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The customer has no idea I often wonder whether the customer ever has the slightest inkling of what a serviceman goes through in tracking down some of Murphy's most vicious creations. But even as I ponder, I know the answer: the customer's thinking is light years away, across a chasm that can never be bridged. And what brought on that little burst of philosophy? Answer - one of the most frustrating and least satisfying exercises I have experienced for a long time. It included just about every negative aspect one could imagine. Quite apart from the technical problems, there were difficult customer relations. These involved, among other things, a reluctance to reveal the set's previous history and a threat to bring consumer affairs into the situation. And I need hardly add that the whole exercise was a complete dead loss financially. The job started out routinely enough; a phone call from a new customer enquiring as to whether I could service her TV set. When I said I would be happy to do so, she said it was a Sanyo colour set which had stopped dead in the middle of a program. Since there didn't seem to be much point in probing deeper right then, I simply suggested she bring the set in to the shop. When she turned up a couple of days later the set turned out to be a Sanyo model CPT 6603, which first appeared about eight years ago. A quick check confirmed that it was quite dead. I made some attempt to delve into the set's history but the customer seemed strangely ignorant in this regard. But I also sensed that she didn't like being questioned. When I opened the set the first thing I found was a blown mains fuse, a 2A slow-blow type. Based on experience, I mentally nominated three possibilities: a faulty degauss thermistor, a faulty horizontal output transistor, or a faulty switching transistor in the power supply. Of these, the last named was the most common. The power supply board mounts vertically on the left hand side of the chassis (from the back) supported by a metal frame. Releasing a screw on the main chassis frees the frame and allows it to be swung to one side. The power supply board can then be freed from the frame, giving access to the copper side. The switching transistor, Q361 (2SC1046L), is mounted on this board on a fairly substantial aluminium heatsink, about 50mm square. This, in turn, is mounted clear of the board by small spacers but is still close enough to allow the base and emitter pins of the transistor to pass through the board to the copper tracks. Knife & blow lamp ~t1<~ ,... .:SU,- 1: AL.~ SE.NSEO SHE. DION~ L-lKE. ge.1N6 qUE~TID~Et>... 62 SILICON CHIP It was as I swung the board clear that I received my first surprise; something which, with hindsight, should have warned me that trouble lay ahead. One glance told me that someone had been there before me. And unless I was very much mistaken, he had been wearing a striped apron and had attacked the board with a butcher's knife and blow lamp. Well, you know what I mean. It was a mess. Excessive soldering heat had destroyed the original cop- . ~Well, to cut it short, it was the same situation as before; a blown fuse, a shot switching transistor, and a couple of faulty diodes. But why? I would have liked to have blamed coincidence but deep down I knew this was clutching at straws. There just had to be a more fundamental reason. I went through the motions of replacing all the components and soon had the set running again. But there was no way that I could return it with the threat of another failure hanging over it. At the very least I owed the customer an explanation for whatever time it was going to take to find the real fault. So I rang the number and this time found myself talking to the lady's husband. And, after giving him a brief resume of the situation, I found him to .be as outgoing as his wife was taciturn. I learned that the set had already failed in a similar fashion on three occasions, and had been repaired by another serviceman, whom he named. Well, that explained a lot of things. I knew the serviceman he mentioned and realised that I erence is the 2S0380 which has the should have recognised his hanhighest ratings of the lot. diwork. He is reasonably clued up I took the precaution of making technically but is sadly lacking the the first test at a reduced voltage skills necessary to handle today's via the Variac, increasing the components. But it explained more that that. voltage gradually in case there was anything else amiss. But everything The fault was obviously a "stinker" went smoothly and I soon had the that had already defied three efset running at normal voltage. forts by the other serviceman until, I ran it for several hours that apparently, the customer became day, spent some time the next day fed up with the situation and sought cleaning up the butchery as best I out someone else - Yours Truly. could, then ran it for a couple more None of this did anything to cheer hours. Then I rang the customer meup. (I learned later of a further comand told her it was ready. She called in later, paid the bill - it was plication. The husband and the seraround $70 - and collected the set. viceman were golfing buddies. One can imagine the conflict that situaAnd that was another one down. tion created in the husband's mind. It's not that simple And what of the domestic situation? Oh yeah! I wouldn't be writing While I sensed a lack of agreement, this had it been that simple. A cou- I couldn't decide whether he was a ple of weeks passed and then the henpecked husband or she a roosterpecked wife). lady was on the phone with the complaint that the set had failed But right then I had to content again and, according to her, it was myself with pointing out, as the same fault as before. I was well diplomatically as possible, that he aware of the significance of that might have saved a lot of time and remark but I wasn't going to argue trouble - and money - if I had at that stage. I simply invited her to known the set's history in the first bring the set in again. place. And I added that it might _ _._, per tracks to the transistor pins and these had been replaced with short lengths of heavy scrap wire, clumsily soldered into place. And I could see that several other components had been removed for testing or replacement. There was no damage here but one could see where he'd been. I made a mental note to clean things up later. I didn't want anyone seeing that mess after I hq.d handled the set. But first I had to fix the fault. It took only a moment to confirm that I had picked it in one; Q361 was a dead short between collector, base and emitter. This type of failure is not unknown in this model and I had seen it often enough before to know that any one of three diodes D362, D363 and D364 - could also have been destroyed (sometimes all three). In this case it was D362 and D364. Repairs were simple - fit two new diodes and a new switching transistor. In place of the 2SC1046L I use a 2S0350 or a 2S0380. Both have higher ratings than the 2SC1046L and my pref- DECEMBER1988 63 SERVICEMAN'S LOG take some time to find the real fault. Fortunately, they had a spare set. Circuit checks So it was back to the bench. I connected the CRO to the switching transistor collector, hoping I might detect some abnormality to account for what it now transpired were five transistor failures in a few weeks. Unfortunately, this only revealed that the waveform was a copybook version of that shown on the circuit. At this stage, I suspected that something might be generating the odd spike or spikes but, unless I was very lucky, I wouldn't be watching when they occurred. In the light of this there was little I could do except let the set run and hope that when it failed again - as I felt sure it would - it would provide some clue. In fact, the set ran for several days with no signs of trouble. Then, one afternoon, I left · the workshop for a few minutes and returned to find it dead. I pulled the faulty transistor out, found a couple of the diodes which had gone with it, and replaced these as a matter of course. But there seemed little point in simply 64 SILICON CHIP fitting another transistor and waiting for it to fail in a few days. I had to make some effort to find the cause. More in desperation than anything else I decided to examine the heatsink mounting and the associated connections to the board, in case an intermittent connection might be generating spikes. So I unbolted it from the board and carefully inspected the mechanical assembly. I drew a complete blank; there was nothing to arouse the slightest suspicion. But the exercise did prompt another thought; what about the mica insulating washer between the transistor and the heatsink? The best view of it in situ revealed nothing so I pulled it out for a closer look through a jeweller's loupe. And there it was; what I had mistaken for a tiny spot of discolouration which often occurs in mica - was in fact a pinhole. It was so small that I doubt that more than the point of a pin would have passed through it. Well, we seemed to be getting somewhere at last. I fished out a new washer, examined it under the glass just to make sure, then fitted everything back together. The set came good immediately and I settled down for another soak test. Another splat It was a brief test. The set hadn't run for more than a couple of hours when there was a sudden "splat" and it went dead. My heart sank: not only had I not fixed the fault but I had probably done in another transistor. And these are not the 10-cent variety- they run closer to $10, even at trade prices. Closer examination brought some consolation; the mains fuse had blown, but the transistor and diodes were still intact. What was more, there was a clear mark where the flashover had occurred; from the transistor case (collector) to the heatsink, across the edge of the mica washer. And it was then that I realised that the new washer was smaller than the one it had replaced. Both were nominal TO3 washers but the original had been a good 5mm larger all round than the transistor case, while the replacement was a neat fit. I must confess that I was somewhat puzzled by this turn of events. Was the design so critical that only an oversize washer would withstand the normal voltages in this part of the circuit? There seemed to be only one way to find out; fit an oversize washer and try again. After some searching I found one; larger in fact than the original, being close to 8mm larger all round. So this was duly fitted and, muttering something along the lines of "Beat that, you b ... ", I switched on again. And again the set came up and played normally. For a couple of hours. Then there was another splat and this time it took out the transistor, plus one of the diodes. And again the weak spot was evident; the bolt holes in the washer were marginally larger than the previous ones and this was where it had flashed over. I was feeling pretty desperate ·1: ·ER.. UNIT I I ' C:307 RJO< 560 K c:i ,o CJII '"""' '"""' TC~'l9 '305 560K ,., i t C387 100., F . , ,,,'~ m -!- POWER-SUPPLY UNIT ':' ') OEFLl:"CTI ON Fig.1: the power supply circuit for the Sanyo Model CPT 6603 receiver. An obscure and frustrating fault caused repeated failure of the main switching transistor, Q361. and disheartened by now, but there seemed to be nothing for it but to heed the obvious lesson and try again. After a lot of searching I found a large washer with small holes, put everything back together and tried again. This time things looked a lot more promising. The set played for several days, during which time the lady customer was on the phone showing obvious signs of impatience. I explained that, hopefully, the fault had been fixed but that I wanted to run the set for a couple more days, just to be sure. Somewhat grudgingly, she accepted my explanation. So I ran the set for a few more days and then, with no hint of trouble, rang the lady and told her she could collect the set. And I did explain that I was making an additional charge - about $30 - purely to cover material costs - ie, the transistors. I would not have dared to try to charge for my time. She seemed to accept this and duly collected the set and paid the bill. I heaved a sigh of relief as the set went out the door. An irate customer It was a misplaced gesture. The set had been gone only three days before the lady was on the phone with the news that it had failed again. And, understandably, she was pretty irate. For that I could hardly blame her. From where she stood she had spent a lot of money (including whatever the other serviceman had charged), experienced a lot of inconvenience, and seemed to be getting nowhere. But what upset me was her threat to take the matter to consumer affairs; in fact, she intimated that she had already contacted them. Assuming that this was not just bluff, and knowing consumer affairs, I imagine that she would have been advised to take the matter back to me, then contact them again if I refused to co-operate. Since I had never had any such intention I wasn't particularly worried about the threat but I was an- noyed. I felt that she might at least have consulted me first. And I spelt this out, adding that I was perfectly willing to stick with the problem, at no additional cost, until it was solved. She calmed down a little after that and agreed to bring the set in again. Naturally it was exactly the same fault, except that there was no evidence of a flashover from transistor to frame. I'd beaten that but it was now clear that this was only a symptom. And it was also clear that I needed help. So I rang the Sanyo service department and eventually contacted a technician who seemed to be familiar with the fault. We had a long discussion and out of it came a list of components which experience had shown were likely suspects. These were: C361 (2200pF) in series with R361 between emitter and collector of Q361; diode D361; capacitors C332 (22µF) and C351 (220µF), connected to terminal 13 of the converter transformer (T361); and the main filter DECEMBER1988 65 Problems? . SERVICEMAN'S LOG ' .. 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ELECTRONICS PTY LTD PO BOX 191, LIDCOMBE, NSW 2141 66 SILICON CHIP W~'S\½ER M\CA ~£'RE. WAS A G~~~ MARK ~~E 1ME: FLASHOVE."R \.\AD CXCOR~E.'t); FROM -rME -r'R.AN~\S"f"OR CASE. 1'0 "ffie. HEP«'SINK, AGRO~S 1'\E. E-'PGE 0~ 'THE MICA WAS\-\~,, ... capacitors, C310 and C311, both 100µF. But he'd kept the best bit until last. A prime suspect was the trigger transistor, Q321. This should be replaced as a matter of course, along with any othe~_r of the aforementioned components which tested less that 100% or were doubtful for any reason. I thanked him very much and then, as he wished me luck, he joked; "If that doesn't fix it, - you're on your own." A comforting thought! But at least I now had a few leads to follow. I started with C361 and D361. Both checked OK but I replaced them anyway; after all, if a minor component has to be pulled out to be tested, it is just as easy to fit a new one in its place . When I came to capacitors C332 and 351 it was obvious that these had been replaced by the previous serviceman. Not only were they brand new but, for once, I was grateful for his style of soldering; at least I knew what he had tried. Similarly, the main smoothing capacitors, C310 and 311, had obviously been removed, though I couldn't be sure they'd been replac- ed. So I pulled them out and checked them. They both tested OK, well on the plus side in fact, so I refitted them. That left only the trigger transistor, Q321. Since my predecessor seemed to have been following the same path, I wondered whether he had received the same advice and had replaced this also. If he had, we were back to square one. Fortunately one glance at the board reassured me. There was no way that Butcher Bill could have removed that transistor and put it back the way it was. So out it came and in went a replacement. Then I fitted yet another switching transistor, put everything back together, and switched on again. It was no surprise when the set came up with perfect picture and sound; that had happened many times before. The real test was whether it would last. I ran the set every day and all day for the next fortnight and it never missed a beat. But as vital as this test was, I couldn't convince the customer of that. In spite of the fact that they had a spare set, she was on the phone every couple of days wanting to know when it TETIA TV TIPS Kriesler 59-4/5/6 Telecommander Remote Control Symptom: At switch on will select channel 6 sound only at low volume. No picture . When any channel button is pressed, sound increases to normal but still no sign of picture . Cure: IC1320 {TDB1030) faulty. Although IC1380 does the channel selection , it is IC1320 that does the actual switching and a fault in this chip cuts off the tuning voltage. would be ready. And no amount of explanation on my part seemed to penetrate. But at the end of a fortnight I reckoned that the point had been proved, at least to the extent that it could ever be proved, and I returned the set to her. I made no charge and urged her to contact me if there was further trouble. This was accepted with moderate grace but without any enthusiasm. Well, that really is the end of the story, except that several months have now passed and I have checked the situation several times during that period. And each time I have been assured that all is well. So it would seem that Q321 was the real culprit. Of course it's easy to be wise after the event but it is now obvious that the flashovers around the transistor were merely symptoms of the fault that was generating destructive spikes. The fact that I fitted washers marginally different from the original simply served to produce a nice red herring; one which cost me dearly. So why did Q321 generate those spikes? I haven't a clue - and neither, it would seem, does Sanyo. Suffice it to say that it is a well documented fault and one worth noting. Perhaps this story may save someone else the run around. Another mystery And while we are talking about mysteries, here is another one to mull over. It is about a brand new National colour TV set, model AW A Thorn 3500 Series Symptom: Very low, wedge shaped picture with very bad convergence. All voltages normal and vertical output waveforms also normal. Looks very much like shorted turns in the yoke. Cure: Break in track on convergence board, between Plug 25 Pin 6, and L7 54. This track passes close to one end of R751, a known source of dry joints and the break was at the closest point to the resistor. TC2959, only recently released. This is a most impressive set, featuring a 74cm (29in) screen, a frighteningly elaborate remote control system, and a price tag of about $2000 to match. It also boasts the best picture resolution that I have ever seen on a colour set. Shortly after its release the makers issued a modification sheet concerning a corner purity problem. This was due to insufficient degaussing current due, in turn, to the type of degaussing thermistor originally fitted. It recommended fitting an alternative type, Part No. ERPF5BOM12OG. I have modified a few of these sets and the modification works well, even without any external degaussing. So I wasn't worried when one of my regular customers phoned to say that they were having trouble with this model set, the customer's description of the fault being that " ... the colour's gone all funny". She explained that the set had been working fine when they had gone away for the weekend but had "gone funny" when switched on after they returned. I imagined that it was just another modification job. In fact it turned out to be one of the worst purity problems I have ever seen; the whole screen was a bright random pattern involving every colour of the rainbow. There was no way that this was the problem I had anticipated. Nevertheless, I went through the motions of checking the thermistor, confirmed that it was the old type, Circuit Notebook ... continued from page 59 plugpacks up to 15VAC are suitable. Either way, the source should be able to supply about 50mA or more. The circuit operation relies on the fact that the LM317 maintains 1.25V (nominally) between its adjust terminal and the output terminal. By connecting a resistor between these terminals it can thereby be made to function as a precise constant current source which is independent of input voltage or load voltage. Two resistors in series, 22n and 2.20, set the constant current at close to 50 milliamps (actually 51.6 for a regulator voltage of 1.25V}. Up to six cells in series can be charged. Charging time is 14 hours (unless otherwise specified). If you have 400mAh AA cells they should be charged for 14 hours at 40mA. To set the ch.arging charging current at 40mA, change the 2.20 to 8.20. 116 TV TECHNICIANS & SERVICE COMPANIES Have your PHILIPS V ARICAP TUNERS (ELC2060) Repaired or Exchanged Only $17 .00 ea. Ten or more $15.00 ea. Quick Service 3 Months Warrc1nty T.V. Test Equipment Shorted Turns Tester (Silicon Chip, Sept. 88) $65.00 ·+ Tax Hi-Voltage Probe 0 · 50kV. $70.00 + Tax Send Faulty Tuners Cheques and money orders + P&P $2.00 or C.O.D. -:r.v.Tu IIERs) 216 Canterbury Road, Revesby 2212, Sydney Australia Phone: {02) 77 4 1154 DECEMBER 1988 67 presto! - the problem vanished; I had a first class picture with as high a degree of purity as I have ever seen. So what was going on? My first reaction was that there had to be some factor in the customer's home which I had overlooked, such as audio speakers which might have been moved close to the TV set. But even that assumption posed as many questions as it answered. In order to change the thermistor, I had pulled the set into the centre of the room, where it was quite clear of any other objects. So why, in that situation, had my degaussing wand lost its magic? Spurious magnetism and fitted a new one. All of which had no effect whatever on the problem. This didn't surprise me and I simply reached for the degaussing wand and gave the front of the set a once over lightly. But that didn't help either. When I switched the set on again the ef~ feet was as bad as ever, although I did get the impression that the actual pattern had changed. So I tried again, being more careful this time and since the back was still off the cabinet, I went right over the inside, poking the wand in wherever it would go. Still no joy. I went through the ritual several times but seemed unable to make any impression. The only noticable effect was that, if I left the set running, I could observe a changing pattern but with no hint of reduction. Faulty picture tube That really rocked me. There was only one possible explanation in my experience and that was a faulty picture tube. The fault is rare but I have experienced it. It is caused by a faulty spot weld securing one corner of the shadow mask, allowing the mask to move back and forth and upset the critical adjustment of the whole system. And a 68 SILICON CHIP symptom is that any attempt to degauss the set simply changes the pattern. It was also a situation which called for some diplomacy. The people involved were very understanding types but I hesitated to suggest that their brand new $2000 set had a faulty picture tube. While it would be fixed under warranty, they'd probably worry about a picture tube failure for years to come! So I simply suggested that the fault needed workshop attention for a few days. They still had their old set, a Philips K9, which would serve in the meantime. And so the set finished up in the shop. I left it for a couple of days, partly to think about the problem and partly to organise a discussion with the National service department. It was just possible that they may have already encountered a problem with this picture tube. But when I put it to them I drew a blank. While they were well aware of such a possibility, they had not had any such problems. So I was on my own. At the first opportunity I gave the set another try. It displayed exactly the same pattern as before and, on an impulse, I gave it another once over lightly with the wand. And Nevertheless, I rang the customer and asked her whether there was such a speaker system and whether it might have been moved close to the TV set. Yes, they did have such a system and one speaker normally sat alongside the TV set. In fact, it had been sitting there when I called. I realised later that if it had had a mouth it would have bitten me. I was just too intent on the TV set to notice it and, as I said, I had pulled the set clear anyway. More to the point the lady was quite adamant that nothing which might be involved had been changed. The set was working perfectly when they left the house on the Friday night, the house was empty until they returned on the Sunday night and, at first switch-on, the set was faulty. There was nothing for it but to return the set to the customer and test it in its original environment. The answer - nothing. Nothing untoward, that is. The set behaved perfectly, even though I had taken the trouble to ensure that it went back exactly where it had been, relative to the speaker. Finally, I moved the speaker all around the set and did everything I could, short of shoving it inside the cabinet, to create the fault. Nothing worked; nothing I could do would re-create the problem. And the set has been running for many weeks now with no hint of trouble. So the mystery remains. Any ideas, anybody? ~