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You've got to be Sharp in this gaille This month's main story has many claims to fame - if 'fame' is the right word. It set something of a record in time taken to obtain a spare part and also a record for patience on the part of the customer. And on the bench it made another record bid; no less than four faults when only one was suspected. The story started when I was called to the local school to do a routine repair job on the PA system. It was the commencement of a term and I encountered a new principal who had just been transferred from a school in the far south of the state. The full significance of this last point was not appreciated until much later. After I fixed the PA system, we began chatting and he soon raised the matter of VCRs. Did I service these devices? When I said I did he explained that he had a VCR which was faulty and that it had had a checkered history of service. In fact, he had some very nasty things to say about the serviceman concerned which I would not dare repeat in print. But briefly, it appeared that it had been serviced on two or three occasions for various faults and after the last service, the present fault had appeared. So could I fix it? That was something of a leading question but sticking my neck out, I said yes assuming it was capable of being fixed economically. And feeling that the industry owed him something, I emphasised that if the repair appeared not to be economical, there would be no charge for the advice. He accepted my offer and added that he didn't care how long it took to fix the thing - just so long as it was genuinely fixed in the end. I doubt if either of us guessed how long it would really take but he was 48 SILICON CHIP as good as his word; not once did he pressure me. Naturally, I needed to know the nature of the complaint and as much of its history as possible. He was a bit vague about this. Basically, it was a failure to reproduce colour but more importantly, it was intermittent. I tried to determine whether this was a recording fault - was it only on tapes he recorded or also on pre-recorded tapes? but it seemed that he had not noted this distinction. Anyway, he duly delivered the machine to the shop. It turned out to be a Sharp VC-483X, a model of about four years old and one for which I had a manual. At the first opportunity I set it up, pushed in a tape and made a recording. There was no colour on playback and none when the tape was played in another machine. On the other hand, the machine would play a pre-recorded tape in full colour. OK, so we had a fault somewhere in the chroma circuitry. My next step was to open the machine and check for anything obvious. My first stop was the Y/C (luminance/chrominance) board which sits above the drum and transport mechanism. It is mounted component side down and is covered with a clear plastic sheet. Bodgie pot And did I find something "obvious"? One glance was enough; someone, presumably the previous N\"( FIRST s-rz::>p Wf=I\S 11-tE Y/C (LUM\NANc.£. CHROMINA~C£)SOAR'D WI-'\~ Sl"t'S A'SOV~ ~ 1'RUIV\ic-~, ~Ar-J\~N'\, MOUl\r\""ED COIY\f>~ S\De; t>OWN &GO~Wt~A a...€A~ 1)f.A~C 'SH"1'" serviceman, had attacked the copper side of the board and made an unholy mess of it. There was also a small trimpot which had been added and which obviously didn't belong there. Backtracking through the circuit and wiring patterns provided some clarification. The bodgie pot was a replacement for R508, a lkn pot used to set the choma record level. In its original form, it is a much smaller unit (about 5mm in diameter) and is normally mounted on the component side of the board. But it wasn't just the bodgie pot that shocked me. It was obvious that a number of other components in this part of the circuit had been removed and replaced with new components, and equally obvious that it had been done by someone who had never come to terms with proper soldering techniques in modern equipment. In short, it was a mess, with great blobs of solder around the various joints, plus a fair share of surplus blobs and slivers scattered around the board. The wonder was that the consequences had not been more spectacular. As to the reason behind this mess, one can only speculate. My guess is that it had started with the loss-of-colour fault, possibly intermittent, and that the serviceman had changed or tested all these components, including the pot, in an effort to track it down. That's fair enough as far as it goes. My real gripe concerns the shockingly poor manner in which it was done. Cleaning up The first thing to do was to get stuck into the board and clean it up. This I did and even if I do say it myself, I was able to get it back very close to its original condition. The only sour note was the bodgie pot for which I had no replacement and which had to be ordered. But at least I had eliminated any potential shorts and dry joints. Then I tried the machine again. And lo and behold, it worked; a perfect picture in full colour. Of course I didn't trust it; it was known to be intermittent and while I wanted to believe that I might have corrected something in the cleanup, that seemed like a long shot. So I simply kept on playing it at odd intervals while I waited for the replacement pot. I rang the owner and explained the situation. He wasn't at all con- cerned and in fact, I gained the impression that he was so cheesed off with the machine that he had mentally written it off. I felt I would have only had to suggest that he scrap it and he would have done so. Of course, I had no intention of making any such suggestion. The lack of the replacement pot meant the machine was now more or less pushed to one side. I tried it on odd occasions and it always worked but after a few weeks, pressure of other work prevailed and I abandoned even this ritual. I did keep needling the supplier about the pot though, but always without result. This went on for nearly six months - just as well the customer wasn't in hurry. Then one day a package of other spares arrived and there among them was the pot. "Aha!", I thought. "Now to get this one off the shelf". Normal Rate - I opened the machine again, fitted the new pot, tidied things up where the old one had been, and tried running a tape. No joy. The cassette carrier accepted the tape and put it on the deck but after that nothing happened. Fault number two. It wasn't a hard one to find. The idler wheel which drives the supply and takeup reels had packed it in. It was probably about due for replacement anyway and six months sitting unused on the shelf was the last straw. Anyway, it wouldn't drive the takeup reel and the protective circuit shut the system down. That didn't seem to be any pro- ly, if it runs at all. Then C813, a 1 O!,tF 35V electro, bypasses the same Vee rail and if it is open or low value, the result is weaving verticals. TETIA TV Tip is supplied by the Tasmanian branch of The Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, Tasmania 7015. $35 per hour. No charge for kits that can't be repaired. 3 month Warranty on repairs . Construction - fixed or hourly 1 2 Month Warranty on Manufactured Kits. Custom Designing, Manufacturing, Large or Small Quantitites. HYCAL INSTRUMENTS Design, Manufacture, Repair of Electronic Equipment. (02) 633 5897 Unit 4, 62 Great Western Highway, Parramatta, NSW 2150. Idle idler TETIA TV TIP National TC2202 (M8 chassis) Symptom: Set reluctant to turn on. Sometimes starts with a squeak, sometimes only squeaks. When it does get started it shows severe hum bars or horizontal wriggle . Cure: There are actually two different faults, closely associated with each other. R804, a 4. 7k0 SW resistor, feeds Vee to the horizontal oscillator. If this is dry jointed the oscillator runs erratical- FIX-A-KIT Kit Repairs - $15 per hour. Trading hours: 8am to 3pm Monday to Friday. D.DAUNER ELECTRONIC COMPONENTS WE STOCK A WIDE RANGE OF ELEC TRONIC PARTS • for Development • Repair • Radio Amateur • Industrial Electronic • Analog and Digital WH ILE STOCKS LAST NEOSID HELIX FILTER for UHF now available COAXIAL RELAY 28VDC $32.00 TRANSMITTER VALVE 5786 $18.00 Come and see. Showroom: 51 Georges Crescent, Georges Hall, NSW 2198 (Behind Caltex Service Station In Blrdwood Road) Phone 724 6982 TRADING HOURS: Monday to Friday 9.00 a.m. to 4.00 p.m. Saturday from 9.00 a.m. to 12.00 noon. N OVEMBER 1989 49 SERVICEMAN'S LOG -CTD blem. I had a spare idler on hand and it was a relatively simple job to fit it. Then I made another attempt to run a tape, only to realise that the machine was now completely dead - there wasn't even a clock function and when there is no clock it is something pretty fundamental. This was fault number three and it was a beauty. I went immediately to the mains fuse, F901, which is about as fundamental as you can get. But it was intact and that's when the fun started; a real chaseme-Charley effort. There is no way that I could describe the whole operation in the space available; nor could the reader follow it without a manual. Instead, I will content myself with a broad picture, with a few component ident codes tossed in for the benefit of my colleagues who do have a manual. IN ~HO'ln", The power supply (PWB-P) generates several voltages - including two 9V and two 13V supplies - which appear at plug PA. I checked all these voltages, hoping to find at least one that had failed. But no luck; they were all as prescribed which was a bit of a setback. One of these supplies - the 13V rail on pin PA4 - goes to board PWB-1 (IF Tuning Circuit) and appears at plug IEl. I followed this because the 13V supplies a package (RUNTKO157GE) on this board which is a power supply circuit in its own right. It delivers several voltages, some positive, . some negative and some AC. Again I drew a blank; all voltages were as marked. At this point pressure of more urgent jobs forced me to put the machine back of the shelf. But I did contact the owner rr w~~ F-\ N\E.sS, wrn-\ G~EA, '8LO~~ OF SOLDE.R":'; .. 50 SILICON CHIP and bring him up to date. He waved off my explanation: "No worries, mate - when you're ready". Patience indeed. It was several weeks before I was able to tackle the machine again. Still convinced that it was a voltage supply problem somewhere, I set about tracing and checking each supply rail in turn to its ultimate destination. Blind alleys I will spare the reader the agony of the blind alleys I followed; suffice it to say that there were several. But I eventually began tracing the 9V rail which commences at pin 9 of the power supply plug PA. This is shown as going to EB4. (Where the heck is EB4 ?) Well it turned out to be on board PWB-E, the mechanical control circuit, portrayed on a 3-section foldout sheet. The 9V rail comes into it on plug EB (pin 4) and rather interestingly, had now increased to 9.2V (good news for the perpetual motion inventors). According to the circuit, this rail goes straight from the plug to a device marked 'PR' but with no indication as to what this is. From there it runs all over the place, feeding a variety of components, and also exits from the board via at least five other plugs. So it is obviously a very vital rail. Having established this much on the circuit, I began checking this voltage on the board. Fortunately, I was able to pinpoint a couple of places where it should have been, only to find that it was missing. Well that was something. Now I was looking for the device 'PR' which, while shown hard against the plug on the circuit, was nowhere near it in real life. I finished up back-tracking along the copper pattern until I finally found it. It turned out to be a safety resistor, with voltage on one side and none on the other. So that was it - except that there was nothing on the device, the circuit, or in the parts list to indicate its value. Fortunately, most safety resistors fall into a fairly narrow category between 0.50 and 20 and with a nominal 0.5W rating. So, as a temporary measure, I fished out a similar device for another machine, fitted it, and we were back in business. Which was a considerable relief after all that effort. But why had the resistor failed? It had obviously been overloaded, rather than failed spontaneously, yet there was no sign of distress in the replacement. Naturally I was concerned that there was another intermittent somewhere but there wasn't much I could do about it, except give it a good workout. (In fact there was a good reason for the failure, which I will discuss later). So for the moment, I simply set it up in a corner of the bench, connected to a monitor, and ran it at every opportunity. The first day I ran a 3-hour pre-recorded tape through it. No problems. The second day I recorded a 3-hour tape, then played it back. No problems. The third day I played another pre-recorded tape. It ran without any problems for about two and a half hours. Then - a noise bar. Oh no! - fault number four. Sweaty brow As I wiped my brow in exasperation, I recognised the first clue; it was a stinking hot day and my workshop doesn't have air conditioning. I switched the machine off, let it stand for about half an hour, then ran it again. The fault had cleared, confirming my suspicion that it was a thermal problem. I rang the owner and asked him if he had ever experienced anything like this fault. He was quite emphatic that he hadn't and it Wil,S then that I realised the significance of his previous location; well south and somewhat elevated. The chances of him encountering the same thermal conditions as I had would be slim. In addition to the thermal clue, the nature of the display provided another clue. The noise bar was drifting slowly down the screen rather than remaining stationary as is more usually the case. This suggested that it was due to a loss of lock in either the drum motor or the capstan motor. The question was, which one? It was a 50-50 chance of course but I had a feeling in my bones that it was the drum motor. ,oo 11-1~ IVlACHIJ\le;:. W~'S N'OR6. OR t-6S1S ~SHE.'D A-S\t:>6-••• My first move was to connect the CRO to the appropriate test points - TP5 for the drum and TP3 for the capstan - and check the waveforms against those given in the manual for adjustment of the tab pots (R724 and R777) for correct operation. These waveforms came up virtually spot on - or at least as far as I could tell by comparing them with the idealised drawings in the manual, a point of some importance as it turned out. Of course, the machine had now cooled and so the test was not really conclusive. I had to leave things there for the day and the following day was a lot cooler, meaning that I had some trouble re-creating the fault. I considered applying artificial heat but rejected the idea. Instead, I wanted the fault to occur as a result of the machine's own internal heat distribution, at least for my initial investigations. Incidently, bulk heating can sometimes produce deceptive results due to innocent components being heated to a level far above that which they would experience under the worst real-life conditions. It's a point to watch. Anyway, the fault eventually appeared without any help and while waiting for this to happen, I had delved a little deeper into the circuit. The control signals for the drum motor come from IC708, on board PWB-A (Servo Still). The phase control signal comes out on pin 1, passes through transistor Q705, and is then applied to the drum motor. The frequency control signal comes out on pin 8, is processed by trani;;istors Q703 and Q702, and also goes to the drum motor. Since I had gleaned little from the CRO patterns at the te~t points, it occurred to me that it might be worthwhile checking these two circuits. I chose the collector of Q702, which goes directly to the drum motor. The only snag was that there are no waveforms in the manual covering these circuits so that I had no way of knowing whether what I was observing, under no-fault conditions, was within tolerance or not. The only obvious clue was that it changed when the fault occurred. Then it occurred to me that while I had the fault in my sights, it might be worthwhile trying to find the heat sensitive component with the NOVEMBER 1989 51 071 ◄ 0X01 ◄ 2a R71 t C701 '"'"' + i.z~ ..-Ei-c-+---N,-, R710 1,0v1 0702 '"" ·•ro 2SC~~ croo .o, 0703 2SA733 Fig.1: the drum control circuitry for the Sharp VC-483X VCR. Transistor Q702 appeared to be excessively heat sensitive. But was it? aid of some freezer spray. I started with Q705. No response. Q703 reacted similarly. But when I hit Q702 (2SC945) the reaction was immediate and dramatic; the fault vanished instantly. Further confirmation was provided by applying some heat to Q702 with the soldering iron, which quickly re-established the fault. I quickly fitted a replacement for Q702, then gave the machine another series of long runs. All seemed well at first but the second day was another hot one and after many hours operation, there was the fault again. And again, a smidgin of freezer was all that was needed to cure it. So what could I do? Run a plastic pipeline from inside the machine and give the owner a can of freezer, with instructions on how to use it? This and similar wild Heath Robinson ideas went through my mind, even while I was trying to figure out a genuine cure. I went back to the CRO check point [TP5) and had another look at the pattern. I still felt that it was as near as one could wish to the drawing in the manual but decided to try adjusting pot R724. With the machine still in fault condition, I turned the pot a few degrees one way with no result, then a few degrees the other way. And bingo, everything came good although it's 52 SILICON CHIP worth noting that the effect on the CRO pattern was virtually negligible. Well, that really did cure it. I ran the machine over many days, for hours at a time, and did everything I could to promote the fault. In all, it must have chalked up about 30 hours of use and it never flickered once. So what was the true nature of the fault? Was the first 2SC945 really faulty? Or was the original setting of R724 marginally off; perhaps just enough to cause trouble when the drift of some other component values became significant? Or had the previous serviceman had a fiddle with that as well as butchering the board? I'm inclined to suspect the latter, although there is little doubt in my mind that the first transistor was more heat sensitive than the second. Pertinent comments Finally, I feel that I should add a couple of pertinent comments. First, several checks with the owner have confirmed the cure; it hasn't missed a beat since and he is delighted. I have a permanent customer there. Second, why did the safety resistor fail? Shortly after I wrote the main part of this story I had another VC-483X in for service. It turned out to be a faulty idler wheel, similar to the one described above. That was simple enough and a replacement was quickly fitted. The only snag was that the machine was now completely dead; not even a clock readout. Of course the penny dropped and I went straight to the safety resistor. Sure enough, it had failed. Why? It was my own fault really. In both cases I had neglected to turn the machine off before working on it and, in removing the cassette carrier to get at the main deck, it is very easy to short one of the supply rails on the small board on the left hand side of the cassette carrier. And oops! - one safety resistor destroyed. That's one I learned the hard way. And what of the intermittent loss of colour which started all this? That was almost forgotten in the hassle of tracking down the other faults. And for a very good reason; it never appeared again and that includes my monitoring right up until writing this last part of these notes. From this I can only conclude that it was a faulty joint, either present from the start or created by the previous serviceman and which was corrected when I cleaned up the board. And that in itself is a rare enough event to be worth noting. ~