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Beating an intermittent It is not often that an intermittent fault can be regarded as predictable. Which is one of the reasons we hate them; they waste a lot of time but seldom teach anything. Well this month's story is different - I've had four so far and, after the first one, they were a snack. Perhaps I'm stretching things a little to imply that these were true intermittents, but they did exhibit the characteristic that sometimes they would work and sometimes they wouldn't. So as far as the customer was concerned, they were just that. What's more, it was serious enough to fool at least one professional service organisation. The story started with a call from the local video store proprietor. As well as video cassettes, they stock a range of video recorders and players for rental, and I service these units. In this case the ·device was a Rank Arena VCR, model RV340, and the complaint was that it would not rewind or fast forward. No mention was made of any other fault and I suspect that this was because it was seldom used in a situation where the fault would show. In fact, it was used mainly in the shop to check or display cassettes, rather than for rental. (It ultimately transpired that this machine was marketed under at least four labels. As well as the Rank label it appeared as the JVC model HR7200EA, which is the origin of the beast, as General Electric GE V6900, and as Ferguson 3V29A Ferguson being associated with EMI-Thorn in the UK.) I was at a disadvantage with this machine in that I had not encountered it before, and had no manual for it. On the other hand most machines are broadly similar and I felt reasonably confident about tackling it on the basis that it was probably a fairly straightforward fault. Time enough to worry about a manual if the fault turned nasty. I'm not sure how old this particular machine was, but I would suspect at least five years. It was a top loading type and probably among the last of these before the front loading types appeared. Initial checks I pulled the cover off, checked for any obvious faults, like foreign objects or mechanical damage, then loaded a tape into it to see what would happen. The machine accepted the tape and when I pressed the play button, it began to play. The only snag was that the takeup spool was not being driven and tape began to build up as it came away from the drum. I hit the stop button before the automatic shut- J;IGH~1 54 SILICON CHIP down functioned, then tried to make the machine wind the surplus tape back into the cassette. It wouldn't, which more or less tallied with the owner's complaint. My first reaction was that this machine looked very like a Sharp and, based on experience with the Sharp, I suspected the tyre on the idler wheel; the wheel which is driven by the reel motor and which toggles between driving the supply reel disc and the take-up reel disc. When the tyres deteriorate they no longer provide the necessary drive to these discs. I had some spare tyres on hand and it wasn't a big job to fit a new one. Then I tried the tape again, only this time I simply checked for forward and rewind functions. Neither operated and I realised that I had a more serious fault. The most likely explanation now was that the reel motor was not running, but confirming this was not as easy as might be imagined. The idler wheel is driven directly from the motor shaft and this is quite small, precisely ground, and revolves at high speed. It is also difficult to see. I tested it by touching the blade of a screwdriver against it and feeling for vibration. There was none. Dead reel motor So was it a faulty reel motor, or a faulty circuit robbing it of drive voltage? Clarifying this involved inverting the machine and removing the approriate covers. Then it was a short step to establishing that, yes, the reel motor had "had it" . Reel motors aren't cheap; they can range from $40 to $100 according to make, so I conferred with the owner. He didn't hesitate and said go ahead. From there on it was routine: order the motor, give the machine a routine clean, drum, heads, etc while waiting, then fit the new motor and give it a test run. It came through with flying colours and produced a first class picture. But rather than accept it on the basis of a few minutes performance I decided to give it a long test run. I had no particular reason to do this, other than the fact that the owner wasn't in a hurry and I had a spare monitor available. And so it ran for several hours, and it performed perfectly. Then I turned it off, mainly because I needed the power point. And this is most important: the machine was turned off at the power point rather than by its own on-off switch, which would have left it in the standby condition, with clock running. Some time later I gave the machine another run, a shorter one this time, and again it performed perfectly. But when I switched it off this time, I left it in the power point with power on, switched off at the machine, the clock running and everything ready to go. TETIA CORNER Sony KV1800-AS Symptom: No sound or picture. 11 0V rail up to 135V. There is no line drive out of the "D" board but the set will run normally if the line oscillator is powered momentarily from an auxiliary supply. Cure: C531, a 1 0µ,F 160V electrolytic capacitor , open circuit or dried out. At switch on, this capacitor charges from the 11 0V rail and feeds a momentary burst of power into the 18V rail. This starts the line oscillator after which the line output stage supplies the normal 18V rail. It sat like this for a couple of hours, then I decided to play it again, not so much as a test but because I had a tape I genuinely wanted to play and check. And this was where things went wrong. The machine went through the loading procedure - ie, wrapped the tape around the drum - but after a moment, shut itself down. I went through the exercise a couple more times but with the same result. So there was nothing for it but to pull the top cover off, load the tape in once again, and try to see what was malfunctioning. It wasn't hard to find. The machine loaded the tape around the drum correctly, started the drum motor, and was all ready to play except for one thing: the pinch roller had not closed on the capstan. As a result. the reel motor did not start and the machine, sensing that something was wrong, shut down. Two step pinch roller I went through the exercise again and observed the pinch roller action more closely. It actually moves in two steps, starting some distance away from the capstan and making its first move as the tape is being loaded but stopping slightly short of engaging the tape and capstan. Then, when the tape is fully loaded, A l'llll. "1988 55 it moves into contact with the capstan. Or that was what was supposed to happen. In reality, it baulked at that last movement. And, incidentally, that final movement signals other functions, such as the reel motor operation, to commence. This was a serious setback. I had seen enough of the mechanical setup, plus the electrical system as represented by innumerable cables, plugs and sockets and solenoids to realise that this was no job to tackle without a manual. I rang the owner, explained that we had a second fault, and asked a few discreet questions to try to confirm a theory I was nurturing. The result was inconclusive, although he recalled that it had failed to operate on odd occasions and, yes, this could have been in the circumstances I suggested. Hot theory Well, at least he hadn't shot down my theory. Whatever it was, I suspected that the fault was temperature conscious. If the machine was plugged in cold and a tape loaded immediately it would play it for as long as it lasted - up to four hours. But remove the tape and try to play another one and the machine's internally generated heat would prevent it. Similarly, if the machine was simply left for a time in standby mode, its own internal heat would then stop it playing a tape. As used in the video shop it was seldom plugged in until needed, so the fault had gone unnoticed except for a few occasions when it had been left on. And since it came good the next time, it was not of much concern. I went on to explain to the owner that I feared it was going to be a difficult job, aggravated by the fact that I didn't have a manual. This latter now appeared to be essential and I wasn't sure I could get one. Obtaining a manual was complicated by the fact that I was still under the impression that it was a Rank machine. It was only during a visit by a rep. from a parts distribution firm that I learned its real origin - JVC. More to the point he said he was sure he could get one, but warned that it could take time 56 SILICON CHIP I F\N\~~EP U? Wrn-\ ~E T\>JO IV\~C. \·H ~E.S 'S'rR \'?~ED l'l P...~ ED; CS\~~'<- S\DE. ON \r\~ "BE~C..~.. · and would be fairly pricey, which is par for the course for these manuals. I told him to go ahead. And he was as good as his word - in more ways than one. He obtained a manual, but it took over three months and it cost $65. I didn't qibble over the price. As I said before, that kind of figure is typical for such manuals, but I do wonder if the general public realise the kind of "incidental" costs like these that a serviceman has to meet. I'd hate to speculate on the capital cost represented by the stack of manuals in my workshop; costs which must ultimately be passed on to the customer. On the bright side was the fact that it turned out to be a first class manual; very comprehensive and virtually devoid of Japanese English. And that's a plus in anybody's book. Back to the fray So back to the problem, which was pretty confusing. The fact is that the inside any video recorder is the most complex arrangement of levers, cams, gears, belts, pulleys, clutches, springs, shafts and pushrods that anyone can imagine. Add to this sundry switches, plus the occasional photocell and exciter lamp, and you have a combination against which Heath Robinson's collections of strings, springs, weights, and lighted candles pales into insignificance. More to the point, one can never be sure, initially, whether a particular pushrod is pushing an adjacent lever, or being pushed by it. It's all very complicated. The obvious place to start was with the malfunctioning pinch roller. This is operated by the pinch solenoid which is controlled by two transistors, Q4 and Q5. It is an unusual arrangement in that Q4 feeds the whole of the solenoid winding, while Q5 feeds a tapping. My initial reaction was that this was a way of providing the dual action of the pinch roller, as previously noted. In fact I was wrong. The first movement of the pinch roller is purely mechanical and only the final, shorter movement is enabled by the solenoid. (I will deal with the reason for the two transistor arrangement later.) But this was of little consequence. The important point was that the roller was not operating for the simple reason that the solenoid was not being activated. There followed a long and involved chase through both the circuit and the machine to find out why. This was prolonged by the need to set up the hot and cold conditions needed for "fail" or "perform" behaviour. I won't bore you with the details. Suffice it to say that I cleared the ,,,i.., ,"r1, I / I 1/ f I I r1 I I I '7"1 ® / / I f'\1, I I I t:.J L--v / / / Loading motor-++--~' \ Capstan motor Capstan belt ------ Capstan flywheel '-- Capstan Take-up clutch belt Take-up clutch Take -up idler Take-up reel disk / Supply reel disk Reel idler Reel motor Reel idler Fig.1: general layout of the video recorder, with the loading motor and associated gearing shown dotted in the top left corner. The pinch roller (not shown) is adjacent to the capstan. transistors and finally established that they were supposed to get a signal from what is called the "after-loading switch"; (AL) SW S002 on the circuit. So I had to find this device and more importantly, what was supposed to activate it. To tell the truth, I never did lay eyes on it. Its location is indicated on some of the drawings, but it is totally hidden by other components. To replace it would probably mean stripping down most of the deck. Faulty twins At this point I put the machine aside, partly to give myself time to think, and partly to attend to more urgent jobs, the owner having indicated he was not in a hurry. So it sat on the bench for a couple of weeks, until, in fact, another customer appeared with the GE version of this machine. His was a long tale of woe but, in essence, it boiled down to exactly the same symptoms as the Rank. So now I had two nasties to deal with. Which meant two failures if I didn't crack it, or two service fees for the one effort if I did. (That's how servicemen think!) I finished up with the two machines stripped naked, side by side on the bench. After a number of test runs I confirmed that both not only had the same symptoms but that behaved in exactly the same way in regard to the pinch roller. So it really was the same fault. So what now'? I was still trying to work out what activated the after loading switch and, after numerous tests in both the fault and no-fault condition, I latched onto a particular lever. I think it is called the pinch roller arm push plate and is shown as ·having one end in the area of the after loading switch, with the other end attached to a spring in close proximity to the pinch roller arm. It is made with two slotted holes which fit over two studs, and it moves back and forth for the length of these openings during the loading and unloading sequences. (I later concluded that it provides the initial movement of the pinch roller.) I picked on it partly because of its proximity to the after loading switch and partly because I noted that, in the no-fault situation, it moved over the full length of the slotted holes but, in the fault condition, only part of the way. In an effort to clarify things I set up a fault condition, then pushed it gently the rest of the way when it baulked. The pinch roller snapped home immediately and the machine played, but the picture was poor / \I'll/I. Hlll8 57 loading ring was obviously what activated the after loading switch, it was clear why the system failed. I was able to prove this by gently prodding the loading ring the remaining distance when it baulked. Everything came good immediately and the machine produced a perfect picture. . . : Unloading c>: Loading Slack belts Wormwheel gear Worm gear Take-up loading ring Loading motor Loading belt Fig.2: exploded drawing of the loading rings, driving gears, and loading motor. Note the direction of rotation. due to an apparent tracking problem. VCR revision To follow the story from this point the reader will need to have some idea of how the loading mechanism works in a typical VCR. When the cassette is inserted it is located over the two reel discs (supply and take-up) with the tape in front of two guide rollers. The protective cover of the cassette is then opened and the two guide rollers then move away from the cassette, one each side of the head drum, drawing the tape with them and wrapping it around the drum for a little over half its circumference. It was the mechanism associated with these guide rollers which took my attention next, and their operation is most easily explained by Figs.l, 2 and 3 taken from the manual. The guide rollers are moved by two loading concentric rings which rotate in opposite directions. These are driven by the loading motor, via a wormwheel and a gear train which drives the upper ring directly, and the lower one via a relay gear to provide the reverse direction. 58 SILICON CHIP As shown in Fig.3 the guide roller assembly sits above the slide ring which, in turn, sits over the loading ring, being coupled to it via a spring. Now notice particularly the slotted holes in the slide ring, and the pins on the loading ring which sit inside them. During the loading operation, with the upper ring rotating clockwise, these pins would occupy the positions shown, until the guide roller assembly reached the end of its travel as dictated by stops called pole guides. At this point the loading ring(s) would continue to rotate until the pins reached the other end of the slotted holes, thus putting the spring under extra tension and driving the guide roller assembly hard against the pole guides. This is to ensure that the guide rollers remain rigid and hold the tape accurately against the drum. Failure to do so can cause tracking errors - as I observed when trying to brute-force the system. At least that was what was supposed to happen. What was really happening, under fault conditions, was that the pins moved only about two thirds the way along the slots. And since this final movement of the But why was the system baulking? Putting it through the loading process a couple more times, in fault condition, produced the answer. The loading motor . continued to run briefly after the system baulked, with the belt simply slipping on the pulley. A little extra tension on the belt, artificially applied, cured the problem, albeit temporarily. I suspected the belt was the culprit, either worn or aged. Starting with the Rank, I pulled th~ belt out, along with the other two; the capstan belt and the take-up clutch belt. All three were in poor condition and needed replacing, even though the other two had not given trouble. Since I had none in stock I put several sets on order. Incidentally, the loading belt is the hardest one to get out. The only way to get it out is to remove the complete loading motor assembly, which is relatively simple, involving only two screws. Then a circlip is removed from the end of the worm gear to allow the shaft to slide out and release the belt. It is easy enough to fit the new belt to the motor assembly while it is out of the machine, and easy enough to slip the assembly back into place. But unless the correct associated procedure is followed the machine will not function. There is not much in the manual covering this operation and I finished up doing it on a suck-it-and-see basis. In fact the procedure is simple enough in theory but, in practice, needs about four hands. All that is needed is to pull both guide roller assemblies hard back against their stops in the unloaded position, and hold them there while the loading motor assembly is slipped into place and the two securing screws fitted. I won't try to explain how I did it with only two hands! Slant pole Guide ro ller Supply pole base €,; 0 Supply loading ring Pin I ' slide ring Fig.3: detailed drawing showing the supply slide ring (note tension spring) and the guide roller assembly driven by it. Next I tackled the GE machine. The situation here was the same; a faulty loading belt. But the interesting thing was that the other two belts were brand new, having obviously been replaced during a recent service. But why had the real culprit not been replaced? Too hard I suppose. Naturally, after the battle with the Rank, the GE machine was all plain sailing, and I had it up and running in short order. Which helped make up for some of the time expended on the Rank. Since then I have encountered two more machines with the same fault, which has put the balance sheet for this episode back in the black. Post mortem But there are some points still to be clarified. The most important one is the reason for the thermal sensitivity of these machines when the loading belts deteriorate. My first reaction was to assume that the elasticity or size of the belt changed with the temperature. On reflection, however, I'm inclined to reject that theory. I don't believe that the temperature change was great enough to affect the belt material in this way. I'm more inclined to the theory that it is metal expansion that is involved; that somewhere the tolerances on some metal parts are just too fine. This is not evident while the loading belt can provide adequate drive, but shows up when the latter begins to wear. Unfortunately, I can't be sure either way. Next point: why does the pinch roller solenoid employ two transistors and a tapped winding? It ap- pears that the solenoid needs more pull to snap the pinch roller into place than is desirable when it is in operation. So Q4, which feeds the full winding, is turned on the whole time the machine is playing. Q5, which feeds the tap, receives only a 330ms pulse at the moment the solenoid is turned on, to boost the action. And that about sums it all up. It has been a long story, but I think a detailed explanation was justified. So keep an eye open for these machines and these symptoms. My experience could save you a lot of time and headaches. (Since this story was written the Serviceman has reported that another three similiar jobs have come in for repair so it looks like it will be a common fault. Ed). that this MHA was installed when television was introduced in southern Tasmania in 1960. Since then it has worked - and worked - and worked. I was able to help in regard to locating a new 6ES8 and this put the MHA back in working order again, hopefully for another 27 years. Out of curiosity I dug out my old valve manual from under a pile of forgotten literature and looked up the 6ES8. It was a variable-mu twin triode RF amplifier designed for series operation in TV tuners. In this mode it required a supply of 180V, together with 6.3V for the heater, and a variable negative AGC voltage. I did not see this antique MHA, so I can't be sure how these voltages were supplied. My guess is that there was a 240V feed up the mast to a transformer and solid state rectifier. (Yes, power diodes were available in the 60s.) The RF output was on 3000 open wire feeder continued on page 90 j) Back to valves To finish off, here is something in lighter vein from our colleague, J.L., of Tasmania. He writes: I had a call recently from a colleague in a bush town about 80km south of here. He asked if, by chance, I had a 6ES8. Now I have to admit that I asked him to say that again while I put my brain into gear. I haven't heard type mumbers like 6ESB for so long that I have almost forgotten what they mean. Then I recalled that these were common RF amplifier valves in 1960 model TV sets. I quizzed my friend as to why he was messing around with these old sets. It transpired that he was not servicing a TV set at all, but a mast head amplifier. He went on to say ,~CHNlC.lAN TR't'\NG ro GE", :Be.L,.°\"S ON o.. 1\i'HIL UJ/l/l 59 INPUTS X = FUSIBLE LINK CONNECTED OUTPUTS Fig.24: a generalised circuit for a programmable logic array. MSI circuit form. Most of those circuits are extremely sophisticated and perform not only addition but also subtraction, as well as many other logic functions. Such circuits are used as the basis for an arithmetic logic unit (ALU) in digital computers. Programmable Logic Arrays While a high percentage of digital applications can be implemented with the combinational logic circuits just discussed, there are also many applications that require special circuits. Those special circuits can often be made from the available combinational circuits, plus random gates and inverters as required. While the resulting circuit usually performs the desired function, a good number of chips must be U8- Serviceman' S Log continued from pag-e 90 which is, I believe, still the original material. When I think of the times a solid state MHA gets blown out by lightning, or zapped by some illegal CB afterburner, I wonder why they ever stopped making valve type MHAs. My friend was lucky to have asked me for an old valve. I did not throw away my valves when I stopped using them. They are stowed under the house, out of the way and almost forgotten. I still have some 6J8s, 6U7s, 6B8s, 6V6s, and 5Y3s. Can anyone remember what they were used in'? 90 SILI CON CI-111' ed. These chips take up a lot of space, consume power, require larger circuit boards and occasionally are not fast enough. All those problems can be overcome by using a programmable logic array (PLA). A PLA is an LSI or VLSI circuit consisting of multiple gates and inverters arranged on a chip in such a way that they may be randomly connected to perform almost any logic function. Semiconductor technology now permits manufacturers to quickly, easily and inexpensively manufacture custom circuits using PLAs. Other PLAs are field programmable. That is, the designer may specify his own circuit, then implement it himself with a PLA. Such circuits make it possible to replace MSI functional combinational circuits and all And another query: the twin triodes in the 6ES8 were described as being run in "cascade", meaning one under the other, like a waterfall. In my dictionary, a waterfall is described as a cascade. I have never found anyone who can explain why an electronic cascade is spelled "cascade" . Do you know'? Thank you J.L., for that little piece of nostalgia. I can't offer any explanation as the origin of the word "cascade", but I doubt whether it has any particular linguistic significance. I imagine it was nothing more than a sudden inspiration by someone groping for a term to describe the new circuit concept. Regarding the supply of power to the old MHAs. I cannot recall any systems where 240V was run up the mast, and it would have been a rather complex and expensive setup. As I recall, the most popular arrangement used the feeder as a supply line, power being fed up it at some convenient low voltage typically 32V if I remember correctly - to a transformer in the amplifier which supplied the required voltages. As to whether anyone can remember the valve types you mention: yep, I sure can; they are part of our history! ~