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SERVICEMAN'S LOG How many symptoms from one fault? I believe it was Henry Ford who made the profound (?) statement that “history is bunk”. But someone else, whose name escapes me, made the rather more realistic statement that “he who ignores history will be made to relive history”. What has all that to do with servicing TV sets? Well, it turned out to be singularly appropriate in regard to the story I’m about to relate, though I doubt whether either of the afore­ mentioned philosophers was thinking of anything so trivial (to them) as TV servicing. It all started with a Teac CT-M515S colour TV set, a 51cm model about three years old and featuring stereo sound, Teletext, and remote control. The owner’s complaint was straight­ forward enough; it was completely dead. And so it appeared to be at switch-on – no sound, no picture and no light on the screen. Until I advanced the brightness control, that is. Then the real symptom became obvious. There was no vertical deflection, the set displaying the classic thin bright line across the centre of the screen. A routine voltage check immediately produced a vital clue – there was no 12V rail. The 12V rail is derived from pin 4 of the horizontal output transformer (T402) via a 0.68Ω fusible resistor (R423), diode D404, a 6.2Ω 3W resistor (R422), zener diode ZD402, and a 1000µF filter capacitor (C421). The immediate cause of the supply rail failure was ZD402, which had broken down and taken out R422. But that was not all. The vertical output IC (IC401 - TDA2653B) had also been de­stroyed. Which had come first and destroyed which? There were no clues on this but I regarded it as of secondary Fig.1: part of the horizontal output circuit in the Teac CT-M515S colour TV set. A 12V rail is derived from pin 4 of the horizontal output transformer, via 0.68Ω fusible resistor R423, diode D404, 6.2Ω resistor R422, zener diode ZD402, and 1000µF filter capacitor C421. 40  Silicon Chip importance any­way. More to the point, replacing those three components was all that was needed to get the set working again. And it worked very well. I gave it a thorough once-over, made some minor setting-up adjustments, let it run for a couple of days, and then returned it to the customer. And that should have been the end of the story. Here we go again It wasn’t, of course. A month went by and the set was back in the shop. Well, that was bad enough but the really nasty part was that it was the same components which had failed. Which meant that I had treated only the symptoms, not the cause. And I had to find the cause. I replaced all the damaged components again (it was becom­ing a costly exercise) and the set came back to life. But of course I couldn’t leave it at that; I had to find what caused all this destruction. In general terms, I suspected an over-voltage condition of some kind, either high amplitude short term, or lower amplitude continuously. I couldn’t do much about checking for the former but at least I could check the latter. So I made a complete voltage check, looking for any values which were even marginally high. This achieved nothing directly; all values were virtually spot on. But it did help indirectly, even though I did the right thing for – initially – the wrong reason. While making these checks, I paid particular attention to the high tension rail. This rail is derived from pin 5 of the switchmode transformer (T901) via D904 and normally sits at 113V, as measured at test point TPB+. However, between diode D904 and TPB+ there is a network of three transistors: Q907 which is directly in the HT rail line, Q906 which controls Q907, and Q905 which controls Q906. I didn’t recognise this network immediately. I assumed it was a voltage regulator and, on this basis, wondered whether a fault here could have been responsible. I was clutching at straws but decided to check all three transistors. And I struck oil! Q907 was short circuit. But grati­fying though this was, it didn’t altogether make sense. If it had ceased to function as a voltage regulator, why did the rail still measure 113V? Why hadn’t it gone high? I took another look at the network and realised my mistake. It wasn’t a voltage regulator at all. Instead, it was a switching network, used to switch the set on and off via the remote control system. In greater detail, the switchmode supply runs continuously while ever the mains supply is on. The remote control switches Q905 which in turn switches Q906 and ultimately Q907 in the HT rail to turn the set on and off. In addition, the remote control switches various signal paths. It would be no problem to replace the transistor but would this bring me any closer to the real problem? Well, it did. Deep down in the brain cells, something stirred. Mr Ford’s disparaged history was proving to be anything but “bunk”. Rather, a whole lot of historical bits were coming together. So much so that I began deriding myself for not realising sooner what might be wrong. I went straight to C909, a 47µF 25VW electrolytic on the base of switching transistor Q904 in the power supply, and reefed it out. I replaced it with a high temperature, higher voltage type and modified the mounting somewhat to keep it as clear as possible from heat sources. Been there, done that So what was the connection? This switchmode power supply is virtually identical to one produced by Siemens many years ago – almost back to the beginning of colour TV in Australia – and which has been used by many manufacturers since then. It was used in some early HMV receivers (C211, C221 series, etc) and more recently in the Fujitsu-General FT-1411 and FT-2011 receivers and the Sanyo CTP6626, among others. And it was memories of the Fujitsu-General FT-1411 which stirred first. It all happened many years ago and, Fig.2: the switchmode power supply in the Teac CT-M515S. The HT rail comes off pin 5 and goes to switching resistor Q907 at top right. C909 is at lower left. what with my memory cells being somewhat sluggish these days, I had completely forgotten it. But as I recall it now, the complaint was that it could not be switched off properly via the remote control. And I use the customer’s term “properly” because, while there was no picture, there was still a raster on the screen; ie, full line structure but no video. It looked a simple enough problem initially. The setup was almost identical with that of the Teac – a transistor (Q606) in the HT rail (109V), controlled in turn by Q605 and Q608, the latter fed from the remote control system. And Q606 had gone short circuit. (As an aside, Q606 was a type 2SC2335, which is the same type as Q906 used in the Teac). Anyway, the problem was easily fixed – a new 2SC2335 and the set was back to normal. The trouble was, the set bounced. I thought it was just bad luck the first time but when it bounced again I knew I was in strife. I won’t bore the reader with all the details as to how I finally cracked it but, as I recall, it was a combination of good luck and some physical evidence. The physical evidence was signs of corrosion around two electrolytics in the power supply, C607 and C620, both 100µF/25V. They were connected in series to give 50µF and fed the base of the switching transistor, Q604, in August 1996  41 was all that was needed to put the sets back in operation and minimise the recurrence of the fault. Which is pretty much where we came in. And, no Mr Ford, history isn’t bunk; it’s a very good teacher. The money-hungry customer the same manner as C909 in the Teac. I can’t explain the reason for the series arrangement. If it was to increase the voltage rating, it wasn’t a very good effort; there was no resistor network to equalise the voltage distribution. Anyway, I substituted a 47µF capacitor with a higher voltage rating and a high temperature rating and that finally solved that problem. Then there was the Sanyo CTP6626 which uses an 80P chassis (or more correctly, there were several sets with the 80P chassis). And, once again, this uses what is virtually a Siemens type switchmode power supply. In fact, this story goes back even further and would have been my first encounter with this particular fault. In this case, however, the story of one fault is essentially the story of them all. Apart from minor variations 42  Silicon Chip (some sets were intermittent), they all produced the same symptoms from the same fault. It was real beaut at the time because I quickly learned to handle the situation. But it did little to prepare me for the variations on the theme which occurred in the Fujitsu-General and the Teac several years later. In essence, the problem presented itself as a destroyed horizontal output transistor (Q451), caused by a dramatic rise in the main HT rail due, in turn, to the failure of capacitor C314. C314 was a 47µF electrolytic capacitor in the power supply and fed the base of the switching transistor (Q304). And, in some cases, Q304 would also be destroyed. Replacing the faulty transistors and substituting a high temperature electrolytic, mounted as far away as possible from any sources of heat, My next story, as fate would have it, is also about a Sanyo TV set: a fairly old set, a model CCC-3000, a 34cm “Cosmo” port­able, using an 80P chassis and, yes, the same Siemens type power supply. But the story is just about as far removed from the power supply problems as it could be. The customer was a European gentleman with only a limited grasp of English. But his grasp of money matters suffered no such limitation; he was as sharp as they come. So this story is nearly as much about customer relations, charges and the eternal problem of quotes, as it is about technical problems. Inevitably, of course, the two are interwoven. The basic problem was simple; the set had been dropped. Not particularly hard apparently – there was no obvious external damage – but enough to put it out of action. Right from the start, and simply on the basis that the set had been dropped, the gentleman wanted me to quote him to repair it. As a basic rule, I don’t quote for repairs and certainly not on the basis of such vague information. I will try to assess a particular situation, based on the best available evidence, but at best this is a guest­ imation. There must inevitably be a number of “ifs” and “buts” included in such an assessment. As a colleague once put it, “you don’t really know what a job is going to cost until it’s finished – and it’s a bit late then to quote for it.” An exaggeration? Perhaps, but there is lot of truth in that too. Anyway, I explained that could not quote him for the job and set out some of the reasons. I told him I charged so much an hour for labour, plus the cost of any components which had to be replaced. The best I could do was switch the set on and try to assess how much damage had been done and, therefore what kind of cost might be involved. And, as I pointed out to him, I didn’t even know whether the picture tube was still working. This didn’t seem to make much Fig.3: the switchmode power supply in the Sanyo 80P chassis. It’s similarity to the Teac supply is evident, both being based on an early Siemens circuit. impression but I switched the set on anyway. The result was more promising than I had expected. The sound came up immediately and, as the tube warmed up, there was some signs of life on the screen – a bright horizontal line. Well, this meant that the tube was intact, the power supply was working, and the horizontal output stage was working. In fact, most of the vital parts were working except the vertical output stage. On this basis, I told him that I thought the most likely fault was a cracked board. I couldn’t say how serious this might be. It might be possible to repair it or, if it was too badly damaged, the only alternative would be to replace it – assuming a replacement was available and the cost could be justified. My most favourable assessment, therefore, was that it would involve at least two hours work. And that assumed that no compon­ents had to be replaced, which I felt was a fair bet. That still wasn’t good enough; he insisted that I open the set, on the spot, determine the exact nature of the damage, and give him a firm quote for a repair. I was equally insistent that this was out of the question and that the situation was not negotiable; take it or leave it. He hummed and hawed about this but we finally reached a compromise. I agreed to quote him for two hours labour. If the job was going to cost more than that I was to contact him and give him the choice of either going ahead with the job or abort­ing it, in which case there would be no charge. I wasn’t particularly happy with this arrangement but felt fairly confident that I could work within it. So it was on to the bench and off with the back. My immediate impression was that it had obviously spent most of its life near the ocean, because there was considerable corrosion on the metal parts. But I was looking for cracks. There was nothing immediately obvious and I removed the main board for a closer inspection. The high risk areas would be near the horizontal transformer and where the board is supported by the cabinet. And this was where I found it; from the transformer to the edge of the board. It was a very fine crack, about 10cm long, and not at all easy to see. In fact, I suspected that at least some of the copper tracks were still be functioning, though obviously not very reliably. Anyway, it looked like a fairly straightforward job, desp­ite the fact that a number of tracks were broken. There was a fair amount of work involved in cleaning the board of dust August 1996  43 Serviceman’s Log – continued and grime, plus the original green varnish, until I was back to bright copper. Then it was simply a matter of flowing solder over the breaks. Sometimes, if a crack is bad enough, I fit a wire bridge but I didn’t consider it necessary in this case. In fact, the end result was very satisfactory, both visually and mechanically. via a 6.8kΩ resistor (R410) to pin 15, which is marked as 12.7V. And this voltage was spot on. There wasn’t much left to suspect, except the IC itself. Had I destroyed it in some way while making tests? I hoped not but the only way to prove the point was to replace it. I pulled it out and, because of possible doubts The big test Unfortunately, when I switched it on, the result wasn’t satisfactory at all; the vertical deflection stage was still not working. My first reaction was to suspect that a supply rail had been lost, perhaps because of a crack I had missed. I pulled out the circuit diagram and began checking all the rails which, at least at their starting points, appeared to be correct. So I began tracing them. And, since I don’t like running a set for long periods with a fault like this, I would switch it on briefly, check a voltage, then switch off while I lined up another check point. Then suddenly, when I switch­ ed the set on, the white line had vanished. And not because I’d cured the fault but because the set was now completely dead. This was a really revolting develop­ ment; instead of finding the fault I had seemingly created anoth­er one. I went over the board again, looking for any missed cracks, but drew a blank. There was still the full 110V on the main HT rail from the power supply but no secondary voltages from the horizontal output transformer. And the CRO confirmed that there was no horizontal activity of any kind; nothing at the output stage (Q451) and nothing at the driver stage (Q450). Further checks revealed that the voltage on Q450’s collec­tor was high. Instead of the indicated 64.7V, it was sitting at the full rail value of 110V. It was obviously turned off and the CRO confirmed that it was not being driven from pin 3 of IC401, which contains both the horizontal and vertical oscillators. So why wasn’t IC401 working? This IC takes its voltage from the 110V rail 44  Silicon Chip why were there no other symptoms due to the cracks? Without backtracking and identifying every broken track, I can only guess. However, it is possible that there were other symptoms which were masked by the vertical failure. There may have been no video or no colour, for example. And why did IC401 then suffer a further failure? This may have been due to my test routine but I don’t think so. Closer examination of the IC revealed quite a lot of corrosion on the pins, particularly where they enter the plastic body. The pins were firm enough mechanically but it’s possible that some corro­ sion had made its way inside the body. As a check, I refitted the original IC in the socket and gave it a bit of a bashing for good measure. But it was complete­ly dead. As I say, these are questions for which I have no an­swer. Ungrateful customer about my diagnosis, fitted a socket to the board and plugged in a replacement IC. And that fixed it. Not only did the set come back to life when switched on but the vertical scan had also been restored and we had a full picture on the screen. Unanswered questions All of which leaves a lot of unanswered questions. If the vertical failure was due to a fault in IC401, rather than the crack in the board, what had caused it to fail? Was the set running when it was dropped and was there a voltage surge when the copper tracks fractured? And More to the point, in practical terms, the job had now gone outside the terms of the cost agreement. As well as the two-hour labour charge – which had been exceeded but which I would carry – the customer was now up for an extra $20 for the IC. Sticking to the agreement, I rang him and advised him of the situation. More aggro; he didn’t want to go beyond the origi­ nal labour charge. I refused to budge. I pointed out that I had kept my part of the agreement and it was up to him to keep his. And I added the clincher – if he didn’t want to pay the extra $20 I would put the old IC back in the set and he could come and col­lect it, no charge. That did it. Knowing that I had the set running on the bench but that I could easily disable it was too much. He agreed to pay the extra charge, albeit reluctantly. It was over a week before he turned up and during that time the set never faltered. But would you believe it, when he came to collect it, he tried to beat me down again. I didn’t even argue with him; I made it clear it was take it or leave it. He took it – and I hope I don’t see it or him again. Some customers are really SC not worth the trouble.