made. I found nothing, but had left the set running during those few minutes. Suddenly my nose told me that something was getting hot somewhere and a few seconds later I pinpointed the source. A thin curl of smoke was rising from a capacitor which forms part of the output stage assembly; C464, a 560pF, 2kV unit connected between the collector and the emitter of the output transistor. It was a blue plastic encapsulated unit which had suddenly developed a brown spot that grew larger as I watched it. Well, that was the breakthrough. I pulled the capacitor out and then, before refitting the output transistor, turned the set on and checked the HT rail again. And this time it came up just above the nominated value, exactly as it had done the first time. Which I reckoned proved the point. At a more practical level I needed a replacement capacitor. My stocks didn't run to an exact replacement, but I did find a 560pF unit with a 3kV rating. This was fitted, the original output transistor wired back in, and the set given another try. And this time everything worked; the set gave forth sound, the HT rail came up spot on, and a first class picture appeared as the set warmed up. So that was that and the set was duly returned to a happy customer. But it is worth speculating on what caused the sequence of events just related. Fairly obviously, the capacitor was faulty all along, being unable to withstand the peak level of nearly 900V generated by the output stage, and pulled the HT rail down accordingly. But with the output stage removed no such voltage was generated, and all it had to withstand was the 114V from the HT rail. Initially, at least, it was able to do this but it must have been on the point of breaking down completely. My test run with the replacement transistor must have been the last straw; by the time I tested the set without the output stage for the second time, it was breaking down at 114V and went up in smoke. Which is all very satisfying from a technical point of view, but most of my diagnosis was wasted; a few more minutes running on the bench would have produced the curl of MARCH 1988 51 SERVICEMAN'S LOG smoke and revealed the fault with no effort on my part. That's the luck of the game and it had a happy ending anyway. A failure to erase My next story involves a much more frustrating experience. It concerns a Sanyo beta video recorder, type VTC 5005, that belonged to a regular customer. The first intimation of the trouble came via a phone call from the lady of the house. The fault was rather unusual in that the machine would record the video signal satisfactorily but, on odd occasions, would fail to record the sound or erase any previous sound track. She went on to enquire whether this was a common fault, whether I knew what would cause it, and, of course, was it going to be expensive to fix. I had to reply that it was a fault I had not encountered before, that I could only assess the likely cause in broad terms, and that it was almost impossible to estimate the cost. However, I did promise not to let costs get out of hand without consulting her. In the meantime I suggested that she bring the set in. So the lady turned up at the shop a couple of days later with the machine and, commendably, she had thought to bring a faulty tape with it. In the event it didn't help a great deal, except to confirm the symptoms she had described, but even that was useful and I wish all my customers could be so thoughtful. I tried the machine while she was there, but I need hardly add that it behaved perfectly. This surprised neither of us. I have not had much experience with this machine, but I do have a manual and I fished this out immediately. In particular I concentrated on the section designated "VD-1 Audio Circuit"; the section which handles the audio signal taking it from the record/replay head, or feeding it to it, as required - and which also incorporates the bias and erase oscillator. It was this latter which interested me most because it was this function that was failing somewhere along the line. But exactly where was the real question. It could be in the oscillator itself, it could be the erase head, or it could be the flexible lead and plug and socket which connects the head to the board. The record/replay head seemed an unlikely suspect, in that there had been no replay problems as such. And if the bias line to this head had failed somewhere on the board I would have expected that the system would still erase, but fail to record anything but a weak and distorted signal. So the odds were strongly in favour of a failure within the erase circuit, or failure of the oscillator itself. :~-. ,. ~ ...-......,.~ 1\-\~'s ~E LUC.Ko~ "Me: GAME. & \T ~~b F\ l'\A1)P'f e.rJl>\NG ~N'(W~'l ... 52 SILICON CHIP ~~ But I needed to be sure. And when you think about it, this isn't an easy function to monitor. Simply letting the machine run in the record mode is of little value, since any failure will not be apparent until the tape is replayed, by which time the fault will most probably have vanished. But, in any case, the time involved in such an approach would be quite unacceptable. As depicted on the accompanying circuit the erase/bias oscillator (near the bottom) consists of transistor Q2007, transformer T2001, and a few minor components. The secondary of T2001 feeds the erase head directly from pin 5, and supplies the bias for the record head from pin 6, via C2027 and tab pot VR2003. The erase head is connected via plug and socket S2002 and the record/replay head via S2001. Fortunately, the circuitry involved is fairly easy to reach. In fact the audio circuit, as shown separately in the manual, is really a part of the complete video/audio board, and the print side of this is directly accessible when the main covers are removed. After that, removal of a few screws permits the board to be swung up, giving access to the component side. For a start I set the machine up in the record mode and connected the CRO to the active terminal of the erase head, this being about the most convenient access point. This confirmed that the oscillator was functioning, at least for the present, so I decided to leave it running in this way, while I went on with other jobs, simply glancing at the CRO from time to time to check what was happening. Initially, this approach paid off. After it had been running for some time I checked the CRO and realised that the erase signal had vanished. Unfortunately, before I could make any further tests the system came good. I wasn't too worried at this stage; I blissfully imagined that, since the fault was obviously in a mood to happen spontaneously, a little prodding, freezing, or heating would encourage its reappearance. I should have known better. I tried every trick in the book; I froze every component likely to be involv- .-----------------------------------■ ..---------il----> R2001 C2009 L?K I AUDIO IN I GNO 0 -L?µ/16 I R "'8al I >- a: "'"' 52001 -- H ... 2 : R-iN~EAD R-P HEAD 02001 2so1011 SWITCHING Q2002 2501011 SWITCHING u "'"' 2~ 3-- h I a- 20 22K R203', ~ Q.1 (0) a/ ?: I (8 -L) Q2003 2SC945 SWITCHING -T ,R r2001 ~~µ" i2 03? 02006 2SC2603 SWITCHING c~~ '"''N' REC/ PLAY OUTPUT LEVEL (0-2) ' -v TP2001 ,02 1 I --- ,..,____-+____,·,~~<. >"' R2036" 8.2K ijg o,- ~ iL u a: r P12002 R2039 R2038 'OK 10K - 02004 2SA1016 SWITCHING g~io1 2SD43B ------------------------------------ Fig.2: relevant section of the audio circuit of the Sanyo VTC 5005 VCR. Note the erase head plug at bottom left. ed and blasted the whole area with a hair dryer. I tugged the leads, wriggled the plugs and sockets, and prodded every component with everything short of a sledgehammer. Nothing produced the slightest hint of a fault. Had I not known better, I would have been prepared to swear that there was absolutely nothing wrong. All I could do now was to continue the tests as before. And this I did, for several hours each day for the next couple of weeks. But not once did the CRO pattern so much as flicker, even though I repeated some of the previous brute force tactics from time to time. It was a stalemate. I would have been happy to leave things set up for as long as was necessary, but this would hardly suit the customer. In fact they had already made a couple of polite enquiries and I sensed that they were becoming impatient. Among other things, I realised that they used the machine more for playing prerecorded tapes from the video shop than for recording off-air programs. The upshot was that I explained to them what I had discovered so far [which wasn't really very much) and that, until the machine elected to fail again - which it appeared to be stubbornly refusing to do at this stage - there was very little chance that I could make any progress. So it was agreed that they take the machine away and make such use of it as they could, until such time as the fault worsened. Weeks went by, then several months, before I saw them again. Then the lady contacted me with a completely different problem and I took the opportunity to enquire about the recorder. "Oh, it's going fine", she replied, "whatever you did seems to have fixed it." I know the feeling: after that length of time I could almost kid myself that I had done something to fix it; almost, but not quite. Deep down I knew it was only a matter of time before the gremlin would strike again. But more months went by and a couple of discreet checks on my part produced the same answer, "It's going fine. " Then came the day when the lady was on the phone with a tale of woe. "The recorder is really playing up now. It is almost impossible to record anything." This was the best news I had heard about the machine so far and my enthusiasm probably showed when I suggested she bring it in immediately. She lost no time in responding. Before disturbing anything I loaded a tape into the machine and made a brief test recording. Sure enough, the fault was there. So, very gently, I removed the covers and connected one of the CRO probes to the active lead at the audio erase head. This confirmed that the fault was still present and I reached for the second CRO probe with the idea of checking progressively along the board. At which point the system suddenly came good and there was no more I could do until it decided to play up again. Fortunately I didn't have to wait very long. When it failed this time I very gently removed the necessary screws and lifted the MARCH 1988 53 SERVICEMAN'S LOG board so as to provide access to the component side. This didn't disturb anything and the fault remained. I reached for the second CRO probe and approached the print side of the board in the vicinity of the two pins which mate with socket S2002, and which carries the leads to the erase head. By just touching the board, and before I could make an electrical connection, I cleared the fault. Suspect plug and socket I immediately suspected the plug and socket assembly, and this seemed to be confirmed when I wriggled the lead and the plug and found that, by stressing the assembly in a certain way, I could make the fault come and go. At last it looked as though I was getting somewhere. In this setup the plug on the lead is actually the female connection, the male contacts being two pins soldered into the board. I was specially suspicious of the female contacts, particularly where they made contact with the cable. This is a crimped connection and it is not 54 SILICON CHIP unusual to find the crimping does not penetrate the insulation properly, resulting in an intermittent connection. Removing contacts from plugs of this type is a little tricky. They are held in by a small tongue or barb punched into the contact, and which is depressed when the contact is inserted into the plug. It springs up when the contact is fully inserted and effectively locks it in place. To remove these I use a long, thin, pointed probe with which to TETIA CORNER Thorn 9904 (Q Chassis) Symptom: No luminance . Sound and colour OK. A normal picture appears briefly if the set is switched on again quickly after switching off. 12V rail reads high at 15.5V. Cure: D231 (EQA01-12S) 12V zener diode open circu it. This diode sets the 1 2V rail and the higher voltage when it fails blanks the video output from IC201 . depress the tongue and thus allow the contact to be withdrawn from the back of the plug. With both contacts out I examined them carefully. As far as I could see they were quite OK, but I also checked them with an ohmmeter. Again they seemed faultless, even when the wires were vigorously tugged and wriggled. Nevertheless, I put them through my own "extra crimping" process. This involves applying pressure to the stem of the contact, in the valley where it is supposed to punch through the cable insulation. To do this, I use a pair of cutters, with one blade lying the length of the valley. I know it sounds drastic, and it certainly looks risky, but it is merely a matter of applying a judicious amount of pressure. And it does work. But it didn't help much in this case. I re-assembled the plug, fitted it back on the board and gave the whole setup another wriggle test. It behaved exactly as before; pressure on the plug or tension on the cable could make the fault come or go. And, since I felt that I had cleared the plug, the next likely suspect was the pair of pins on the board. Conned I examined the joints where these pins were soldered to the board, even though I had already been over them once before, but could see nothing suspicious. Nevertheless I resoldered them, just to make sure. But this achieved nothing either and I was forced to the conclusion that there was probably nothing wrong with the plug, socket, and cable assembly; that I had been conned by their sensitivity to pressure. And I had been conned in more ways than one. It had seemed so obvious that this was where the fault lay that I hadn't even bothered to make the other obvious check which I had set out to make at the beginning; whether the oscillator itself continued to function when the waveform vanished from the erase head. I quickly made amends, connecting the second CRO probe directly to the oscillator circuit. Then I wriggled the plug and socket assembly again, created the fault, and established that it was the oscillator that was failing, not the circuit to the head, in spite of the symptoms. That much established, the next likely possibility seemed to be a faulty component in the oscillator circuit and, by now, I would have been quite happy to replace every component in this part of the circuit if it produced a quick cure. After all, there was only one transistor and a handful of resistors and capacitors. The transformer, T2001, was about the only special item. But before taking that drastic step I decided on another freeze, heat, and bash routine. After all, the thing was much more touchy now than it had been when I tried this before, and it might just work. And so I set to, with the machine in the record mode, both CRO probes connected, and the fault condition evident, this having been achieved by much wriggling of the aforementioned erase head plug. I drew a blank with the freezer, and similarly with the heating, so I reached for the sledge-hammer actually the butt end of an insulated alignment tool - and began prodding. Nothing happened until I came to C2029, a 100µ,F 16V electrolytic, when the lightest touch caused the oscillator to come good. Which was very encouraging, except that no amount of additional prodding, freezing, or heating of this component could reverse the procedure. So what did it mean? Was the capacitor faulty, or was this another furphy like the erase head plug where, apparently, vibration and pressure was being transmitted to the fault somewhere nearby on the board? In any event, it seemed logical to remove the capacitor, check it as thoroughly as possible and, if any suspicion remained, replace it. But it didn't come to that because, as I unsoldered one of the lugs, the solder came away much too readily and I was convinced that I had uncovered a dry joint, one that had defied my visual inspection. Closer inspection of the lug confirmed my suspicion. It had been tinned during manufacture but I was convinced that the solder on ... the board had never really "wet" it. Most likely it was a "cold" joint; one where the temperature of the lug had never reached the melting point of the solder. I measured the capacitor as a matter of course, and it was well within tolerance. I cleaned the lug, tinned it again, put it back on the board, and made sure it was well and truly soldered to the pattern. Then I checked the whole system again. I wasn't really surprised when it came good immediately, because it had done that many times. But I was gratified to find that wriggling the erase plug no longer had any effect. Considering how touchy it had been before, I felt it was very significant. But there was more to it than that. The capacitor's position on the board was slap alongside the two pins which mated with the erase plug. So wriggling the plug could easily have aggravated a dry joint at the capacitor lug. I ran the machine for several hours a day for a week or so after that, and it never missed a beat. Of course it had done that before too, so I couldn't be one hundred percent sure that I had fixed it. The only thing I felt sure about was that I had found a dry joint, even if the only evidence I had was my own observation when I unsoldered it. And so the machine was returned to the customer, with a strong emphasis on the need to contact me immediately should the fault show again. Thafwas several months ago and a couple of check calls have confirmed that there has been absolutely no sign of the fault. Only time will tell though. I'm keeping my fingers crossed. ~ RCS Radio Pty Ltd is the only company which manufactures and sells every PCB & front panel published in SILICON CHIP, ETI and EA. 651 Forest Road, Bexley, NSW 2207 Phone (02) 587 3491 for instant prices 4-HOUR TURNAROUND SERVICE MARCH 1988 55