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SERVICEMAN'S LOG It was all a long time ago I have two quite unusual stories this month. For how long & by what devious means can one keep a TV set working? And for how long should one keep customers’ records? The other story emphasises how lost we can feel when away from our own workbench. The first story really started over 11 years ago. It was little more than a routine job then and I certainly did not imagine that I would be resurrecting it after all this time. But I did make notes about it; not only in the normal way for my own benefit but also for a colleague who expressed some interest in it. So I was able to recall the events in reasonable detail. It was all brought back by a recent phone call. A lady at the other end of the line introduced herself – the name didn’t register immediately – and went on to remind me that I had fixed her TV set, an HMV, “some time ago”, and that it involved a problem with the green in the picture. My memory isn’t particularly good on some-time-ago jobs (one handles so many jobs) but the mention of the green problem rang a bell. I asked her if she was living at a particular address at the time – it’s funny the way one’s memory works at times. Yes, that was correct. And the whole story came flooding back. In particular, I remembered how accurately she had de­scribed the symptoms; far more so than most people. And I also registered that it was more than some time ago; it was long time ago. In fact, I had begun to link it with other events and was thinking in terms of six or eight years, which wasn’t that far out. But despite the fact that I remembered the set and its symptoms, I could not recall the fault itself or what I had done to fix it. Anyway, I regarded all that as being of little importance and asked what problem she had now. To my surprise, she replied that the HMV set was the problem. Was that set still going after all these years? Yes, it was but, on her own admission, apparently not very well. Her son, who works for the ABC in another state, had visit­ed her recently and commented that the picture – and particularly the colour – was rather poor. In short, he realised that gradual picture deterioration had crept up on Fig.1: the RGB driver stage in the HMV 12642. The green driver stage, X01, is at the top of the diagram & is direct coupled to the green gun cathode. Doctoring this stage restored performance. 56  Silicon Chip her over the years and suggested that it was time she bought a new set. And that was the purpose of the call; not to initiate any service to the set but to seek my advice regarding a new set. In particular, she needed to know which brands I handled on a war­ranty basis because she wanted to be sure I would be available to service the set. Such loyalty can be quite touching at times. So that was more or less it. I nominated the brands I handle on a warranty basis and left it to her to choose which ever one she fancied. She thanked me and we left it at that. How long ago was it? And that should have been the end of it. But I couldn’t get the set out of my mind. Just how long ago was it? So I began searching through my old files. It turned out to be quite a search but I eventually found the relevant file and a copy of the invoice, dated September 1st, 1982 – over 11 years ago at the time of writing. But that wasn’t the real surprise; that was only appreciated when I read my notes about the fault and its cure. The set was a 48cm HMV, model 12642 (which was also market­ed as a JVC 7765AU), and it was about five years old at the time. The fault, as described by the customer, was that the picture had lost its green content. And I had noted that this customer was more astute than most; her description was completely accurate. In greater detail, she explained how the set had turned on a display of multi-coloured fireworks on the screen. This display lasted for a few seconds and the set then behaved normally; except that there was no green. As recounted in my notes, there was initially some confusion on my part as to the exact cause. At first I suspected some kind of green gun failure but, in fact, I found I was able to produce a green image using various brute force tactics. This led me to suspect a circuit fault, whereby the tube was not receiving adequate green signals, and I spent some time following this lead, only to finally conclude that it was false; that the fault had to be in the tube. More specifically, I con­cluded that the fireworks display had been caused by a fragment – probably loose cathode material – causing a momentary internal short. And this had damaged the green cathode, reducing it’s emission to the point where it was virtually cut off at the normal bias level. So was it worthwhile fitting a new tube or was the set a write-off? It was a marginal situation and the lady was not very happy about spending a couple of hundred dollars on a replacement tube, much less the cost of a new set. Finally, I decided to take a punt on a mild form of butchery. As matters stood, the tube and – potentially – the set were both a write off. But suppose I were to doctor the bias voltage; could I brute force the green back to a normal level? And, if so, how long would such a trick last? The first question was easily answered. Reference to the circuit (see Fig.1) shows that the collector of the green driver transistor (X01) is directly coupled to the green gun cathode and sets it at about 142V. By shunting R01, a 1.2kΩ resistor in the emitter circuit of X01, the current through X01 would be in­creased, its collector voltage reduced, and the bias on the green gun reduced. And it worked. After some trial and error, I settled for a 5.6kΩ resistor across R01, which produced virtually normal green performance. The second question was another matter. I had no way of knowing how long it would last but I reckoned a minimum of 12 months would be a reasonable guess. Anything after that would be cream on the custard. And so, on that basis, I felt that this ap­ proach was justified. It would give the customer time to assess her financial situation and decide on a new tube or a new set. I was most careful to explain what I had done; that it was a short-term measure that could only be justified on a nothing-to-lose basis. Assuming she was happy, I would leave it like that. She said she was and so I did. As I recall, I checked the situation about a year later and the set was still going strong. And that was the last I heard of it – until now. But another 10 years? That’s a lot of cream! Curiosity killed the ... Having sorted out all those memories, another thought oc­curred to me. Would the lady let me borrow the set, to check it and satisfy my own curiosity as to just how well it had stood up over all those years? So, a week or so later, when making January 1994  57 a call in the area, I took a punt and knocked on the door. When the lady answered I explained that I was simply making a courtesy call in case she needed any more advice about a new set and to ask whether I could have a look at the old set out of curiosity. She greeted me warmly and invited me in to see her new set. It turned out to be a 48cm Samsung and she was very happy with it. Then a tray appeared carrying the inevitable cuppa accoutrements – teapot, cups and saucers, and chocolate biscuits. And so we sat and chatted for a while. Eventually, I raised the matter of the old set again. Yes, of course, it was in the back room. And I could have it if I wanted it; she was a loss to know what to do with it. I went through the motions of protesting but she was adamant; take it away. And so, after a pleasant interlude, I came away with the old set. I didn’t regard it as valuable in the financial sense but I valued it for what I might learn from it. Back at the shop, I lost no time in setting it up. In most respects, its performance was first class – a good sharp picture, excellent geometry, normal sound, no tuner problems and no noisy controls. It was quite remarkable for a 16-year old set, with only one service job in that time. But the colour – yes, that was crook. I fed in a colour bar pattern and, as 58  Silicon Chip in the original case, the green was very weak. Also, the red was flaring but the blue was about normal. Just for the heck of it, I went through the motions of grey scaling and this improved things a little. But it was still very poor. Picture tube rejuvenation So what now? Did I have any more tricks? Well, there was one other possibility but it was a Sydney-or-the-bush approach. What about a spot of picture tube rejuvenation? There are various devices for this but the basic principle is pretty much the same. Normally, the heater is run at its rated voltage, although some authorities recommend overrating it some­what. I prefer not to. After that, a voltage of between 600V and 700V is applied to the grid for about two seconds. This typically creates some brief fireworks around the cathode area and the idea is to keep applying short bursts until this activity ceases. It’s very much a gamble. Sometimes it works and sometimes it doesn’t. And even when it does, there is no guarantee as to how long the effect will last. But what did I have to lose? So I set up the booster and went over each gun in turn. Significantly, I needed to give the green gun about eight bursts before all the fireworks ceased. For the others, one or two bursts were sufficient. Next, I went through the grey scaling process again. It was much improved now; almost good enough, in fact. But there was one limitation which is common with weak tubes, whereby the grey scale tends to vary with the setting of the brightness control. In this case, turning the brightness down would increase the green level in the lowlights, while turning it up had tended to emphasise the red. The best that one can do is try to balance things at what would be regarded as a typical brightness viewing level. So this was what I did. I then let it run in the workshop for a couple of days to see how it would hold, this being the critical aspect of tube boosting. And it did shift, again involv­ing the green. I gave the green gun a second short boost and grey scaled it again, which again improved things a little. So that’s where it stands at the time of writing. I hope it might stabilise a little more with time but I know I’m being optimistic. So of what use is it now? Well, not much for serious bench work, although it could serve as a loan set in an emergency. But what I am really hoping is that I might score a suitable tube from a set written off for other reasons. If that should happen, I may have a set that’s good for a few more years – but not 11! Serviceman’s holiday And now for a complete change of scene – literally. After several months of planning, I recently set off on a leisurely trip up the New South Wales north coast on what was partly a holiday and partly a business trip. Among other things, I had been invited to stop over for a few days with a family I have known for many years but had not actually seen for quite some time, although we have kept in touch. And I had little doubt that they would have an array of electrical and electronic jobs lined up for me. And so I had packed as much gear as was practical – multi­meter, soldering equipment, small tools and an assortment of likely minor components. Outside of that, I could only hope. And so I found myself settled in and we spent some hours mulling over old times and catching up on all that had been happening. But then came the practical problems of the present, in the form of a video recorder which had begun to play up just a few days before I arrived. It was a Sharp model VC-505X, a model with which I have had very little experience and, without a man­ ual, I was starting behind scratch. The problem itself was that, at times, the ma­chine would go into the play – or record – mode briefly, then shut itself down. And it was similarly erratic in fast forward or rewind modes. Anyway, the recorder was taken out of its cabinet and set up on a small table. Since the problem was a mechanical one, there was no need for a TV set connection at that stage. I pulled the cover off, pushed in a tape, and set it running. It loaded and ran normally the first time and for the next several tries but then suddenly baulked. And the primary reason was immediately obvious; the take-up reel had stopped and the take-up reel sensor had shut the machine down. We then tried the fast forward and rewind functions, with the same results. Well, there was obviously a fault somewhere in the reel drive mech- anism and this was the first hurdle. As I mentioned earlier, I am not very familiar with this machine; all I knew was that it used an unusual reel drive system. Most machines use a rubber-tyred idler wheel and this is supported on the end of a short arm which toggles to one side or the other and engages the wheel with the appropriate reel drive. Instead of the tyred wheel, this machine has a gear wheel mounted on the end of the arm. This toggles from side to side in a similar manner and engages a matching gear in the reel drive train. But that’s not all; as well as moving from side to side, the arm carrying this gear also moves up and down. This movement is probably necessary to ensure smooth engagement of the gears. That much established, I took a breather. In addition to the main problem, I was also aware of several routine things that needed to be done. One was a general clean-up of the capstan, heads and guides, which were a trifle grotty. Another was to fit new belts. There are only three belts in this machine; one from the capstan motor Protect Your Valuable Issues Silicon Chip Binders These beautifully-made binders will protect your copies of SILICON CHIP. ★ Heavy board covers with 2-tone green vinyl covering ★ Each binder holds up to 14 issues ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A11.95 plus $3 p&p each (NZ $6 p&p). Just fill in & mail the order form on page 101; or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. to the capstan drum and flywheel and two associated with the loading motor. They weren’t in bad nick but since it would probably be long time before I serviced the machine again, it seemed prudent to replace them if possible. Of course I didn’t have any such belts with me, or any cleaning alcohol. So, when the family announced a visit to the nearest large shopping centre, I jumped at the opportunity to try for what I needed. Medicinal alcohol Well, I was lucky. I found a TV service centre which had suitable belts and an obliging chemist who dispensed a small bottle of medicinal alcohol. Thus equipped, I completed the various routine tasks without incident but the mechanical problem remained a mystery. The problem was that the mechanism would not function at all unless there was a cassette in it and when there was a cassette in place, it was impossible to see the mechanism. The solution was crude but effective. My friend had several old tapes on hand which had been put aside due to wear or other faults. 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The result was a crude version of the commercial transparent dummy cassette I use in my workshop. With this in place, it all became clear. The reason the gear was not engaging was simply because the arm holding it was not lifting through the full distance. Well, that took me one step closer but the reason for this failure still had to be determined. In the event, this was the easy part. Closer examination revealed that the vertical shaft on which the gear arm moved up and down had become gummy due to some kind of lubricant, applied either during production or subsequently. A good swabbing down with alcohol cleaned this away and that was the answer. I chose not to lubricate it again; it was a plastic bearing moving up and down on a metal shaft and I seriously question the need to lubricate such a simple movement. Broken tape So that was that problem solved. But hardly had I heaved a mental sigh of relief than another one landed on my plate. It was a broken tape. More specifically, it was a recorded program 60  Silicon Chip which, while of no longterm value, was of considerable immediate interest. But they had seen only the first 15 minutes or so of it when the tape broke. As nearly as I could work out, the tape had been running for about 15 minutes when it became necessary to stop play for some reason. But then the recorder went cranky and would not start again. In the process, it had formed a small tape loop before the machine shut down. This loop then became tangled and the tape broke when the cassette was ejected from the machine. While not a common problem, it is not the first time a customer has turned up with a broken tape, begging for help. On my own bench it would be no problem. After the first couple of incidents, I invested in cheap tape splicer. It is a simple jig which holds the tape with the broken ends overlapping. An angled slot then guides a sharp blade to make a clean cut through both layers. The two ends now left butting together are joined with an adhesive patch. In spite of the jig’s low cost, it works quite well and has helped several customers. Prior to that, I had resorted to a more primitive approach. This involved making a simple overlap joint using an acetone-based adhesive, on the understanding that this joint would be used only to play the remainder of the tape and that it would not pass over the heads or capstan. Crude though it was, this idea worked too and it seemed that I would have to resort to a similar trick in this case. My friend’s workshop yielded a tube of clear acetone based adhesive and I went to work on the break, finishing up with reasonably neat joint. But I had no time to test it. It was time for me to leave and I wanted the joint to be left overnight to set properly. And so I departed, with an invitation to stop over again on my way back in about a week. I had an idea that there might be more jobs waiting for me then but I didn’t anticipate anything like what actually happened. It appeared that the jointing operation had been a complete success and the tape had been played to its conclusion. But when another tape was subsequently played, there was trouble. Although it was a 3-hour tape, at the end of two hours the take-up reel was chock-a-block full, to the point where excessive tape had fouled the bodywork, stopped the reel, and shut the machine down. Crinkled tape The basic reason was easy to see – the tape was crinkled, taking up much more space than normal. But why? I could only assume that something in the tape transport path was damaging it. So off came the cover and I went straight to guides, heads and capstan. And one glance at the capstan was enough. It looked as though it hadn’t been cleaned for years, even though I had cleaned it thoroughly only a few days before. More specifically, it displayed the two characteristic dark oxide rings (one tape width apart) that normally occur after prolonged use. Even more puzzling was the fact that attempts to clean it using alcohol and a tissue proved fruitless; it simply wouldn’t budge. Again, this is what one would expect from severe fouling over a long period – often requiring that the capstan be careful­ly scraped to remove the rubbish. But why after only 3-4 hours of playing? Anyway, I attacked it with the first thing handy – a fine screwdriver blade. I know that sounds drastic but it isn’t really, unless one is woefully heavy handed. And in this case only the lightest touch was necessary; all the fouling moved as one piece and came away as a tubular shell with the oxide rings at each end. The capstan was now as clean as I had left it. So what was it? The adhesive I had used to join the broken tape. And why? Because the tape had been rewound and, contrary to my advice, the join had passed over the capstan. How it happened I don’t know, and diplomacy dictated that I not stress the point. No real harm had been done apart from the damaged tape and nobody was upset about that. So it all ended happily in the long run. But I think the lesson is that trying to improvise away from one’s own workbench can be fraught with danger. It doesn’t always pay to be SC too clever.