Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN’S LOG Lightning strikes thrice A popular superstition when I was a youngster was that various calamities – natural or manmade – always happened in threes. Well, that was according to the adults who knew all about these things. I never did believe it but a recent experience does have me wondering a little. The story is really about video recorders; and I use the plural term deliberately because it involves no less that four machines, all of the same brand – but different models – which turned up on the bench in quick succession. And three of them had suffered lightning strikes. See what I mean! The first one was a Panasonic NVJ1A, a run-of-the-mill model now about four years old and no longer in production. It is owned by one of my regular customers who, incidentally, is someth­ing of a computer buff, a point of some interest as it tran­spired. He came into the workshop one morning, pushed the machine across the counter with the cover removed and said, “Will you put a fuse in it. It’s stopped and the fuse is blown”, pointing to the offending component. Well, he wasn’t wrong; one glance at the blackened glass was enough to confirm that. But it was obvious from the way he spoke that he firmly believed that the fuse was the only thing wrong. And when I gently suggested that this was an effect, rather than a cause, he was quite reluctant to accept the idea. There is only one answer in such cases. I fished out an appropriate replacement, fitted it and applied power. Splat! – one fuse destroyed. After that, he didn’t argue. But it was only then that he told me about the storm and the fact that his power main had apparently been struck. And so he agreed to leave the machine with me. But I warned him that, if he was lucky, the damage would be confined to the power supply. If he wasn’t, it could be a lot more serious. I didn’t attack it immediately. There were other jobs ahead of it, the owner had indicated that he was not in a hurry, and I was not at all familiar with this power supply. I had a manual but had not had occasion to service this section before. All I knew was that it appeared to be fairly conventional switchmode supply. Strike two In any case, before I could get it on the bench, what should turn up but another Panasonic recorder; an NVFS90A. This is a very much up-market Super VHS model, with all the bells and whistles one can imagine – and a price tag to match. But for all that, its power supply is almost identical with the NV-J1A. And it had also suffered a lightning strike; not from the same storm but 40  Silicon Chip Fig.1: the power supply circuit for the Panasonic NV-FS90A video recorder. The components to be checked out included the mains fuse (F1101), the bridge rectifier (D1102), the starter circuit across the output of the bridge rectifier, the auto vol­tage selector IC (IC1101), and safety resistor R1125. from one a couple of days later. In this case, however, there were no obvious symptoms and the fuse was still intact. Its owner was anxious to get it fixed as quickly as possible. In all these circumstances, I decided to let the FS90A jump the queue. After all, I had to familiarise myself with the power supply on one machine or the other, so it might as well be on the more urgent one. The power supply is a self-contained unit which is housed in a metal box. It sits in the rear lefthand corner and is easily removed by undoing a few screws. The box itself measures about 150 x 50 x 65mm and the bottom cover can be sprung off quite easily, although the top can present problems. Not all models have the same top. Some, including these two models, cover all the box and are quite easy to remove. In other models, they are only about 50mm long and are quite tricky to remove. To make matters worse, there is nothing in the manual covering this procedure. The secret lies under a small plastic label, marked “AC IN”. This can be peeled off and it is then possible to see how the cover and body are slotted together. In all models, the construction is quite compact and this makes them a little diffi­cult to work on but not seriously so. As I have already noted, the mains fuse (F1101) was intact and there were no other visible signs of damage. Well, it didn’t take long to establish that bridge rectifier D1102 was open circuit; a rather strange fault in the circumstances – I would have expected a short circuit. Anyway, that’s how it was. And, since I didn’t carry this item in stock, it would have to be ordered. But what else could have been damaged? The next major component after the bridge rectifier is a small IC (IC1101), which is described as an auto voltage selector – more about this later. There was no conclusive way to test this IC without a new rectifier but I made a few resistance measurements across the appropriate pins and, judging by the transistor configurations shown within the IC, I suspected that it may be faulty also. So that was added to the order list. So what about the NV-J1A machine? Like the NV-FS90A, it would almost certainly need some replacement parts. Closer exami­ nation revealed one important difference between the FS90A and the J1A; the J1A was fitted with a protective thermistor (D1101) across the mains, a refinement the FS90A lacked, in spite of its up-market price tag. I hoped that this might have prevented further damage. And yes, this had broken down and taken out the fuse. So maybe that was all that was needed. I had a replacement on hand, fitted it, and tried again. No joy; another fuse for the garbage bin. I moved on to the bridge rectifier and confirmed that this was the real culprit – it had broken down. So another bridge was added to the order. Next to consider was the auto voltage selec­tor IC, which was a different type from that in the FS90A. Be­cause the J1A had apparently suffered a heavier wallop than the FS90A, I also ordered a replacement for this IC, just in case. So the order was duly despatched and a couple of days later I had the spares on the bench. Naturally, I went straight to the FS90A and fitted the new rectifier. I made a quick check but no joy. Well, I’d suspected the auto voltage IC anyway and it was easy to fit the new one, so that was the next step. In fact, when I pulled the old one out and compared measurements with the new one, my suspicions were confirmed. Unfortunately, when the new one was fitted there was still no response, so it was back to basics. Measuring August 1994  41 So that was something I had learned – the hard way – and is well worth jotting down for future reference. It could save an embarrassing bounce. Back to the J1A across the bridge rectifier showed 300V plus, which seemed logical, and so I moved on to pins 3 and 4 of the auto voltage selector IC, which should have had essentially the same voltage between them. In fact, it was zero. From there it was no great effort to find the culprit. R1125, a 2.2Ω safety resistor in the negative line from the rectifier, was open circuit. Well, that was no great problem; 2.2Ω safety resistors are a readily available component and I always have stock on hand. A new one was fitted and the machine came to life. A thor­ ough workout confirmed that all functions were performing cor­rectly and I gave the machine the usual clean and lube routine before ringing the owner with the good news. A nasty bounce He came in the following morning, which was a Saturday, and collected it. And I naturally assumed that that was end of that story. Not so – he was on the phone first thing Monday morning with the bad news. The thing was dead at first switch-on. It wasn’t a good way to start the week. All I could do was ask him to bring it back in and assure him that I would sort it out. Back on the bench, it didn’t take long to find the reason; the 2.2Ω safety resistor I had fitted had failed. I could find no obvious reason for this, so I fitted another one, switched on, and the machine came good. I put it through several on/off cycles – the previous failure had obviously occurred at switch-on – and it came good every time. 42  Silicon Chip I left it for a while to attend to other jobs, then tried again. Bingo! It was completely dead. And yes, it was the 2.2Ω resistor. But why? With no obvious clues I realised I needed help. I could spend a week trying to puzzle this out and more than likely be no further advanced at the end of that time. I rang one of my con­tacts at Panasonic and put the problem to him. His reaction was immediate. “What kind of safety resistor are you using?” I replied that it was a standard 2.2Ω 1W type such as one buys at the (electronic) lolly shop; the kind of thing everyone uses. “But not from Panasonic?” “No – does that matter?” “You fit a Panasonic type and you wont have any more trou­ble.” Sensing a certain amount of incredulity on my part, he went on to spell out the difference (more on that in a moment). In any case, I had little option; I ordered the type he nominated – several, in fact – and when they came to hand fitted one to the FS90A. And that really was the end of it; the machine has given no further trouble. The crucial difference So what is so magical about the Panasonic component? It is a wire wound type, as distinct from the more usual metallised types. And, although it is rated at only 1W, as are the other types, it is capable of withstanding a much heavier switch-on surge. And this particular power supply does have a heavy switch-on surge. This, in turn, is a byproduct of the auto voltage selector system. In the meantime, I had gone back to the J1A. I had already replaced the thermistor, established that the bridge rectifier had failed, and suspected that the auto voltage selector IC might be faulty. In fact, this latter suspicion proved to be correct. So the rectifier and IC were replaced and, on a hunch, I checked the 2.2Ω safety resistor. I wasn’t really surprised to find it had failed and was grateful that I had the correct replacement type on hand. And with all that attended to, the machine came to life. More importantly, it operated in all modes – an important point, because lightning strikes don’t always stop at the power supply. They can pick on odd components anywhere in the system and create faults which can be very difficult to track down. So the owners of both the FS90A and the J1A were lucky; the damage in either case could have been much more serious. Strike three Machine number three was an NVFS65A. It had also suffered a lightning strike and, as it transpired, had suffered more damage that the other two. And, of course, with the first two under my belt, I was feeling pretty cocky about this one, the power supply being essentially the same. But pride goes before a fall. I went through the same rou­tine: the fuse, the rectifier, the safety resistor and the IC, all of which needed replacement. But it still wouldn’t deliver voltage and I began digging deeper into the circuit. I didn’t have much luck initially. Panasonic suggested some likely components to be either tested or replaced, as did a colleague who was also familiar with these units. But none of the suggestions helped. So I was at a temporary dead end. I say temporary, because I was quite confident that I could track down the trouble, given time. But the owner had other ideas. He called in to see how things were going and I told him quite frankly that the job had proved far more difficult than thermistor across the mains and the surge protector at the power point. Yet it was still damaged. On the other hand, nothing else in the house was damaged, nor was there any indication that any of the surge protectors had been activated. And by “activated” I mean destroyed because, as far as I know, all these surge protectors are sacrificial devic­ es; once activated they have to be replaced. Fig.2: the power supply for the Panasonic NV-J1A video recorder is basically similar to that in the FS90A but also features a surge protection thermistor (D1101). I had originally anticipated. “Well,” he said. “What about fitting a new power supply?” This was the last thing I would have suggested. I told him that yes, I could do that, but it would prove pretty expensive. He shrugged his shoulders. “Hang the expense. If that will solve the problem, go ahead and fit it”. So that’s what was done. In fact, Panasonic didn’t supply a complete replacement; just a new board to fit in the metal case. And that put the machine back in operation. But while having the machine fixed was gratifying in one sense, it was somewhat frustrating in another. I was still keen to know what the problem really was and anticipated that I could probably retain the old board and solve the puzzle at my leisure. But that was not to be either. Before I could even raise the matter the owner indicated that he wanted to take it and, since it was his property, there was nothing I could do about it. So that one must remain a mystery. Surge protectors Still on the subject of lightning strikes, I mentioned earlier that the owner of the J1A was a keen computer buff. The significance of this is that he had become more than usually aware of the risk of power line surges, of whatever origin, to his precious computer equipment. As a result, he had fitted surge protectors to most of his power outlets, including the one normally used for the video recorder. So the recorder had two levels of protection; its own Machine number 4 Machine number four was a model NV-L20 and it came in shortly after the first three. It was completely dead also but there was no suggestion of a lightning strike. On the other hand, the failure did look as though it might be linked to a mains shut-down, at least indirectly. As the owner explained, the machine had been operating normally immediately prior to a planned maintenance shut-down by the supply authorities. All householders had been warned and there was no great hassle involved. However, when power was restored a couple of hours later, the machine was dead. I didn’t attach much importance to this initially, writing it off as mere coincidence. And it might have been too but what I found made me think. With the previous jobs still fresh in my mind I went through the power supply in short order; the fuse, safety resis­tor, and rectifier were all intact, with the usual 300V plus out of the rectifier. Nor could I find anything obviously wrong with the voltage selector IC. With all those items cleared, suspicion fell on the starter network. This consists of four resistors (R1102, R1103, R1123, and R1124) across the rectifier output and a 1µF 400V electroly­tic capacitor from the junction of R1103 and R1123. The arrange­ ment is broadly similar to many starter circuits used in TV sets. And a common fault is an open circuit or high value resistor. In this case, however, all the resistors checked out OK, leaving the capacitor as the prime suspect. And so it proved to be. When I pulled it out and checked it, I could get no reading at all on the capacitance meter. So that was an easy one; a new capacitor and the machine came good. SATELLITE SUPPLIES Aussat systems from under $850 SATELLITE RECEIVERS FROM .$280 LNB’s Ku FROM ..............................$229 LNB’s C FROM .................................$330 FEEDHORNS Ku BAND FROM ......$45 FEEDHORNS C.BAND FROM .........$95 DISHES 60m to 3.7m FROM ...........$130 LOTS OF OTHER ITEMS FROM COAXIAL CABLE, DECODERS, ANGLE METERS, IN-LINE COAX AMPS, PAY-TV DECODER FOR JAPANESE, NTSC TO PAL TRANSCODERS, E-PAL DECODERS, PLUS MANY MORE For a free catalogue, fill in & mail or fax this coupon. ✍     Please send me a free catalog on your satellite systems. Name:____________________________ Street:____________________________ Suburb:_________________________ P/code________Phone_____________ L&M Satellite Supplies 33-35 Wickham Rd, Moorabin 3189 Ph (03) 553 1763; Fax (03) 532 2957 August 1994  43 SERVICEMAN’S LOG – CTD I gave it a routine clean and lube and returned it to the customer. But where did the mains shut-down come into this? I have no doubt that the capacitor was on its last legs anyway and it is important to appreciate that power is applied to this part of the circuit at all times, whether the machine is being used or not. Only when power is turned of at the mains is the voltage across the capacitor removed. My theory is that this voltage contributed to a certain amount of “forming” of the capacitor – enough to maintain a small amount of capacitance which was sufficient to allow the system to start. Removing this voltage for a couple of hours was the last straw that sank the camel’s hump. But, of course, it’s only a theory. Auto voltage selector Finally, this might be as good a time as any to expand on the auto voltage selector system. It is designed to allow the machine to work on a very wide range of voltages, although there is no mention of this in the manual. The specifications simply say “230-240V, AC, 50-60Hz.” However, a practical test confirmed that it can be plugged into 110V (which I have available in the workshop) and still operate quite normally, without any adjustments. There is nothing especially new about this concept; it has been around in many TV sets for several years now, although the particular circuit configuration was new to me. Also, it is not something we think much about in this country, being blessed with a 240V 50Hz standard which is used virtually everywhere through­out the continent. But from a manufacturing and marketing point of view, the advantages are obvious. Not only does the one power supply suit all countries but it is even useful within some countries. This would be particularly so in countries like Japan, which has a variety of power supply systems in different areas. While the 110V 60Hz system is On Sale Now At Selected Newsagents Or buy direct from SILICON CHIP Price: $7.95 (plus $3 for postage if ordering from Silicon Chip). Order today by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 979 6503; or send cheque, money order or credit card details to PO Box 139, Collaroy, NSW 2097. 44  Silicon Chip the one most commonly used, many areas use 220V 50Hz. I also understand that there are some 220V 60Hz and 110V 50Hz systems. It all adds up to a real nightmare, not only for manufacturers and distributors but also for consumers who wish to move from one area to another. And to further complicate matters, the same type of power outlets are often used for both voltage systems. It doesn’t take much imagination to visualise SC the problems this can cause.