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SERVICEMAN'S LOG A tale of two Sharp VCRs VCRs can fail for all sorts of reasons but I recently had one that really takes the cake. Fortunately, not all jobs are like that one, with most being quite routine. Just when I’d thought I’d seen every­ thing in the servicing game, along comes something to really set me back on my heels. Spilt drinks or other liquids are common reasons for TV and VCR failures but monkey urine? –you’ve got to be kidding! Of course, out of the thousands of servicemen in Australia, it had to be yours truly that got saddled with the job. The story started out innocently enough. Some months ago, the local vet brought in a mid-drive Sharp VCR with the complaint that it stopped working after about two seconds on “play”. The set was a 1992 VCA34X which looked to be in good condition – at least from the outside. Unfortunate­ ly, it didn’t smell quite so good and had a quite distinct “pong” of stale urine about it. However, seeing that it had come from the animal surgery, I assumed that this “pong’ had been picked up from something in the air. The fault description turned out to be quite accurate and when I removed the top cover, I could see that fast forward and rewind were OK. The play mode was a different matter, however – the arms loaded the tape properly, the drum motor started and the capstan motor started but it only did a revolution or two before stopping and unloading the tape. It all looked OK, so why didn’t it work? Unfortunately, I don’t have the service manual for this model but the deck was a very common type; only the electronics were different. So the first question was “is this a mechanical or an electronic problem?” I decided to inspect the whole machine carefully 30  Silicon Chip and turned it upside-down to remove the lower cover plate. Immediately, it was obvious from the severe corrosion where the smell was coming from. It was also obvious that this was where the problem lay. Fairly obviously, the machine had been sitting in a pool of urine, perhaps up to 12mm deep. And although this had long been cleaned off by someone else, the corrosion was abundant to see up to the high level mark. I was about to phone the vet and tell him that all was lost and that he should get a new VCR but then I had a little think to myself. To fix or not to fix Perhaps this wasn’t a hopeless case after all. First, all the major motors were actually turning and secondly, someone else had already done a pret­ ty good job of cleaning up the mess in­side. Added to that, all the bottom printed circuit boards looked OK and so I concluded that the job was worth investigating fur­ther to see if the ma­ chine could be salvaged. At this stage, my main suspect was the drum motor as it seemed hesitant to start and its speed appeared to be intermit­tent when it was strobed under a neon mains light. I also discov­ered that, occasionally, when the video was switched from EE mode (ie, Tuner) to the Play mode, a picture could almost be seen although not always in sync. My next step was to fire up the CRO and check the output of the PG (pulse generator) head. This revealed a pulse generator signal but it was fluctuating. Also there was 12V on pin 3 of the plug to the drum motor. From this evidence, it seemed likely that the drum stator board or its com­ ponents had corroded and so I decided to remove it to see if anything could be done. The connection to this motor is via a special 6-lead flat printed circuit ribbon cable harness whose end is just pushed into a receptacle. When I removed this, I found it to be badly cor­ roded. I cut about 12mm off, scraped away the white varnish covering the tinned strands, cleaned the socket and reinserted the harness. When I powered up and pressed “Play”, the motor spun up quickly and continued to play correctly. I then checked the other sockets in this area but they were all OK. In hindsight, I should have suspected a cable connec­ tion but everything looked fine until the cable was actually removed. I returned the video to the vet with a warning that despite cleaning it, he would probably have further problems in the long term as it is very hard to prevent a chemical reaction of this type from continuing. He apologised for the state of the VCR – apparently a monkey (would you believe it) had had an “accident” in that corner of the room and even though he had got a friend to check the VCR out, he must have missed that particular socket. Just how long the machine will last is anybody’s guess and I didn’t press the vet for any further details on the monkey or what it was doing there. Let’s just say that it doesn’t pay to have a monkey monkeying around near a VCR. Another Sharp VCR I thought no more about this repair until a few months later when another similar Sharp VCR came in. This time it was a slightly older model, a VCA105X, and the complaint was that it wouldn’t play. The unit looked in good condition throughout and this time there were no unpleasant odours! As with the previous unit, this too would load up, the drum motor would spin and the capstan motor would spin a few revolu­tions before it would unload and stop. This time, I did have the service manual and it also had a troubleshooting guide for the exact symptoms being experienced. The first step is to check the head switching pulse applied to pin 3 of IC801 (IXO491) and the PG (pulse gen­ erator) signal applied to pin 4 of servo control IC701 (IX04313GE). Well, the PG pulse was there but there was definitely no head switching pulse (HSWP) . In fact, all the voltages and inputs seemed cor­rect going into IC701 but nothing was coming out of pin 28 and there appeared to be no short circuit on that line. In view of these symptoms, I felt that the problem had to be electronic and, at this stage, considered IC701 and/ or the system control microprocessor IC801 as the main suspects. Before replacing these devices however, I first checked out all the B+ lines and the various clock signals but could find nothing wrong. The HSWP also went to the Y/C module and onto the head preamp board. I tried disconnecting these boards in turn but there was still no sign of a pulse. With reluctance, I ordered IC701 first as I knew these ICs would be ex­ pensive. It arrived a few days later and I wasted no time in fitting it. Unfortu­ nately, it made no difference, so that was expensive mistake number one. I was now faced with the prospect of having to replace the main micropro­ cessor (IC801), which was even more expensive. Before ordering it however, I tried replacing the mode select switch in case it wasn’t engaging quite correct­ ly – to no avail. Finally, I went ahead and ordered IC801. It too arrived after a couple of days and I went about the laborious task of removing the old 64pin IC and soldering in the new one. And that was expensive mistake number two because it also made ab­ solutely no difference – there was still no head switching pulse. Murphy was really working overtime on this job! A new approach I was discussing my expensive folly with several colleagues when one said that he had recently obtained the same model as a trade-in. What’s more, he offered to lend it to me so that I could pinpoint the location of the faulty parts without spending any more mega­bucks on use­ less guesses. He was also of the strong opinion that it was an intermittent capstan motor that was caus­ing the problem as this occasionally occurs February 1997  31 Serviceman’s Log – continued on this series of decks and gives many similar symptoms. Well, this was indeed a stroke of luck and I started by swapping over the capstan motor but that too made no difference. By now, I felt that my original diagnosis – that the fault was electronic – must be correct. As a result, I began swapping all the electronic circuitry between the two machines, board by board, but again this made no difference. After changing the last board, I was forced to conclude that I was looking at a mechanical/motor problem. In fact, it had to be the drum motor, even though all the pulses and voltages from it were correct and it looked as though it was reaching the correct speed when viewed under the strobe light. I couldn’t be certain of this, how­ ever, as it was unloading as soon as it apparently reached the correct speed. In fact, the machine did not even have time to switch from EE mode to Play mode, so no picture was available. Anyway, I proceeded to replace the drum motor and try again. Unbelieva­ bly, it worked this time but I couldn’t under­stand why. Out of curiosity, I measured the output from the new motor and it matched the old one exactly. How could this be? To solve this mystery, I replaced the old motor and in the process noticed that the ribbon con­ nector cable had come away from its 32  Silicon Chip hardened plastic support and that the tracks were somewhat loose and frayed. I didn’t pay much attention to this until I retried the old motor which, to my amazement, was now working. So what was the answer? As it is not possible to prove, I can only speculate that the ribbon connector cable was either shorting or not making a proper connection with the socket. I guess I should have remembered the symp­ toms of the earlier repair and examined this connection more carefully first. Bread & butter The next morning, I faced up to two TV sets that were awaiting my attention. The first was a 34cm Toshiba 144R8A made by Samsung (a P54S) and it was quite dead. Often, a set of this size is not really worthwhile repairing as they are so cheap to buy. It all depends on how difficult the fault is to fix and sometimes it can be quite difficult to decide whether to go ahead or not. On the plus side, the faults in this chassis are fairly well known which does reduce the amount of time spent in diagno­sis. In the 34cm model, most of the problems are in the power supply, due to the electrolytic capacitors drying out. This causes the chopper power transistor Q801 (2SC3552/BU508A) in the switchmode power supply to blow, which also takes out the fuse and/or R801 (5.6W 7W). In this case, the transistor had tak­ en out the resistor and so I replaced these parts and four electros (C808, C813, C812 & C811) all at once. When I switched it on again, the set was ob­ viously struggling to fire up and was making funny noises in the power supply. I immediately switched it off again to prevent another failure of the chopper transistor and then started to check for shorts on the B+ rails. The line transistor Q404 and capaci­ tor C413, a common culprit, were both OK. In fact there were no shorts and no, or extremely small, output from the chopper transformer. Checking the voltage across C807 confirmed there was 340V out of the bridge rectifier and I checked and cleared resistors R806 and R807. The oscilloscope confirmed that the circuit was oscillating though the waveforms were incorrect and Q801 was getting hot. I changed IC801 which is the main IC in the power supply but that made no difference. And that really only left the trans­ former (T801). Removing it, I tested it with a shorted turns oscillator/tester which indicated that pins 1 and 3 were shorted. This test is not always conclu­ sive as I don’t know what frequency the circuit is designed to resonate at. However, I decided to take a punt – a new chopper transformer was ordered and it subsequently proved that my diag­nosis was correct. The repair cost really made it quite a margin­al exer­ cise, however. Sony KV2064 The second set was a Sony KV2064 and the customer thought that it had to be the on/off switch because it was intermittently dead. Of course, power switches can sometimes be faulty but it never ceases to amaze me that many people think that a TV set consists of a tube, a valve, a switch and a fuse. That’s it – it has to be one these items which is faulty if the set doesn’t go! As it turned out, the owner of the set was a very heavy smoker and there was a film of nicotine over every surface and component inside the set. The switchmode power supply had 240V AC coming into it, which cleared the on/off switch, and there were no fuses blown. This of course left only the tube and the valve! A close examination of the power supply circuitry revealed that the start­ up resistors were the likely culprits. There was over 300V on the collector the chopper transistor (Q602) and the four high-value 0.5W resistors were all discoloured, although only one (R602, 330kW) was open circuit. I changed them all for high voltage types, reworked the solder just in case and switched on. It was all an anti­ climax – everything worked perfectly and the set now started every time it was switched on. All I have to do now is explain to the customer that it wasn’t the on/off switch. The reluctant NEC Later that morning, a lady dropped in her NEC video with the symptoms that it wouldn’t fast forward or rewind and turned itself off. When I eventually got the chance to look at it, I found that there was virtually little or no take up torque, which suggested a reel idler problem. Being an N9083A, this machine was a later (1991) version of the N9000 series, with the reel drive being a sub-assembly which is fairly easy to remove from under­neath. The main problem is due to two tyres within this assembly which are supplied in the VBK83 tyre and belt kit. However, one of the tyres is an integral part of an idler assembly and the replacement will not stay on satisfactorily without gluing. You can either purchase the entire sub-chassis assembly for around $55.00 trade (part no. 016192683 – this not shown in the service manual) or forego replacing that one tyre. It’s a real nuisance that it is not available as a complete pulley idler on its own. It is also worth noting that earlier models also require a modification to the white CR slider – this part is now black (part no. 016457582) and costs around $2.00. One can only hope that the customer clearly understood that the cutdown repair would not last as long as the original. Still, I spelt it out as clearly as I could. This afternoon saw a dead IBM monitor arrive. It was PS/1, manufac­ tured in June 1993, series 028-004. I have seen quite a few of these units with blown mains fuses (F601 2.5AT), usually without any apparent cause. In one case, however, it turned out to be the chopper IC itself that intermit­ tently blew the mains fuse. This particular unit also had a dud mains fuse. I replaced it, checked the rest of the supply for any obvious faults and then, just to be on the safe side, switched the set on with a 200W globe in series with it. Initially, the globe went bright, then dimmed and came on again at half about luminance. The first current surge was obvi­ ously due to the degaussing circuit kicking in and there was obviously still quite a lot of current being drawn at the end of the degaussing cycle – at least 0.75A. However, the compliance plate said the full current drawn was 1.4A, so I tried it without the globe and the fuse held and the screen gave a good picture. For the rest of the day, I cyclically switched the monitor off and on for periods of 15 minutes, without further problems. In the end, I could only ad­ vise the customer that the cause was probably either due to a mains surge or fatigue in the old fuse. All we can do now is keep our fin­ SC gers crossed. February 1997  33