Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG Sets aren’t made of rubber, but... Nobody likes to have a set bounce. But let’s face it; it’s an occupational hazard. It happens to all of us sooner or later but it’s still a blow to our professional pride &, poten­tially, to our reputation. Occasionally, a set bounces by reason of our own careless­ ness or lack of experience with a particular brand. But most of the time, it is just plain bad luck. A second fault occurs short­ly after the set is returned to the customer, probably producing similar symptoms, and the customer expects an explanation. To be fair, most customers are reasonable but once in while one will go off his brain. And it sometimes takes fair bit of diplomacy to quieten them down. But they are not the worst. The worst ones are the ones you don’t hear about, except much later on the grapevine, when the damage to your reputation has been done. Naturally, all those thoughts were prompted by a recent experience. In fact, none of these nasty things happened but they could have, and it served as a reminder that this threat is always there. The story is about an AWA model C3423 colour TV set, a 34cm model which is actually made in Korea by Daewoo. It belongs to one of my long-standing customers. His complaint was straightforward enough – distorted sound on all channels – and I imagined the cure would be quite simple. And initially, this appeared to be the case. When checked on the bench there was no doubt about the validity of the complaint; the distortion was really severe. And, as I had expected, the cause was simple enough; fai­lure of one of the two transistors in the audio output stage. These are designated on the circuit as Q601 and Q602 and both carry the type number KTC2230Y. In this case it was Q601. Fortu­nately, I had a replacement in stock but it appears that a 2SC2230 is, as far as I can determine, the same device, the KT prefix and Y suffix being a Korean version. Anyway, I had the specified type number, so I simply fitted it. And that cured the fault. I finished the job late in the afternoon, and left the set running on the bench for an hour or so until I closed the shop for the night. When I switched it on again the next morning, it performed quite normally and so I rang the customer with the good news. I subsequently unplugged the set and pushed it aside when I needed the bench space but later turned it back on again to demonstrate it to the customer when he called in. It’s back again Fig.1: the audio output stage in the AWA C3423 colour TV set. The audio drive comes from pin 3 of IC101 (top) & is applied to the base of Q602 which apparently operates as a single-ended class-A stage, with Q601 as a cascode. The output appears at the junction of Q601 & Q602 & is fed to the loudspeaker via a trans­former. 56  Silicon Chip So that was another job finished – or so I thought until it bounced. A couple of days later, the owner was on the phone with the bad news that the sound was still distorting. He was quite reason­able about it though, because he realised that it wasn’t exactly the same fault as before. While the original fault was obvious the moment the set was switched on, the set would now run normally for an hour or so and then would gradually begin to distort. At the end of about two hours, it was really bad. And I gathered that the owner had prob­ ably been trapped in the same way I had been, by initially using the set for relatively short periods. So the set finished up back on the bench. Initially, I let it run for about two hours, by which time it was quite intoler­able. I then decided to check the audio feeding the output stage, on pin 3 of IC101. This was easy enough to do using a small audio signal tracer and it confirmed that the signal was perfectly clean at this point. My next thought was to make some voltage checks but I didn’t have much to go on. The circuit is one of those that a colleague calls “a street directory with no street names”; or, in this case, no voltages. Well, there was one, the supply rail to this stage, at 103V. Assuming this figure was correct I reckoned there would be about 50V across each transistor. It also seemed reasonable to expect that there would be around 0.5V or 0.6V between the base and emitter of each transistor. So in spite of the circuit limitations, I was able to build up a fair picture of the likely voltages. After allowing the set to cool down, I switched it on again and confirmed that these voltages were correct. The supply rail measured the indicated 103V rail, there was roughly 50V across each transistor, and there was about 0.5V between the base and emitter of each transistor. Having confirmed this, I let the set run until the distortion reappeared, then made another voltage check. It was a different story this time. While the other voltag­ es remained as before, the base-emitter voltage of Q601 had dropped significantly. I left the meter connected and let the set run. The voltage continued to drop as the distortion increased until, after about two hours, it had dropped to a mere 0.05V. Well, that was a clue but that was all it was; I still had to find the cause. Fortunately, there is only a handful of components in this section: six resistors, six capacitors, and the two transistors. I was inclined to ignore the transistors. After all, Q601 had just been re- placed and the chances of two failures in a row seemed remote. But statistics can let one down. I had more spares on hand and it was only a few minutes work to change both. And that promptly ruled out that possibility; it made no difference. The resistors did not seem to be a high risk but were easy to check anyway. And again I drew a blank. That seemed to leave only the capacitors – two low value plastic types and four electrolytics. Of the latter, C608 (22µF) served as a decoupler for the 103V rail. However, I couldn’t relate a fault here with the observed symptoms. All things considered, including the change in Q601’s base-emitter voltage, the most likely suspect was C610, a 3.3µF cou­pling capacitor to the loudspeaker. It was an electrolytic, of low value, and in what appeared to be the fault area. It was simple matter to pull it out and test it. Its ca­pacitance measured 3.3µF as marked and there was no significant leakage. But it was just as easy to fit a new one anyway, whereupon the set produced good clean sound. More importantly, it April 1995  57 continued to do so for the rest of the day, after which I consid­ ered the point proved. So I’m not sure what was wrong with the capacitor. Normal­ly, there are three likely faults in a capacitor: loss of ca­pacitance, leakage and internal series resistance. Since it appeared to have correct capacitance and no leakage, that left only internal resistance, which is not quite so easy to measure. On the other hand, there seems little doubt that it was a temper­ature sensitive fault and it is sometimes difficult to duplicate the exact temperature conditions when making measurements. So, all things considered, I’d put my money on leakage. After all, one side of it connects via the output transformer (T601) to the 103V rail and the other side to Q601’s emitter. So, if it was leaky, the effect would be pretty drastic. So it all ended happily. But it was a nasty trap and I’m not sure whether there were two quite separate faults or whether the faulty capacitor was the cause of Q601’s failure in the first place. In any case, I fell into the trap. With the benefit of hindsight I should have given the set a longer soak test. But this is not always convenient and 58  Silicon Chip there were no symptoms to suggest that it would be advisable. How does it work anyway? Finally, having solved the problem, I couldn’t help but wonder about that output stage configuration. It is not an uncom­mon arrangement and I must have looked at it many times in vari­ous makes and models of sets. And despite having replaced faulty components in these circuits, I have never bothered to think much about the arrangement. Until now, that is. It must have been the need to service it twice in quick succession, and the need to work out voltages, which prompted me to start wondering about how it operates. The first point to note is that the two output devices are of the same type number and, therefore, of the same polarity. Compared with the popular complementary symmetry pair configurations, I find this arrangement puzzling. And the more I look at it the more confused I become. I simply cannot grasp how the circuit works. And those colleagues I have consulted appear to be equally as confused. Some made suggestions based on other circuits with which they were familiar but nothing seemed to add up. As already noted, the two transistors are effectively in series in the DC sense and operate from the 103V rail. The audio drive is from pin 3 of IC101 and the output is taken from the junction of the two transistors and capacitively coupled to the speaker transformer, the other side of which connects to the 103V rail. It also appears that the output is at relatively high im­pedance, hence the speaker transformer. There is also a feedback network into pin 2 of IC101. Beyond that, it is not clear how the circuit works. It would appear that Q602 operates as a single-ended class-A stage, with Q601 as a cascode. But the biasing arrangements for Q601 are something of a mystery since the base of this transistor is tied one diode drop below its emitter. So there it is; an ultimately successful job but one which left a frustrat­-ing circuit puzzle. If anyone can throw any light on this circuit, I would be happy to pass it on to readers. In the beginning My next story takes us back a few years; some 20 years in fact, to the beginning of colour TV in Australia in 1975. More particularly, it involves Fig.2: the power supply for the Kriesler 59-1. The two mains fuses (F101 & F102) are at left, while fuse F120 is to the right of the bridge rectifier. TR120 is the chopper transistor. one of the first colour sets of that era. I refer to the model 59-1 made by Kriesler which, in vari­ous modified forms, was popular for many years. And while this particular set may not necessarily be 20 years old, it would be pretty long in the tooth. It belongs to a lady customer who moved into my district a couple of years ago She first sought my assistance about a year ago. On that occasion, the main problem was due to some dry joints, of which this set had its share. In addition, I made a routine modifica­tion to permit the set’s use with a video recorder. It had been a long time since I had done this and I had to dig out the appro­priate modification note to refresh my memory. The modification involves the horizontal oscillator cir­cuit. In greater detail, it involves modifying the time constant of the automatic frequency control (or flywheel sync system). In these early Kriesler sets and in some Philips sets of the same era, before the advent of the domestic VCR, this time constant was relatively long. This was perfectly satisfactory for the highly stable off-air TV signals but was too severe for some video recorders. The modification is relatively simple. It involves the Line Control Unit (CU701) and pins 3, 10 & 11. Pins 3 and 10 must be connected together, while pin 11 is connected to chassis. With that done, and the dry joints repaired, the set was returned to the customer. When it came in this time round it was completely dead and I had a gut feeling that it was power supply failure. There was no life of any kind; not even a hiccup to suggest an overload shutting down the power supply. My first check was at the fuses. The two mains fuses (F101 and F102) were intact, but fuse F120, a 2A type between the bridge rectifier and the chopper transistor (TR120), was blown. So it looked like a fault on the board itself, most likely TR120. Fortunately, I still have a fair stock of boards for this model, salvaged from sets scrapped for other reasons. So it was a relatively simple job to pull out the power supply board and substitute a known good one. This would at least confirm my suspicion and clear the rest of the set. And it did; the set came to life immediately and put up quite a creditable performance, considering its age. Even the picture tube looked as though it was good for a few more years. OK, so the fault was on the power supply board. If it was as simple as I suspected, it would be well worthwhile repairing. Naturally, I went straight to the chopper transistor pins, on the underside of the board. And a quick check with the meter con­firmed my suspicion – it was shot, base to emitter. I unscrewed the mounting nuts, then turned the board over to pull the transistor clear. And this was the first hint of something unusual. One glance was enough to indicate that there had been “a certain amount of mucking about going on”, as one of my colleagues often puts it. Sticking out from under the transis­tor were some pieces of black insulating tape as used by electri­cians. It was now clear that TR120 had been replaced on a previous occasion. This was no surprise – faults of this kind are common enough in all sets. But the nature of the repair was. The insu­lating tape had been used in place of the isolate mica washer that’s used to separate the transistor from its heatsink. In fact, two strips of tape had been used, with one overlapping the other to provide the necessary width. A real shocker Such a bodgie repair was a real shocker. At that stage, I had no idea when, or by whom, the repair had been done. I could only assume that someone had been caught out in the field without a washer and had taken this way out to do a quick repair and avoid a return visit. Well, that would be an explanation, if not an excuse. But it is a pretty rough approach. For one thing, as we all know, insulation tape degenerates with time, particularly in a heated situation such as this. And, in any case, it would provide very poor thermal con­duction compared to a standard mica washer. The standard washer is made as thin as possible, consistent with adequate electrical insulation, in order to provide maximum thermal conductivity, usually aided by a heatsink compound. Insulation tape is thicker and, in this case, there was a double thickness of tape where the two strips overlapped in the middle of the transistor between the two pins. In fact, I took a few minutes off to check these thicknesses with a micrometer. A typical washer is of the order of .005in, while a single thick­ ness of this tape was .008in, making a double thickness of .016in (pardon the imperial measurements; my micrometer goes way back.) So the poor old transistor must have been running much hotter April 1995  59 SERVICEMAN’S LOG – CTD Fig.3: a previous “serviceman” had isolated the chopper transistor using two pieces of electrical tape instead of a proper mica washer. It’s a wonder it lasted as long as it did. than it should have been since the repair was made. Naturally, I fitted a new TR120, complete with the correct washer, whereupon the set came back to life. There had been no other side effects from the failure. But the bodgie repair raises the question as to why this transistor failed. Maybe it was due to fail anyway but there are two far more likely possibilities. One was that there had been an electrical breakdown between the transistor case and the heat­sink, as the tape did not fit too snugly around the mounting bolts. Alternatively, the lack of adequate heatsinking may have finally taken its toll. Who did it? But regardless of the reason, that is no way to repair a TV set. I was curious as to how it had happened so, when I rang the lady to advise her that the job was finished, I raised the matter of the previous service – after all, it did involve the same component. In fact, she was most helpful. It transpired that, before moving into my area, the set had been covered by a service con­tract with a large service organisation. And when she came in to collect the set, she brought all the relevant documents with her, including the job sheet for the service in question. And this produced another surprise. There was no suggestion of an emergency repair in the house, as I had envisaged. Accord­ing to the dockets, it had been taken to the company’s workshop and the job done there. So 60  Silicon Chip how on earth could such a bodgie job be justified? The documents also pinpointed when the job had been done, which was about six years previously. So it had lasted rather longer than I would have expected. But that’s no excuse. What firm was it? No, I’m not saying. I’ve seen and heard only one side of the story. There could be an explanation which completely absolves them, so we’ll let it rest there. But it was a nasty act on somebody’s part. The intermittent VCR And finally, here is a story from a reader, J. S. of Portarlington, Victoria. Here’s how he tells it: After reading the Serviceman’s Log in the August issue of SILICON CHIP about the NV-370 and NV-600 VCRs, it rekindled my memory of an NV-470 I had fixed two months earlier. This was one of those intermittent faults. Don’t you just love those? This particular problem seemed to involve the power switch. At times, one could keep pressing it and get no response whatsoever. Even shaking the whole unit, or prodding the board around the power section, would not revive it. And then, for no apparent reason, it would come good and remain so. Once again I repeated the shake and prod tests, with no result. I waited for it to reappear of its own volition. When it did, I took the covers off and removed and replaced a couple of the 3-pin wire connectors (PJ1003 & P1002 ) on the power section of the board. And bingo, the problem vanished. A dirty connector? I subjected the unit to another shaking and prodding test and, as it did not fail again, I more or less accepted that this could have been the cause of the problem. At that time, I did not have a circuit diagram with which to check the layout. But not being 100% satisfied that the problem was solved, I kept it for further observation. Sure enough, some eight days later it happened again. It was time to get serious and get a copy of the circuit. Thus equipped, I realised that at least one of the connectors I had changed, PJ1003, had little to do with the power supply circuit. On closer examination of the copper side of the board, in the power supply region, I noticed some discoloration, apparent­ly due to overheating, around transistor Q1001 (2SD­1275), the voltage regulator for the 12.7V rail. At the same time, I had my finger on Q1001’s heatsink and as I applied pressure, it sank towards the board. My interest aroused, I wiggled it and watched it from the solder side. The unit was plugged in at the time, and I noticed arcs being emitted from Q1001’s collector and its copper track. Sure enough, the fault could be induced and corrected by wriggling Q1001’s heatsink. Closer examination of the copper tracks around Q1001 revealed that the collector track had broken due to the size of the heatsink. This was attached directly to the transistor body, without any anchoring pins into the board. Q1001’s base and emitter pads were also beginning to lift off the board. I soldered a substantial piece of tinned copper wire to each lead of Q1001 and along their corresponding copper tracks, which gave the transistor and its hefty heatsink a solid base. Hopefully, this will solve the problem for the life of the unit. This fault clearly illustrates that one should always start any diagnosis with a thorough visual inspection. The telltale signs could be very time saving, as in this case, especially as I was looking around and at the fault right from the start. Thank you J. S. for an interesting story. Your point about a thorough visual inspection is well taken. I’ve been telling myself that for years but SC I still get caught.