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SERVICEMAN'S LOG Neither cat proof nor kid proof Many faults in TV sets are due to human factors, or sometimes “cat factors”. Flower vases, cats, boisterous children and TV sets often don’t mix well together. This month, we were blessed with not one but two recent model Sony 53cm TV sets fitted with the current BG-2S chassis and barely out of the egg. It is unusual to be servicing such new sets (particularly two of them), as they would normally be covered under warranty with a Sony Service Centre. But these two came in by accident – literally. The first one was a KV-G21F2 belonging to Mrs White, who very foolishly left a vase of freshly watered flowers on top of the set, while she went shopping. And she left Pookie, the pedi­gree Siamese cat, in charge of the house. Pookie decided, in his wisdom, to leap onto the TV set, presumably on his way to climbing the brocade curtains. Unfor­tunately, there wasn’t enough room for both the flowers and Pookie on top of the TV. The result was inevitable, with most of the water finishing up inside the TV set. The situation was compounded when the lady switched the set on that night, to be confronted by a pyrotechnic performance on the wrong side of the screen, followed by complete silence and the sickly smell of something burning. The result was not only a technical problem but a diplomatic one as well. The lady’s hus­band wasn’t all that enamoured of Pookie at the best of times and now Pookie was really in the dog house – which is a dreadful thing to happen to any cat and doubly so for a sensitive pedigree Siamese cat. There was even talk of finding a new home for Pookie! Anyway, the upshot of it all was that, by the time I ar­rived on the scene, it was pretty horrible mess, with a surpris­ ing amount of corrosion. I suspect that the vase contained more than plain water, probably having been enriched by something designed to keep the flowers fresh plus some natural chemicals from the flower stems. I cleaned it and dried it as much as possible, then made a visual inspection. I was surprised to find that the main fuse (F601) was intact but R611, a 0.1Ω 0.25W resistor supplying power to pin 2 of switch­ mode transformer T601, was open circuit. From there, I traced the circuit from pin 5 of T601 to pin 1 of IC601, an STRS6708. And this IC had broken down completely – it had obviously gone short circuit and taken R611 with it. Before replacing these parts, I made a few checks on the various rails and found a short on the main 114V rail, off pin 13 of T601. This short could be anywhere but I decided that, for August 1998  27 Serviceman’s Log – continued the moment, I would start close to the power supply and check from there. I opened the rail by disconnecting resistor R131 and inductor L802, then bridged this gap with a 100W globe, with a voltmeter across it. With the short isolated, I replaced R611 and IC601 and was rewarded with normal power supply operation and a 114V rail, up to the 100W globe. But I still had to track down the short. This wasn’t hard; the horizontal output transistor, Q802 (2SD1878), was the logical suspect and sure enough, that was it. I replaced Q802, removed the 100W globe, reconnected R131 and L802, switched on and the set came to life. Well, sort of. The first problem I observed was intermittent vertical scan. I let it run for half an hour or so to see what would happen and whether I could pinpoint any likely cause. I couldn’t and the vertical timebase problem only became worse until it failed completely. Again, I picked on what ap­peared to be the most likely suspect – the vertical output IC (IC551, LA7830). I was right once again but I must admit that it took no great mental effort – the whole thing was 28  Silicon Chip messy and badly corroded. Replacing it cured that problem. But I wasn’t out of the woods by any means. I had fixed the most obvious and immediate problems but a prolonged soak test revealed that a few more subtle ones were still lurking. In par­ticular, there was some horizontal tearing and either wrong or no colour at switch-on, although these problems usually cleared after few minutes. By now I was beginning to worry that the soaking might have created long-term damage which would continue to surface long after I had fixed all the obvious faults. The problem as far as I was concerned was how could I possibly guarantee the repair under these circumstances. All I could do was press on and hope for the best. In more practical terms, the chief suspect was the jungle IC (IC300, TDA8375A), a 56-pin monster which had scored a direct hit from the water. Quite frankly, I didn’t fancy having to replace it. It would not only be expensive but the job would be time-consuming as well. Nevertheless, that seemed to be the next logical step so out it came. This revealed several damp patches that had been under the IC, along with some wire links that were already show­ing signs of corrosion. What ever it was that had been added to the water, either deliberately or by accident, it was a pretty potent brew, attacking everything in its path. I cleaned and dried everything and fitted new links to the board. At this point, I was all set to procure a new IC but sud­denly wondered whether this might be overkill; it could be that there was no fault in the IC itself and that the faults had been due to the damp patches. Was it worth taking a punt on that? If I refitted the original IC and the fault(s) reappeared, I would have to go through the whole replacement routine again. But if it fixed the problem, I would have saved the cost of the IC plus the delay in getting it. I took the punt and I won; the old IC behaved perfectly and even after a soak test lasting several days, there were no signs of trouble. The only complaint came from Poo­kie’s master, who was frustrated at the time it took to do the job. But as I explained to him, it was essential that I (soak) test the set over several days to be sure that nothing had been overlooked. And what happened to Pookie? Nothing drastic as far as I know, apart from a few black looks and some nasty comments from his master. I did, however, suggest that the flowers should be moved elsewhere to prevent a repeat incident. The fallen Sony The demise of the second Sony KV-G21F2 was more violent than the first. It had actually fallen over onto a carpeted concrete floor. As a result, the cabinet was cracked and the set was dead. I didn’t enquire as to how this set had met this untimely fate. No explanation was offered but the Morris family has four boisterous young boys so it wasn’t too difficult to imagine what had happened. A close examination on the bench revealed that the main PC board had sustained several large cracks, the most obvious ones being around the horizontal output transformer and the front panel controls. These cracks were all repaired but it wasn’t going to be as easy as that. The large cracks were easy; it was the hairline cracks that were the real problem. There are several approaches to finding these. Visual inspection using a good light and a jeweller’s loupe is one method and it really pays to examine the board carefully adjacent to where large cracks have occurred. The other approaches are basically electrical. If a voltage or a signal cannot be found at one end of a track but is present at the other, the break is somewhere along that track. But pin­pointing it can still be difficult. The light and jeweller’s loupe may be sufficient but it often requires a very sharp point­ed probe which will penetrate the board lacquer. And in the end, it’s back to visual inspection. It took a long time to find and fix some of these hairline cracks and it was a most frustrating exercise. Finally, I was confident that I had found the last one. I switched the set on, there was a rush of sound and the picture came on. My elation didn’t last long – there was a sudden click and then silence. After some detective work, I found yet another hairline crack which had been concealed by paint and was lying between closely packed adjacent tracks. But it was of no immediate help; the set was still dead – only the standby light could be switched on and off with the remote control. So where to now? Were there more hairline cracks which I still had to find or was there a component fault as well? Either way, all I could do was go over the whole thing again to see if I could find a clue. I went back to taws and checked the power supply and rail voltages. The supply was working perfectly and the main HT rail was at 114V and was applying voltage to the collector of Q801, the horizontal drive transistor (2SC3209). And this gave me my first clue – the voltage was too high at that point because the transistor was completely cut off. The reason for this wasn’t hard to find; there was no base drive to the transistor. The set is switched on via pin 30 of microprocessor IC001 (system control), which switches on 9V regulator IC521. Among other things, this regulator supplies 8V to pins 12 and 37 of IC300, the jungle IC which had reared its head in the previous story. The regulator also switches a +15V rail to the audio output IC (IC203), via transistors Q207 and Q208. These circuits were all working but there was no horizontal drive signal (15,625Hz) from pin 40 of IC300, which accounted for the high voltage on the collector of Q801. But was this due to a fault in the IC or was it due to something connected to IC300? I went over everything I could think of that might be the cause. I soon established that the 4.43MHz crystal oscillator reference wasn’t working but couldn’t be sure whether this was a cause or an effect so I put that observation on hold. There is a “hold down” function (HD.SW) associated with pin 27 of IC001, which controls transistor Q801 and whose function it is to shut down the horizontal output stage in the event of a fault. I disconnected the circuit from pin 27 in case it was shutting things down but no joy. Finally, I took the plunge and replaced IC300. And that was it; the set burst into life and everything functioned perfectly. And it kept on working after several days soak testing. And that was about as far as I could push it. There was no way of knowing whether there were any more hairline cracks lurk­ ing in the background. There could be but until a fault develops, there’s nothing more that can be done. I mentioned earlier that the cabinet had been cracked. It was a relatively minor fault and a new cabinet would cost a couple of hundred dollars. In view of the cost, the owner was happy to live with it. So why did IC300 fail? My theory is that it was damaged by spikes, probably from the horizontal output stage, before all the cracks had been fixed. So considering everything, the owner was lucky the set was not written off. And perhaps his four bois­terous boys should be consigned to the dog house along with Pookie! The Akai stereo TV Mr Gavin is an amicable bloke who was somewhat disappointed that his 18-month old Akai CT2167A TV August 1998  29 Serviceman’s Log – continued set had broken down so soon. This Chinese-made model is a 53cm unit with remote control and Teletext. Unfortunately, the owner had found it increasingly difficult to turn the set on with the remote, until eventually it failed completely. As it came to me the set was dead except for the standby indicator LED. This at least told me that the switch­ mode power supply was working, as the 5V rail for the LED is derived from a 7V rail via Q901. In fact there are four voltage rails, the highest being at 105V. My first step was to check this voltage at the collector of Q402, the horizontal output transistor. This was present and was switchable with the remote via Q905. As for the Sony TV in the previous story, the collector voltage on the horizontal driver transistor (Q401) was high, once again suggesting a lack of drive to that stage. This should come from pin 41 of jungle IC IC301. And again, there was no 8.8MHz signal from the crystal clock oscillator at pin 2 of IC301. It fact, there was no voltage at all on that pin, or any­where else on the IC for that matter. In particular, there was little or no voltage on pin 42 which should have been 8.6V. This voltage is normally derived from the 105V rail 30  Silicon Chip via 5W resistor R306 (6.8kΩ). At least, the circuit showed it as a 5W resistor. But if the undersized unit in the chassis was rated at 5W I was a mon­key’s uncle. I pulled the resistor out and measured it, only to find that it had gone way up into the megohm range. I fitted a fair dinkum 5W unit in its place and the set burst into life and everything functioned as it should. My theory is that the increasing reluctance of the set to turn on was almost certainly due to the progressive increase in value of that 6.8kΩ resistor. Just how the wrong wattage resistor found its way into the set is anyone’s guess. Unusual video recorder My final story for this month concerns an unusual video recorder that Mrs Small brought in for service. It was one of the NEC 9000 models; one of a long-running series with many different features and cabinet styles. In this case, it was a model N9034M which is a multi-system variant of the N9034A, both of which were available in Australia, although it was originally designed to sell in the Middle East. The “M” model covers PAL, NTSC and MESECAM (a version of SECAM). That is by way of background. The actual problem involved the power supply, which is designed to automatically operate from any voltage from 90-260V AC (either 50Hz or 60Hz). It’s obviously aimed at the widest possible market and is a set that you could take almost anywhere. An interesting point here is that the power supply con­struction is completely different from the rest of the video. In fact, it’s quite possible that the power supply is not made by NEC at all but by some other manufacturer. That said, the power supply is obviously very well made. It’s built inside its own metal cage and is relatively easy to remove from the body of the recorder. But that’s the end of the easy part; working on it within the cage is a job in itself. The cage is made in two parts, each roughly “L” shaped. One half forms the top and one end, while the other half forms the other end and the bottom. The cage itself functions as a heat­sink, the heavy duty power transistors and other larger compon­ ents being mounted on one end. The remainder is the circuit is mounted on a PC board. But there’s more to it than that. From a servicing angle, this power supply has more tricks than a cartload of monkeys. Any mistakes in the diagnosis of this switchmode unit and you can blow the lot in a nanosecond. When that happens, you are right back where you started, except that you are at least $50 lighter. Unfortunately, the replacement parts are very expensive. At trade level the transistors cost $24 each and the resistors $1.62. The exception is 5W resistor R2 which was listed at $4.15 before it became “no longer available”. A quick check showed that resistor R2 (20Ω) had failed. This resistor is in the bridge rectifier secondary circuit and it blows instead of fuse F1 which is rated at 2A. More to the point, of course, was why did that resistor fail? And that is the tricky part of servicing this power supply. The circuit is such that a failure in one component can create a chain reaction that can take out several other major components, particularly the expensive power transistors. The safest procedure appears to be to make as many individual component tests as possible before applying power. In this case, the basic cause SILICON CHIP This advertisment is out of date and has been removed to prevent confusion. Fig.1: the universal power supply in the NEC 9034-M video recorder can be tricky to service. A failure in capacitor C11 and/or C12 can also take out power transistors TR1 and TR2, along with resistors R2, R10 and R11. P.C.B. Makers ! If you need: •  P.C.B. High Speed Drill •  P.C.B. Guillotine •  P.C.B. Material – Negative or Positive acting of the fault was the failure of either electroly­ tic capacitor C11 (220µF 10V) or C12 (220µF 15V), or both. This causes transistors TR1 and TR2 (2SC6378) to go short circuit, which takes out 5.1Ω resistors R10 (5W) and R11 (1W). From this, it is easy to understand how a newcomer could be trapped by such a circuit. Simply replacing one suspect component at a time is not good enough. One needs to be sure – or as sure as possible – that all faulty components have been found and re­placed before applying power. Fortunately, none of these dire predictions eventuated. Having been forewarned by colleagues, I was extremely careful and the job was completed without incident. So I was lucky the first time. Finally, here is a little snippet from the reception coun­ter. A teenage lad came in with a video recorder and was con­cerned about the loss of the small flap, fitted on many machines, which cover the various controls. Could he buy a replacement and how much it would it cost? My assessment was that it would cost far more than it was worth; that it would be easier and cheaper to simply forget about it. But the lad was obviously not convinced. “But what about the cockroaches?” “The cockroaches? What cockroaches?” “Well, isn’t the flap put there to keep out the cockroach­es?” Well, maybe the flap was performing that function in its present environment but a I doubt whether that was what was in the designer’s mind SC when he specified it. •  Light Box – Single or Double Sided – Large or Small •  Etch Tank – Bubble or Circulating – Large or Small •  U.V. Sensitive film for Negatives •  Electronic Components and •  •  Equipment for TAFEs, Colleges and Schools FREE ADVICE ON ANY OF OUR PRODUCTS FROM DEDICATED PEOPLE WITH HANDS-ON EXPERIENCE Prompt and Economical Delivery KALEX 40 Wallis Ave E. Ivanhoe 3079 Ph (03) 9497 3422 FAX (03) 9499 2381 •  ALL MAJOR CREDIT CARDS ACCEPTED August 1998  31