Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG Well, it looked like that at first It is probably just as well that neither I nor my col­leagues are medical practitioners. At least if we make a wrong diagnosis, we usually get a second chance & the first error can be corrected. My first story this month is about a video recorder – a Panasonic model NV-SD10A which belongs to the local primary school. This is a fairly recent model, released only a couple of years ago. And while it has nothing to do with the story, it is worth mentioning that it is fitted with the very much upgraded “K” deck. This deck is a considerable improvement on earlier decks which, in most brands, employed some five or six belts, and possibly three motors, to perform all the necessary functions. And as can be imagined, the belts were a common cause of pro­blems. The “K” deck has eliminated most of the belts – it uses only one if my memory is correct – and has proven to be a very reliable unit. But that is by the way. This particular recorder had other problems. According to the teacher who brought it to the shop, it could not be turned on. Well that seemed to be a straightforward enough description, even if a little quaint. I would have simply described it as completely dead. And it was too. I plugged it in while the teacher was there and there was no sign of life of any kind; no clock display or indicator lights and no response to any of the function buttons. All of which was deceptively simple. I imagined a fairly obvious power supply fault, or even just a faulty fuse. So the teacher left it with me adding, as he left, that there was no particular hurry for the machine; they had others that could fill in. Power supply checks It was a day or so before I could get at it, then I went straight to the power supply, which is a switchmode type. A preliminary check revealed nothing obviously wrong. The fuse was intact and mains voltage was reaching the right terminals. Next, I checked the various voltage rails out of the supply (45V, 14V, 12V, 12.3V, 5V, -29V, etc). And they were all present and cor­rect. That put a completely different slant on things. If the power supply was delivering all the necessary voltages, then the failure was in some other section. Fair enough but how would a failure in one section shut the whole thing down, causing it to appear completely dead? The most logical answer was that the fault was somewhere in the management section, possibly in the microprocessor (IC7501) – see Fig.1. This determines and initiates the correct sequence of events for any user command. It also drives the clock and other front-panel displays. Of course, this was simply a broad assumption. Exactly where or how this failure was occurring was what I had to track down. What followed was a long and laborious voltage checking procedure. Fig.1: a section of the microprocessor control circuitry in the National MV-SD10A VCR. IC7502 is at extreme right & its output at pin 3 connects to pin 21 of the microprocessor (IC7501) at top left. The 4.7V shown at pin 3 of IC7502 somehow becomes 4.8V at pin 21 of IC7501. 68  Silicon Chip As far as possible, I tried to confirm that the vari­ous voltages at the power supply were being applied to all the points where they were supposed to be. But, in particular, I concentrated on the voltages applied to the microprocessor. This eventually provided a clue. Pin 21, which is labelled “Reset”, is marked as 4.7V but, in fact, was showing only about 2.4V. Back tracking from there brought me to IC7502, a PST7026. This 3-legged device is similar in appearance to a small signal transistor but is rather more complicated, the circuit diagram showing that it contains a couple of op amps and various other circuit blocks. More to the point is its function. I wasn’t sure of this at the time. All that the circuit indicated was that it is fed from one of the 5V rails – shown as 4.8V at one point and 4.7 at another - and delivers 4.7V to pin 21 of the microprocessor. But obviously, there had to be more to it than that. Its real job is a delay function. At mains (repeat mains) switch-on, it pulls pin 21 low momentarily, before applying the indicated 4.7V. This, apparently, is the “reset” function. Before I go any further, a word about some of the voltages quoted. It is not unusual to find quite silly voltage figures on many circuit diagrams. Usually, the differences are only nominal but I have known them to be quite significant and misleading. It was this kind of mistrust that confused matters later on. Nevertheless, I felt sure that 2.4V in place of 4.7V was too great a difference. And, with very little else in the cir­cuit, the IC seemed the likely culprit. It’s a very inexpensive device costing less than $2, so the easiest thing to do was change it. Unfortunately, I had none in stock, so I placed an order. And that was the next hurdle. They were on back order and it would probably be several weeks before delivery. Well, the teach­er had said there was no hurry, but I contacted him and explained the situation. He was quite understanding and confirmed that there was no great hurry at that time. It was just as well because it was over two months before new stocks arrived. Unfortunately, this is not a rare occurrence; it happens all too often these days. When my order arrived, I lost no time in fitting it. When I switched the machine on, it immediately burst into life. The clock and all other indicators came up correctly and a quick check of all the various functions indicated that they were working perfectly. Problem solved? So that solved that problem. Or did it? Out of curiosity, I went back to the output of the new IC, expecting 4.7V, or something close to it. In fact, it was only about 3.6V. And that was about as awkward a figure as one could imagine. While ob­ viously adequate to allow the set to function, it was less than shown on the circuit. So was the circuit wrong and what was the allowable toler­ance on this voltage, or the spread of the IC tolerance? I had no way of knowing, so I simply let the machine run for the next couple of days. During this time, I made and replayed several test recordings and switched the machine off and on at the mains at regular intervals. It never missed a beat. But I still wasn’t happy and was trying to decide what best to do when the teacher rand to say that their situation had changed somewhat. Some programs that they wanted to record clashed in time and they needed all their machines. Could they have this one back some time in the next week? I explained the situation: that the July 1995  69 SERVICEMAN’S LOG – CTD but, worse than that, I couldn’t find it; there was no sign of it anywhere near its electrolytic mate, adjacent to IC7502. Nor could I see it anywhere else on the board. In the end, I had to resort to tracing the copper pattern on the PC board. And that’s where I found it; a surface mounted type behind the micro­processor (IC7501). I put the meter across it in-situ and measured something over 1kΩ. That wasn’t conclusive, of course, so I pulled it out and switched the machine on. It leapt into life and, more to the point, I now had close to 4.7V on pin 21. Problem solved. I’m not sure of the exact role of this capacitor and the recorder seemed quite happy without it. But, of course, it had to be replaced. I didn’t have a surface mounting 0.1µF capacitor, so I substituted a small disc ceramic type which I’m confident will do just as good a job. Or perhaps even better. No, this wasn’t one of my better efforts but I’m telling the story for the benefit of any reader who encounters a similar problem in this model. Life’s little mysteries machine was working, apparently quite reliably, but that I had some reservations about the job. If he wanted to take it and try it in those circum­stanc­es he was welcome. After all, it might just as well have a trial run at the school as on the bench. The machine returns And so the machine was duly collected. And, as I later learned, it performed faultlessly during their special recording sessions and for a couple of weeks thereafter. Then the teacher was back on the phone again with the news that it was exhibiting exactly the same fault as before. So I told him to bring it back in. He was rather worried about the cost but I assured him that I would stand by the job, as the real fault had not been found. Naturally, I went straight to the output of IC7502 and confirmed what 70  Silicon Chip I feared. It was down to 2.5V again. So what now? I took another look at the circuit. Assuming that the re­ placement IC was not at fault, there appeared to be only two things that could be loading the voltage on pin 21 and pulling it down: (1) a fault in the microprocessor (IC­7501); or (2) one of two small capacitors shown connected to pin 3 of IC7502. I tended to discount IC7501, if only because it performed normally when fed with the correct voltage. And that left the two capacitors – C7511 (a 0.22µF 50V electrolytic) and C7510, shown simply as 0.1µF. Well, if it was going to be one of these, it would be the electrolytic. Wrong again; it took only a few moments to pull it out and fit a replacement. There was no difference. So that left only the 0.1µF as the last hope. I wasn’t very confident My next story comes from a colleague and is another of life’s little mysteries. The problem was eventually solved but with no really satisfactory explanation. This, more or less, is how he told it to me. The set was a National model TC2697 and the customer’s complaint was that the picture rolled occasionally. What a horrible word that “occasionally” is – how does one tackle an “occasional” fault? With difficulty might be the best answer. Anyway, all I could do was set it up and let it run, hoping that when it misbehaved I would see it and gain some insight into the cause. So that’s what I did but trying to keep one eye on it while working on other jobs is a near impossible task. It was a couple of days before there was any hint of trou­ble and then it was only a glimpse out of the corner of the eye. Did that picture roll? I wasn’t sure. But patience paid off; eventually it rolled when I was looking directly at it. It flicked one frame then, a few seconds later, it flicked two more frames. After that, it seemed to settle. I watched it for some time but it was rock steady. This didn’t help very much, except to confirm that the fault was in the set, rather than due to local interference. But I was no closer to even guessing what was causing it. In general terms, of course, I suspected a fault in the vertical deflection system, or a sync fault. Following this latter thought, I checked the hold control behaviour. Weak sync pules will normally make the hold control setting quite dodgy but not in this case; it was locking up solidly. I even contrived to run it on a very weak signal and it still locked up positive­ly. OK, rule that one out. The first real clue came by chance. The workshop happened to be very quiet – I wasn’t running the sound –and I was study­ing a circuit on the bench when I thought I heard a faint splat. At the same time, I thought I glimpsed the picture flick from the corner of my eye. I moved in closer and watched the screen directly. Sure enough, the picture flicked again and there was the splat at the same time. There wasn’t much doubt about it now; I had a problem somewhere in the horizontal output stage and this was triggering the vertical stage. Until now, I hadn’t even taken the back off the set, pre­ferring not to disturb anything until I had seen the fault. Now that I had seen it and had a clue, it was time to look inside. I set up a mirror so that I could watch the screen while looking in the back of the set. It was a good setup but didn’t help much. The picture flicked a couple of times and I could hear the splat but I couldn’t see anything. Naturally, I concentrated around the horizontal output transformer and the ultor cap, but to no avail. And again, I hesitated to disturb anything until I had pinpointed the source. This approach continued for several episodes, without any suc­cess. The next step was born of desperation. I closed all the blinds on the windows, turned out the lights and checked the inside of the set again. I didn’t have to wait long; the picture rolled and I heard the splat. I saw the light from the flash but not the flash itself. I had been watching the ultor cap but it wasn’t there. Anyway, after a few more rolls and splats I finally spotted it and it was quite tiny. It involved the horizontal output transformer but in a very strange way. The transformer is a fairly conven- tional type, consisting of a rectangular ferrite core made in two halves. The windings are on one half and the two halves are glued together and held with a strong steel clip, which sits in a groove in each piece of ferrite. And this spring is floating; it is not at earth or at any other potential. But the layout is such that it is within about a millimetre of an aluminium bracket-cum-heatsink, which is mount­ed on the chassis. This bracket carries the horizontal output transistor. And, by some mechanism, a charge was building up on the clip, eventually becoming strong enough to jump the small gap to the bracket. The set would then behave normally until the charge built up again. By what mechanism this was happening I don’t know. The only explanation that seemed reasonable at the time was that any piece of metal in a strong electric field can acquire a charge from it. I understand that this effect can be quite a problem for power supply linesmen working in the proximity of very high voltage power lines. If this was the explanation, then it seemed reasonable to cure the fault by simply connecting the clip to chassis. And no sooner said than done. I prised up one end off the clip, slipped a short length of copper braid under it, and connected it to chassis. And that fixed it – no more sparks, no more splats and no more rolling. I ran the set for a couple of days, positioned so that I could hear as well as see the fault, and felt confident that it would not occur again. The set was then returned to the customer. Complete failure Which was all very fine – except that, about three months later, the set failed completely. The cause – failure of the horizontal output transformer. Yes, I know, you told me so. Or did you? And, if so, I’d still like an explanation. The obvious one would be that the winding had broken down to the core. OK, but ferrite is supposed to be a non-conductor and, if the suggestion is that it could provide sufficient leak­age at the high voltage involved, then this is a new one on me. Otherwise, I might have decided to replace the transformer in the first place. But could I have justified the cost on the basis of the symptoms described. In hindsight, yes, although a new transformer isn’t cheap. The new one cost over $100 but that was all it cost the customer. I waived any labour charges in the circumstances. After all, fair’s fair. Well, that’s my colleague’s story. I must confess that it’s a new one on me. Doubtless he’ll know better next SC time – and so will I. July 1995  71