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SERVICEMAN'S LOG The day my multimeter lied to me! I have had a real mixture of sets this month, including one that bounced from last month and a couple more than 10 years old. They all produced their fair share of frustration but they were all beaten in the end. I do make mistakes. There, I’ve admitted it; I’m not perfect. So shoot me. Well, Mrs Evans may very well have felt like doing just that, with good reason. Last month I described how her Sony KVF29S Z 2 (G3F chassis SCC-G711-A) had no sound and 18  Silicon Chip intermittent east-west pincushion distortion. I had traced the fault as being down to IC303, the 12V switchable regulator. It wasn’t a hurried job. I did soak test it for well over a week before giving it back and it did go for almost a month after that before the original fault re-occurred. While I’m in full flood, I should confess that there was another symptom which I hadn’t really taken any notice of. The picture had looked slightly washed out, as though the tube had low emission which would have been surprising for a set less than four years old. But when the fault was fixed, the picture had improved. So I thought nothing of it. After delicately smoothing down some ruffled feathers and generally eating humble pie, I got back into Mrs Evans’ TV problem. I thought it not unreasonable to apply the same medicine as before, namely replacing the PQ12RF21. It was quite possible that the new one had failed. These PQ12 switchable IC regulators are made by Sharp and come in a series, with a choice of different numbers for the last four digits for which I am unable to find any data. I could obtain PQ12RF11 and PQ12R04 from my local supplier for only $4.00 or so, but to get hold of the PQ12RF21 would cost me $30 at trade price. Why the last four digits should mean such a massive increase in price was beyond me as all the packages looked identical (TO-220 with four legs). As the fourth leg is only the switchable pin, I eventually decided to use a 7812 3-pin regulator as a cheaper test substitute and only to prove the point. It was just as well as it made no difference to the symptoms. Obviously I was barking up the wrong tree. Because I was making voltage measurements where only a few volts seemed to be making a difference, I thought using a digital meter was sensible and more accurate to monitor the voltages. As the 12V rail was down to 11V on pin 2 and varying, I thought I would try downstream and see if the load was too great. I changed the Pin Con- SETS COVERED THIS MONTH • • • • • Sony KVF29S Sharp CX-4814 Sharp VC-H865X Masuda T1092 Bell & Howell VS-IC trol IC2504 (UPC393C) but to no avail. Next, I checked a bit further upstream to find that the voltage on pin 1 was also 11V and the 15V source was down to 13V. I hung a few additional electros on the printed circuit side to see if this would change anything, on the off-chance that there were faulty ones on the component side, but it made no difference. I then checked the main 135V rail to find that it was also low. Indeed, all the voltage rails were low, which would account for the poor picture. I also found that the voltages I read were different every time I switched the set on and off. A bit disconcerted, I continued my quest. I thought that if the secondary voltages were low, the voltage reference must be faulty, so I replaced IC602 SE135. This is a common 3-pin regulator that controls the optocoupler IC600 PC111 to the main switchmode chopper, IC601 STR-S6708. The voltage on pin 2 of IC601 was 73V instead of 64.7V. This made no difference until I changed the optocoupler as well, whereupon the fault suddenly cleared completely and the sound reappeared. Great! But a glance at the multimeter showed that the voltage had now soared to 150V. I quickly shut down the set. This left just the STR-S6708 to replace, which I did. Then just as I was switching the set back on, in the corner of my eye I noticed a spark and I heard a crackling noise. I’m not sure that you would call this a lucky break, because in one sense it wasn’t. I was indeed fortunate in seeing where it occurred but it surely caused some sort of damage – probably expensive! I had of course immediately switched the set off. The spark occurred at the ground (pin 10) of the chopper transformer, T601, and the printed circuit to this pin is bisected exactly at the pin so that it couples the negative sides of C613 and C616 through pin 10 to ground. The spark was caused by an invisible hairline fracture under the lacquer and screen-printed component markings, right on the edge of pin 10 which is a solder rivet joint. Indeed it certainly was an expensive crackling noise because the damage caused was quite extensive, requiring the replacement of all my familiar friends, IC303, IC602, IC600 and IC601. When I had done all this, it fired up correctly but the digital meter was still reading high secondary voltages. I couldn’t bear this. What had I done wrong? I left the digital meter monitoring the 135V rail and used an old analog meter to check the rails, especially the 15V & 12V ones. To my surprise, these read correctly. This wasn’t making much sense any more so I checked all the rails with the analog meter. Guess what? They all read correctly, including the one that the digital meter was showing as 20V higher! The two meters were flagrantly arguing with each other, so I got out Fig.1: the relevant portion of the Sony KVF29S Z2. If you think that the reproduction is poor, it’s not! This is typical of the circuit diagrams that service technicians have to work with. But it’s a positive work of art when you compare it to the PC board component layout (right) which shows the same section. April 1999  19 yet another meter to determine which was right. Fortunately, the analog one was correct which also meant that the set was now fixed properly. I think. And hope! So why didn’t the digital meter read correctly? I’m not certain; possibly because its 9V battery was low. Later on, after I had replaced the battery, I checked the digital meter on a known power supply and the voltages were correct. However, I feel that I can no longer trust that meter. Call me old but I prefer the analog meter. After all, it is WYSIWYG – what you see is what you get! The TV set is still on test as I write because I don’t want to do any more grovelling than is absolutely necessary. As Clint Eastwood’s Dirty Harry once said, “A man has got to know his limitations”. I certainly know mine. The 10-year old Sharp Normally, I don’t touch 10-year20  Silicon Chip old sets but Ms Bell smiled at me so sweetly I was beguiled. She had struggled in with a Sharp CX-4814, which really isn’t very heavy, but she had got this far so I guess I just had to fix it! I certainly helped her carry it into the workshop. The set was dead but there were so many really bad solder joints on the motherboard I wasn’t at all surprised. Anyway, at switch-on I could hear the 15,625Hz timebase whistle, the crackle of the static from the EHT and could see the filaments in the CRT light but that was about it. The 115V, 24V and 15V rails were all OK and running my fingers along the pins of IC201 produced noise in the loudspeaker. It appeared as though we had lost all the small signal circuits. I have had a lot of experience with this series of sets and I immediately suspected I201 (IXO506CE) as I had had many fail on me before. The major nuisance of this IC is that it has 30 legs and you can’t buy IC sockets for it. Instead, you have to use socket strips. The other bugbear is the metal screening cage around it, which makes access poor. I really didn’t want to change this IC as it is also very expensive. So before I started I thought I would just check the voltage rail feeding it in case an unseen dry joint was the culprit. Good move. There was no 12V at pin 4 and after chasing it back all the way to Q603 I could measure 15V on the collector but nothing on the emitter or base. The transistor turned out to be OK and there were no shorts on the emitter or base circuits. The bias that feeds Q603 is derived from the +115V rail via R624 and R645. Both seemed to measure correctly in circuit but on removal R624 (68kΩ) was nearly open circuit. I fitted a new one and the set burst into life. Ms Bell would surely smile on me again. Sigh! Not so the boss. She had noticed me booking in the set rather slowly and reminded me curtly of company policy on old sets. I slyly shrugged it off, saying things were a little quiet so we should take on some of the dross and besides I did have a little expertise on these, from all our rental sets. Just to prove the point about old sets, a young man brought in his Bell & Howell VS-IC TV/Video which would have been over ten years old too. This set is an Australian hybrid of a Sharp X-3434D television and a National N-180EN video recorder put together with a bizarre add-on extension case. Dead flyback transformer The set was dead but I took the job on anyway in case I was accused of being inconsistent. This was a mistake of course, because the headache of this model is detaching the extension VCR and case. It is held on with two plastic screws on the rear and four concealed screws on the side. To make it worse, these screws are concealed with little plastic covers and are three inches deep inside the cover, too dark to see. It took me ages with the aid of a small torch to work out that they were 4mm Allen screws and you needed a really long shaft! Despite all this, I guess the easy part was taking it apart. So was the diagnosis and replacement of the flyback transformer T601 (TRNF1412CEZZ). The really difficult part was getting the whole thing back together, especially as the screw retaining plastic washers had got lost. Trying to get four Allen screws in simultaneously without one falling inside was extremely difficult. In fact, trying to get it all back together took longer than disassembling and repairing it. The other major drama was that the TV set wouldn’t work without being connected to the VCR via an interface which was too hard to juggle whilst disassembled. Things got even worse when I discovered after I had got it all back together that there was no sound. There was nothing for it but to go back in. Swearing and sweating, many hours later I finally diagnosed and replaced IC301 (IXO250CE) and got it all back together again. In the process of dying, the old flyback transformer had punctured the insulation and arced over and killed the IC. The reason I hadn’t picked it up before was because of Fig.2: the trouble that one little half watt resistor got me into (and I'm not just talking about a difficult service job)! Still, she it was worth it in the end! the additional TV mute button on the rear extension! Aptly named . . . Mr Bradley brought in his beautiful Sharp VC-H865X hifi VCR. I say beautiful because not only did it look good but it was immaculate, as though it had just come out of the box. He had really looked after it. This was a great shame because when he told me what the problem was I thought that might well be the end of it. Its fault was that none of the controls work­ed and the display randomly showed different segments. This rang alarm bells with me, spelling out “expensive microprocessor”. Not only that but sometimes they are very difficult to replace, especially if it is a 64-pin surface mount and sometimes it can be either o n e ( Ti m e r or Syscon) or even both ICs. The only way to confirm these large scale ICs is to replace them – but which one first? Anyway, I told him that I doubt- ed that it would be worth fixing and explained, as best I could, my dilemma with the above scenario. Initially crestfallen, he took it badly, so I said I would have a quick look and see if anything else might be causing it; otherwise I would advise him to get a new one. The only things I could check were voltages, clocks, dry joints and cracks and possibly corrosion from the old brown glue – (aptly named gorilla snot.) It didn’t take long to ascertain that all was perfect in the peripheral circuits to the ICs and it really looked as though replacing one or both was April 1999  21 Fig.3: the electros were easy – pity they also decided to take out the switches in this Sharp VCR. the only cure and that was uneconomical. In the course of examining it, I noticed that of all the random displays the VCR was giving, one seemed more persistent than all the others. The “tuning up” symbol was flashing as though someone was pressing the button. I thought it was worth having a closer look on the timer control board behind the front escutcheon. Because of its complexity, it wasn’t easy to remove but finally I got it out. Under the mag lamp I checked for cracks and found none but when I examined the tuning board, I noticed the metal was dull and slightly discoloured. On the copper side the pattern was all corroded around this area. Now I was getting excited – here was a possible cause of all the strife. What had caused all this corrosion? I doubted that it could be from external sources as this location was too far inside the VCR and was localised. 22  Silicon Chip Back on the component side it didn’t take long to find the cause or causes. There were two very small electros, C5020 and C5021 (220µF 6.3VW), which were a complete mess, leaking down onto the SW5001 and SW5002 tuning/tracking tactile switches. Cleaning them up and replacing the switches was easy; finding small replacement electros was a little harder. Anyway, this effected a complete repair and the unit is now back in service. Masuda trouble These days I tend to shudder when I hear the name “Masuda”. It’s not that it sounds like a Japanese food dish, it’s just that it spells T.R.O.U.B.L.E. These sets were originally imported by Brashs and are no longer supported at all. I feel sure that the reason why there was one on my desk to be repaired was a reprisal by the Boss over the Ms Bell affair – which wasn’t an affair and was of course quite innocent. Anyway the written command on the job card was “Dead – fix”; not even a “please fix”. This was a Masuda T1092 which is an AC/DC 27cm remote control TV from Taiwan, also sold under the name of Akai, Aiko, Hanimex, Tandy and Silver. The most common problem with this set is the failure of IC402, a custom-made 3-pin 11V regulator block. However, in this instance it was a different problem. F402, a 5A fuse, was actually glowing and there was no sound or picture. The 11V rail was low and got even lower as it reached the flyback transformer. I switched the set off and started looking for shorts to ground with the ohmmeter but could find none, despite the huge current. It took a long time disconnecting and measuring backwards and forwards between IC402 and T404. The line output transistor was OK and so were all the connections to the flyback transformer. I was beginning to actually suspect the flyback transformer of breaking down under load when, more by luck than judgement, I measured D410 FR605 out of circuit to find it was very leaky. I replaced it with an FR607, a 1000V 6A high-speed diode which actually fixed the problem. However, after the set had been on for only fifteen minutes or so, the diode was amazingly hot to touch. I added another diode in parallel but even then they were still running very hot. I left the set to soak test but it seemed quite stable and a week later showed no further sign of stress. I checked all the voltages again (11V on the anode and 22V on the cathode), reboxed it and sent it home. I couldn’t help wondering why the diodes, now rated at 1000V and 12A, could still be so hot! No wonder the original didn’t stand a chance. What a weird TV! The easy NEC With some sets you can’t help thinking, “this has got to be an easy fix”. The symptoms are clear and logically, the answer has to be equally so. So I thought I had it made when a Thai-built NEC N4850 came in. I think this is an NEC design as the board has PWC3607A printed on it. Fig.4: the relevant circuit section for the NEC N4850. The “A” in the type number for IC601 makes all the difference. Now how easy is this? The fault is: goes dead after one to two hours and if you freeze IC601 (just ever so slightly) it comes good. Probable solution: replace IC601; no need to even look at the circuit. Did that. I ordered the STR50115. It came the next day and I fitted it –piece of cake. Switched on confidently. Goodness gracious me! A complete new set of symptoms – the set was still dead but the two front LEDs were pulsating very slowly. I must have made a mistake. Silly me. I checked everything; it was 100% OK. To be doubly sure, I removed the new IC and refitted the old one. The set came on perfectly and then went off an hour or two later. My conclusion was that I had a duff new one. These things happen, so I ordered another. When it arrived, I slammed it in – the set was still dead and pulsating. I compared the new ones with the original – no difference. It was time to get technical. I dug up the circuit diagram of the set. No clue immediately hit me. There are a couple of modifications to this set, one being to add a .001µF 2kV capacitor across C613 and C612 is upgraded to .0047µF but doing these made no difference. I finally found the reason. It stares at you so obviously in the circuit and you probably picked it straight away – the answer is, of course, the IC. It is marked on the circuit as STR­ 50115A. The A is not always print­ed on the component but it makes all the difference, and fitting it with new heatsink compound (it gets very hot) fixes the problem completely. 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