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SERVICEMAN'S LOG There’s a long, long trail a’winding Well, in the words of the popular song, that’s what it seemed like; a long, long trail through circuits and PC boards in search of the most elusive combination of intermittent faults that has been my misfortune to encounter for many years. It is a story about an Akai video cassette recorder, model VS-35 and the problem was an intermittent sound fault. Not just any intermittent sound fault, mind you. This was an intermittent sound fault the like of which I have never seen before – and I hope I never see again. The machine came to me from a colleague, who felt that he could no longer cope with the problem. And so it landed on my bench along with a “best of British luck” attitude. While there was some history of the problem, I suspected that there had been more than one finger in the pie before it reached me. Anyway, I was saddled with it. The gist of the problem was faulty sound on playback although, as far as I could determine, the sound always recorded normally. To test this theory, my first step was as to make a test recording on the machine immediately after observing the sound problem. This tape subsequently played perfectly on another machine, which seemed to clarify this point. But putting such doubts aside, the extent and nature of the faulty sound was the real problem. Sometimes, at switch on, the sound would be perfect for a few minutes, then become intermit­tent. And when it went intermittent, the whole machine became mechanically sensitive; the lightest tap anywhere could turn the sound on or off, as the case may be. Initially, I couldn’t believe what I was observing. It seemed impossible that there would not be some variation in sensitivity which, in turn, would give a clue as to the general location of the fault. But no; touching anything – boards, leads, the deck itself – produced an equal response. Making a start The setup consists of a large mother­ board, a small chroma board, a power supply board and a preamp­ lifier/ audio board. Initially, to the extent that there was any particular sensitivity anywhere, I sensed that the area around the preamplifier/audio board might be marginally more sensitive. I even reached the stage where I could achieve a response by flexing or tapping the board, without it being sup­ported in any way but simply connected by the various leads. I was convinced that this was where the trouble lay. And I wasn’t particularly impressed by the soldering on the underside. There had been some work done on it but there was still a large number of original joints which were, at least, suspect. I set to and checked every suspicious joint and remade anything which looked even remotely suspect. I spent a lot of time on it and by the time I had finished was prepared to bet my reputation on it. Initially, it appeared that the fault 44  Silicon Chip had been cured. But not for long. After a few minutes it was back just as it was before. Well, not quite; the equipment as a whole seemed just as sensitive as before but it now appeared that the area of sen­sitivity had moved to the motherboard. To the extent, that is, that I could be sure of anything. But if it was on the motherboard the implication was that I was chasing two faults; one which I might have found on the preamplifier board and one still to be found. So was it a case of two faults producing essentially identical symptoms? It seemed like an impossible long shot but I was ready to believe anything. Assuming that there had been a fault in the preamplifier board, and that it had been fixed, the next logical step was to investigate the motherboard. Parts of the underside of this had also obviously been worked on, while other untouched areas needed closer scrutiny. The upshot of this was a major overhaul of this board. Any suspect connection, whether it had already been reworked or not, was tackled. It was a long process but when I finished I felt reasonably confident that I had done a thorough job. And so it seemed. When I replaced the board and set every­thing working the machine came good. There was no sign of the sound fault and the machine was seemingly immune from its previ­ous mechanical sensitivity. I let it run for about half an hour or so, giving it an occasional prod or bash, and all seemed well. Or at least it was until I put everything back into place and prepared to fit the cover. Then it was back into fault condi­tion, exactly as it was before. I won’t bore the reader with all the emotions and rude words which resulted from that discov­ery. Suffice it to say that it was back to taws. The methodical approach By now it appeared that the mass soldering approach had served its purpose. If it had done any good at all, it wasn’t good enough. What was now needed was a methodical approach. A major problem here is how best to convey all the circuit ramifications to the reader. We are talking about several A3 sheets, covering both circuit and PC board patterns. Obviously, reproducing these is out of the question and the best I can do is present a Fig.1: part of the audio preamplifier board in the Akai VS-35. Audio from the A/C head (top, right) goes to pins 3, 4 & 5 of IC700, comes out on pin 16, and then goes to the motherboard via pin 12 of connector WF10. word picture which I hope will help the reader follow the story. First, it is it necessary to visualise the audio signal paths where a fault is likely to be. And there are, initially, two sources of audio signal. One comes from the audio track on the tape, feeding the Audio/Control (A/C) head on the deck. These signals are fed to the preamplifier board, on pins 3, 4 & 5 of IC700, come out on pin 16, and go to the motherboard via pin 12 of connectors WF10 (“A.OUT”). The other source comes from the tuner and the IF system on the motherboard, which demodulates the sound IF and delivers an audio signal. These signals involve a longer path but eventually find their way back to the preamplifier board, where the two audio signals are combined into a single audio path which then reappears on the motherboard. This path eventually goes to the modulator but, on the way, connects to an RCA socket, designated as A.OUT, December 1996  45 Serviceman’s Log – continued on the rear of the chassis. This provides a convenient audio check point and the first thing I did was to organise an external amplifier to moni­tor the audio at this point, which is close to the end of the combined audio line. At the same time I arranged things so that the output from the VCR was fed into a TV set. This simple test confirmed that the fault was present at both the RCA A.OUT socket and the TV set. At this point I had to choose between the two audio paths. I was convinced that, whatever I did, it would be wrong (Murphy would see to that) but I had to start somewhere. So, for better or for worse, I elected to check the line coming from the IF system. The signal from the tuner is pro- cessed to IF level, desig­nated on the circuit as VIF, and applied to pins 4 & 5 of IC1 (M51496P) – see Fig.2. The audio signal then comes out on pin 11 of IC1, which was where I started tracing the signal. For this pur­pose, I used a simple audio probe which I have mentioned in previous notes. This confirmed that the signal was intact out of the IF system at pin 11 of IC1, even though the fault was evident at both the TV monitor and the A.OUT socket. Well, that was a good start. From here, via a long path on the circuit, I traced the audio signal to pin 5 of IC201 and then out again on pin 4. It then went to the base of transistor TR213, the signal from the emitter then going to line “SELECT.A” and thence to pin 11 of a 12-pin connector WF10. And the audio signal was still fault free at this point. Now, from pin 11 of connector WF10 on the motherboard, the circuit goes to a similar WF10 connector on the preamplifier board, then into pin 13 of IC700 and out on pin 16. And, as men­tioned earlier, the audio signal from the A/C head on the deck also appears at pin 16. In short, the two signals are combined at this point and the combined signal goes to pin 12 of WF10 and then back to pin 12 of WF10 on the motherboard. And I still had a clean signal at this last point. I was getting close now because there was not much circui­try left between this point and the A.OUT RCA socket where the fault was obvious. But where was it? From pin 12 the signal goes direct- THE “HIGH” THAT LASTS IS MADE IN THE U.S.A. Model KSN 1141 The new Powerline series of Motorola’s 2kHz Horn speakers incorporate protection circuitry which allows them to be used safely with amplifiers rated as high as 400 watts. This results in a product that is practically blowout proof. Based upon extensive testing, Motorola is offering a 36 month money back guarantee on this product should it burn out. MOTOROLA PIEZO TWEETERS AVAILABLE FROM: DICK SMITH, JAYCAR, ALTRONICS AND OTHER GOOD AUDIO OUTLETS. IMPORTING DISTRIBUTOR: Freedman Electronics Pty Ltd, PO Box 3, Rydalmere NSW 2116. Phone: (02) 9638 6666. 46  Silicon Chip Frequency Response: 1.8kHz - 30kHz Av. Sens: 92dB <at> 1m/2.83v (1 watt <at> 8Ω) Max. Power Handling Capacity: 400W Max. Temperature: 80°C Typ. Imp: appears as a 0.3µF capacitor Typical Frequency Response YOU CAN AFFORD AN INTERNATIONAL SATELLITE TV SYSTEM SATELLITE ENTHUSIASTS STARTER KIT Fig.2: IF signals are applied to pins 4 & 5 of IC1 on the mother board (Akai VS-35) and appear as demodulated audio on pin 11. From there, the signal eventually goes to pin 11 of connector WF10 and then to the preamplifier board where it is combined with the audio from the A/C head. ly to a 680Ω resistor, R276, which is mounted alongside the WF10 connector. There was a clean signal on both sides of R276 so I traced the copper track to the next convenient point, a jumper link about 12cm away, near the edge of the board. And bingo! – there was a faulty signal at this point. The fault was somewhere along that 12cm of track. I pulled the motherboard out and examined that length of track in the minutest detail, using my most powerful glass. I couldn’t pick it so I resorted to cleaning away small areas of lacquer on the track, allowing the probe to make contact, until I narrowed the fault to a small length near resistor R270 and transistor TR214. It was then that I noticed a hole in the board through which a mounting screw was fitted. Suppose someone had been a mite too heavy handed in fitting that screw; could it have cracked the track? Now that the search area had been narrowed to within a couple of centimetres I took a long hard with the glass. And, yes, there was no doubt about it; the finest and faintest of cracks could be discerned but only because I knew where to look. As usual, once a fault is found, the whole thing becomes something of an anticlimax; a spot of solder was all that was needed to bridge the gap. Then, just to be on the safe side, I fitted a wire link anyway. That fixed it. No amount of bashing, prodding, or soak testing produced any sign of the fault. I progressively put everything back in place and there was no sign of trouble. Final­ly, I phoned the customer and told him to collect it. He indicat­ed that he would come around immediately. The unbelievable Now you’re not going to believe this. No sooner had I hung up than the machine went into its act again. It is not enough to say that words failed YOUR OWN INTERNATIONAL SYSTEM FROM ONLY: FREE RECEPTION FROM Asiasat II, Gorizont, Palapa, Panamsat, Intelsat HERE'S WHAT YOU GET: ● ● ● ● ● ● 400 channel dual input receiver preprogrammed for all viewable satellites 1.8m solid ground mount dish 20°K LNBF 25m coaxial cable easy set up instructions regular customer newsletters BEWARE OF IMITATORS Direct Importer: AV-COMM PTY. LTD. PO BOX 225, Balgowlah NSW 2093 Tel: (02) 9949 7417 / 9948 2667 Fax: (02) 9949 7095 VISIT OUR INTERNET SITE http://www.avcomm.com.au YES GARRY, please send me more information on international band satellite systems. Name: __________________________________ Address: ________________________________ ____________________P'code: Fig.3: this is connector WF10 on the mother board. The audio signal was clean until after R276, on the way to the modulator. __________ Phone: (_______) ________________________ ACN 002 174 478 December 1996  47 me; no, that wouldn’t be anywhere near strong enough. Unfortunately, no amount of wailing and gnashing of teeth was going to solve the problem. I just had to get stuck into the monster and start all over again. Nor was it any help to know that the customer was on his way and that I would be under pres­sure to either come up with a quick fix or some kind of an excuse, although I couldn’t imagine what it would be. As it transpired, I had a reprieve. The customer had been delayed and rang back after a short time to say that he couldn’t make until the next day. But a reprieve was all it was; I still had to start all over again. By the time I had delved back to where I started I was shocked to find that the fault now appeared to have shifted back to the preamplifier board. Indeed, I quickly found that by flexing it at one corner I could create the fault. After seeking desperately for some kind of inspiration, I finally decided to look more closely at a number of surface mounted components on this board. I was clutching at straws but it was all I could think of. To tell the truth, surface mount components do not usually cause problems – at least not in my experience. Of course, there are a lot of factors that must be considered at the manufacturing level but, provided due care is taken, a surface mount assembly is extremely reliable. So had I overlooked something? As I said, I was clutching at straws but, after going over this part of the board and re-soldering everything yet again, it did appear that the problem was fixed. Mind you, I could be pardoned for being sceptical. Anyway, all seemed well for a while until I started to put everything back together again. Then the fault reappeared only this time it was pretty clear that it was on the motherboard. What was more, after more prodding and flexing, it appeared that the sensitive area was now back near where I had found the origi­nal crack. I fished out the audio probe and began tracing the audio path as before. But this time there was a difference. Originally, the audio signal had appeared as a clean signal on pin 11 of socket WF10 on the mother board, having originated from IC201. And, after its journey to the preamplifier board, it was still clean when it reappeared on pin 12 of WF10. But not this time – the fault was now obvious on pin 11 of WF10. Now the important point about this is that the copper track running from pin 11 runs parallel to the track from pin 12 –the very track containing the fault which I had originally repaired. Another crack It didn’t take Sherlock Holmes to suspect that there might be another crack in the adjacent track. And so it was; this crack was even more difficult to see and the audio probe gave the only positive indication. Any­way, I bridged it as before and tried again. And this time the job held in spite of all I could do it. I even finished the job before the customer turned up. Con­sidering everything, I would have preferred to give the machine a much longer test but the customer wanted it back as soon as he could get it. Yes, I thought the worst every time the phone rang for the next few days but there was no sign of a bounce. A phone call to the customer several weeks later confirmed that the machine hadn’t missed a beat. Still, I’m SC keeping my fingers crossed! 48  Silicon Chip