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SERVICEMAN'S LOG Of ships & shoes & sealing wax Of ships & shoes & ..? Well, something like that. More exactly, this months column is a collection of items which have accumulated over the past few months, typically letters from readers. Unfortunately, good intentions don’t always work out in practice. For one reason or another, there always seems to be a space problem and so, finally, they simply had to be dealt with in one hit. So here goes – and my apologies to all those con­cerned for the inevitable delays. My first item is from one of my regular contributors of the past, J. L. of Tasmania. We haven’t heard much from J. L. in recent months and I had gained the impression that he has scaled his activities down somewhat. 38  Silicon Chip Anyway, here is his latest story. I had a job last week that was totally wasted on an old bloke like me. The manager of the local gymnasium had asked me to have a look at their cassette deck, the one used by the aerobics in­structors to play the music for their students to puff by. I was told that the deck would only play half a tape and would then slow to a stop, just when the exercises were getting up to full speed! What I wasn’t told was that I would have to do the job on the job, during a class, because their spare deck was not up to continuous use either! They had to swap from one ma­chine to the other to get through a session. I don’t know if you have ever tried to work in a room with 50 young women, each scantily clad in lycra tights and bouncing about all over the place to the thumping beat of loud rock. As I said at the top, the job was totally wasted on me. The deck turned out to be a Tascam Ministudio, a 4-track 4-channel recorder designed primarily as a semipro deck for rock bands and the like. It is quite a rugged unit, which was why it had been chosen for play-only use in the gymnasium. Unfort­unate­ ly, it had “play-onlyed” just a little too often and was now due for some serious maintenance. Flying blind There was no way I could do a proper test of the deck in situ. It was accessible enough, on top of a small wooden cabinet, but there just wasn’t enough space around it for tools and test gear. Quite obviously I was going to have to fly blind with this one. Fortunately, I have had quite a lot of experience with audio tape decks, going back to long before VCRs became common. I also have a useful collection of specialised tools, including a torque meter which proved to be the saviour in the present job. First up, I loaded a 1kHz test tape into the machine and set it playing. With a pair of headphones plugged into the unit’s “phones” socket, I was just able to hear the test tone and con­ firm that it was running at about the correct speed. As I stood watching the frenetic activity all around me, I became aware that the tape was definitely slowing. This was after only about five minutes, so I could just picture all the exercis­ing slowly grinding to a stop. My next move was to replace the test tape with the torque meter. A tape deck in good condition should give a take-up torque reading of around 4050 gram/centimetres. This one was hard pressed to reach 5g/cm, which explained why the tape was so loosely spooled in the cassette that I could see daylight through the layers. I can’t remember when I first learned the usual cause of this problem, so I can’t tell our younger readers just how I first found it. I do know that I have since found hundreds of stretched or hardened main drive belts in both audio and video cassette decks. And so it was in this case. I opened the machine and exposed the bottom of the mechan­ism. I used a finger to rotate the main flywheel, Well, that’s J. L.’s story and it is good to hear from him again. But I can’t help wondering whether he is perhaps coming on a little too strong in his protestions – that he is promising to not enjoy himself just a little too emphatically! The Marantz VCR which should have caused the motor pulley to spin. It didn’t move! When I powered the mechanism, the wedging action of the motor pulley managed to drive the belt but the flywheel barely moved. That was enough – I removed the old belt which had become quite hard and highly polished. I selected a replacement belt about 1cm shorter than the original and fitted it into place. When the machine was reassembled, the torque figure had gone up to 55g/cm – a whisker high but one that will provide a margin for the future. By this time the noise and activity around me were begin­ning to wear me down. So rather than hang around to test the deck, I set it running and made my departure. When I checked next day, I was told that it was working perfectly, so my diagnosis must have been correct. I promised to go back later to complete the cleanup and general maintenance of the unit but only if they could assure me an hour free from the distractions of young women and loud rock! When I related this story to one of my younger colleagues he couldn’t believe that I was more interested in repairing a tape deck than in watching the girls do their aerobics exercises. Apparently, at his gymnasium, they have segregated classes and he would have given his left leg to get into the girls’ class. He tried to convince me that he was only interested in “...all that stretchy Lycra” but somehow I don’t believe him! Next up, is a letter from A. M. of North Turramurra, NSW, concerning a problem with a Marantz 740A VCR. It is similar to the situation described in the Serviceman’s Notes for December 1995. Anyway, here’s his story. Like many readers of SILICON CHIP, I usually read your column first when the magazine arrives. The December issue was, however, a double treat because it contained the solution to a problem that has consumed many hours of head scratching and frustra­ tion. I refer to the power supply fault in the Marantz 740A. The problem first showed itself as a curious sort of “rotating echo” effect in the sound. After some fiddling, I found that if the Audio Play switch which had been set on Mix was set to Hi-Fi or Linear, the effect disappeared. As a result, I dis­missed the episode as an intended gimmick. Some months later, I noticed that the picture was somewhat degraded when the machine was first switched on but that this cleared after a few minutes. This slowly became worse, until it took about 15 minutes for the picture to clear. At this time, I also found that with the Audio Play switch in the HiFi position there was an intolerable frame buzz, while the picture was degrad­ed. The fault was obviously heat affected and the service manual showed that the Video Head Amp PWB-Y was the only common signal path for hifi audio and video. I heated and froze every component on the board but without any result. Incidently, the circuit diagram for PWB-Y and others have been helpfully (?) marked to show the signal paths on playback and record. I assume this was originally in colour but the manu­als as supplied are black and white copies, so that all the path markings succeed in doing is to obliterate the details of the circuit! On the grounds that most faults are mechanical but without any clear idea as to where the thermal effect would arise, I carefully cleaned the heads, November 1996  39 Serviceman’s Log – continued aligned the tape path, and replaced the pinch roller – all of which effected a slight improvement but did not correct the real fault. It was clear that there was no problem with the record function but, when we could no longer stand the playback delay, we bought a Sanyo VHR-310. It was a nice luxury to have two recorders, even if one could record only. You can no doubt imagine my joy on reading your December 1995 column and I wasted little time in diving into the monster again. Your comments about access are spot on but I’ve delved into this device so many times that the operation didn’t take long. I was glad of your encouraging comments about bending the support bracket out of the way. This is something I would normal­ly be reluctant to do but which almost cannot be avoided in this design. Of course I was rewarded with immediate and 40  Silicon Chip complete success. It is not at all obvious why the fault manifests itself in this way, or why the ±9V regulator is designed in a way which is bound to make it more fault prone. Thank you once again for the solution to a most obscure problem and for an always entertaining and informative column. And thank you, A. M. for your interesting report of this exercise. I have no doubt someone else will benefit from your time and effort. The GC181 colour TV set My next letter comes from N. B. of Epping, NSW and concerns a story in the Serviceman’s Notes for January 1989. The set on the Serviceman’s bench was a GC181 colour TV set and it had suffered an imploded picture tube and sundry other damage but without any clear reason for the fail- ure. So here is N. B.’s story about his GC181 TV set. The set was bought in 1977, together with a small trolley that held it about half a metre off the floor. Around five years later, my wife and I were snoozing on a Sunday morning to the usual restful background of our three sons squabbling over which cartoon show they would watch. Suddenly there was a thump, fol­ lowed by absolute silence. After some minutes, our eldest son put his head around the bedroom door and announced that the TV set was dead. Feeling that it would do them no harm to miss the cartoons, I told him to switch it off at the mains outlet. When I later went to investigate, I found that the set had been pushed backwards off the trolley and was lying backwards at about 45 degrees. The middle of its base was against the back edge of the trolley and the extension of the back cover was resting against the wall. The soft old plaster of the Glebe terrace house had been dented by the case but had not come away from the wall. Now, I know almost nothing about how TV sets work but I had been building audio gear since about 1960. I figured that the damage was probably physical and that I might be able to repair it. Taking the back off, I found that the neck board had broken in half. I had on hand some single-strand copper bell wire, so I carefully cleaned the tracks each side of the break and soldered a short length of bell wire across each broken track. The neck board is so small and light that the soldered joint provided enough mechanical strength. However, this did not fix the set. On investigating further, I found that (as in the set you described) the main board was cracked from front to back about 50mm from its lefthand side. I carefully removed the various plugs and boards from the main board, noting where each went, and considered the problem. Obviously, wire links would not provide enough mechanical strength for this board, so I used Araldite first to repair the break. When this had set, I soldered bell wire links across each of the broken cracks and then reassembled the set. Much to my surprise, it worked perfectly. The point is that a fairly minor fall backwards snapped both the neck and main board. If the set had fallen all the way to the ground, it is quite possible that it would have suffered the same damage as your customer’s set. I wonder how much dif­ference there is between an impact that snaps a neck board and one that breaks the neck off the tube. Incidentally, about five years ago the set again fell off the trolley. That time it fell forwards and landed flat on its screen. There was no damage. It still works, although now as a second set. Its only problem is that it really needs new volume and colour pots – the sliders have to be wedged in place with bits of cardboard so that the contacts bear on a relatively unworn area of the track. Not bad for a TV set that is almost 20 years old! Servicing at sea Finally, there is a most unusual story from A. D. of Whan­garei, New Zealand. And this is another of those letters which has languished too long in the “too hard” file, while trying to fit everything in. Here is his story. Intermittent faults are the most annoying things, especially when it’s your own equipment and is currently in use. To set the scene of this story I should explain that I live on a yacht and I picked up a TV set while cruising in Australia. It is a 12.5cm Trakka 15 colour set made by Philips (KA212). I have another TV set, a 25cm model, so the 12.5cm model was stored in a box and didn’t get much use over the winter after I bought it. It was a very wet winter and very damp on the boat. When the set was eventually pressed into service so that I could watch TV in bed, it wasn’t long before it developed a couple of faults. When first turned on, a hissing sound could be heard from the EHT supply but this usually stopped after about five minutes, so I wasn’t too concerned about it. The more serious fault would cut out the picture and sound completely. The channel selector indicators would remain lit, but I couldn’t change channels. Then the picture and sound would return but the set would always revert to channel 1. I thought a few hours in the sun might dry the set out, as my suspicions were that the EHT system was being loaded and that this set might have an overload trip that was resetting itself. But after sunning the set the fault became more frequent and annoying. And it always seemed to fail right on the punch line of the show I was watching. I found that I could sometimes get the picture back by rocking the monitor/TV switch on the back of the set and, since I had stowed the set resting on its back, I suspected that I had damaged the switch. So, finally, the cover came off. This set has two large circuit boards with quite a few discrete components, plus a sub-board with the monitor/TV switch on it. The switch was going to be impossible to remove so I simply linked the respective switch points, as the set was never going to be used as a monitor any­way. I thought that would be it and sat back to watch Fawlty Towers but, five minutes later, it was in fault status again – just as Basil was annoying some German diners. I removed the cover again and this time did some probing with a meter. The sub-board has a number of cables to it: video/audio, in/out and power. I did consider trying to remove the whole sub-board but settled for leaving the meter connected to the 9V supply rail and waited. It read 8.92V when the set was normal but dropped to 4.2V when the fault occurred. And so the tracking down began. I had to find the start of the 9V stabilised rail, which involved checking other supply rails carrying higher voltages. I finally came to a TO220 package with 12V on one pin and around 9V on the other two. So out came the magnifying glass and, sure enough, I dis­covered a dry joint. I can only assume that flicking the switch may have applied extra load to the circuit and temporarily remade the contact. I cleaned the EHT connections, applied silicone grease to the EHT cap and put the cover back on for the final time. I am not a TV technician but an experienced general techni­cian. Well, that’s A. D.’s story of his life on the ocean wave. And I don’t know about not being a TV technician, A. D. I reckon you’ll make a pretty good substitute in the meantime. And that’s the roundup for this month. Which isn’t a bad effort considering that we have been from Tasmania to New Zealand and points in between, and covered everything from aerobics SC to marine electronics. November 1996  41