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SERVICEMAN'S LOG The topsy turvy world of remote control A popular saying in my boyhood household was that “lazi­ness is no good unless it is well carried out”. It was usually prompted by my tricks that made my chores easier to do; a snipe with the implication that making things easier was cheating in some way. By the standards of those days, the modern TV set, with its remote control unit, must surely represent the ultimate in lazi­ness being well carried out. Be that as it may, they are now a fact of life and, to be fair, they offer more than the opportuni­ty for laziness. For the hospital patient and anyone confined to bed, or the disabled generally, they are a godsend. All of which is leading up to a story about remote control units. I could write innumerable stories about these devices; hardly a day goes by but that one of these turns up on the bench. The faults are mostly routine – routine for remote control units, that is, because they have a pretty hard life. They are left on chairs and sat on, which can be disastrous if it is a hard seat; they are dropped on the floor, which is bad enough in itself, but worse if they are kicked or trodden on; and the kids play “catchings” with them. One that came in recently had fin­ished up in the washing-up water in the kitchen sink. Strangely enough, I was able to salvage that one. The lady of the house scooped it out almost immediately it hit the water and, although some moisture found its way in, it wasn’t flooded. Nevertheless, the lady was very diffident about approaching me, convinced that it would be a write off. I thought so too, until I opened it. To my surprise, it didn’t look so bad, so I gave it a solid spray with dewatering compound, the kind of thing used to dry out wet ignition systems. I left it for a while to dry out, then 62  Silicon Chip gave it a try. And it worked. More importantly, several months later it is still working. So that was one of the happier accidents. Conventional faults The more conventional faults include various battery prob­lems. It’s not so much flat batteries though, because most users fit new batteries immediately there is a problem. However, the battery contacts can give a lot of trouble, mainly due to loss of tension. And there can also be problems where the contacts are soldered to the board pattern. This junction is often weak me­ chanically and, after repeated battery replacements, the joint fails. Then there are the inevitable corrosion problems. Sometimes it is a rogue battery that has leaked but more often it is due to exhausted batteries that have been left in a unit that hasn’t been used for a while. It is usually possible to re-tension the contacts and to clean up corrosion. The latter may mean removing the contacts, scrubbing off the corrosion, and re-tinning them with solder. Fractured joints can sometimes be repaired and sometimes not. And there is some pattern of inherent faults. One that comes to mind seems to suffer more than a fair share of keypad problems. Repairs are seldom satisfactory and they are generally written off. Another problem area concerns the ceramic resonator. In most units, the ceramic resonator is mounted flat on the component side of the board, with its leads bent at 90 degrees and taken through the board to the copper pattern. The weakness here is that, in most cases, the ceramic resonator is supported only by its leads. And, eventually, vibra­tion will take its toll; one of the leads breaks. Fortunately, there is usually enough lead left to salvage the situation, after which a dab of glue to secure the ceramic resonator to the board makes for a better-than-new repair. An unusual fault So much for the general background. The story that started all this is something else again. The fault is so unusual I was tempted to hold back on the solution and offer a prize of a free flight to the Moon for anyone who picked it. Fortunately perhaps, some distant Scottish ancestry caused me to have second thoughts. OK, down to the story. This particular remote control is an NEC model RD-309E and is teamed with an NEC model N3420 TV set. It is owned by one of my a regular customers and the complaint was simple enough and fairly typical: “It doesn’t go”. I checked the batteries (two AA cells) and they were OK but closer examination threw suspicion on the battery contact ten­sion, along the lines already mentioned. It was a simple job to bend the springs to provide adequate contact pressure, then replace the batteries and try it with a test unit I have. No joy; it was still dead. So I opened the case and set it upside down (ie, keypad down) on the bench. As can be seen from the photograph, this reveals the component side of the board, with the IC, the ceramic resonator, a transistor, the IR LED and a few minor components at one end. My first check was at the ceramic resonator. The leads were intact but the ceramic resonator was not secured, so a spot of glue was applied to hold it firm. I then set it up for further testing. I have found that the easiest way to work on most of these units is to leave them upside down on the bench and feed them from a variable power supply via a couple of clip leads. This is often more convenient than trying to use the batteries, which may not be very secure when the case is open. So this was the setup I used for this one. And lo and be­ hold, the thing worked. I pressed a number of keys from under­neath and everything seemed to be OK. The only snag was, I didn’t know what I had done and so I decided to press on and see what happened. I put the whole thing back together again, refitted the batteries, turned it right side up, and checked it again. It was as dead as the proverbial dodo. I pulled it open, hooked it up to the power supply, and checked it again. And it worked. So what was I doing wrong? Seeking inspiration, I carefully turned the whole thing over, checked the front panel, and tried again. Once more, it was dead. It didn’t need many such checks to confirm what I now sus­pected; it would work when upside down but not when right way up. Well, I had no idea what was wrong but I suddenly realised that I had probably been caught out once before with the same symptoms, without realising it. A couple of months before, a customer had brought in exact­ly the same model unit. And, initially, the complaint was the same: “It doesn’t go”. And with good reason it appeared. One of the ceramic reso­nator leads had broken but with enough lead protruding to allow the break to be bridged with a blob of solder. After then gluing the ceramic resonator to February 1995  63 Fig.1: view inside the NEC RD-309E remote control, showing the PC board. The components next to the IC, from left, are: the 47µF electrolytic capacitor, the LED driver transistor, the ceramic resonator & the IR LED. the board, I fully expected the device to work. It did too, when I mocked it up on the bench upside down. I assumed that that was the end of exercise, apart from putting everything back together. As it so happened, pressure of other work caused me to put it to one side at that point, the customer having indicated that he was in no particular hurry. When I did finish the job, about a week later, the thing was dead. Over the next week or so, I tackled it several times in between bigger jobs but without success. Sometimes I could make it work, sometimes I couldn’t. Unfortunately, because of the on-again-offagain approach, I didn’t recognise any pattern; I simply assumed it was one of Murphy’s sick intermittent jokes. When the owner subsequently dropped in to see how I was progressing, I gave him a rundown of the above sequence and advised him that an intermittent fault in one of these devices might be more costly to find and fix that the unit was worth. He thought about it briefly, than decided to write it off, and asked me to get him a new one, which I did. And the old one finished up in the scrap box as a possible source of spare bits (eg, the keypad). But now, alerted by the sequence of events with the unit on the bench, I suddenly realised that the supposed “intermittent” behaviour, could easily have followed an upside down/ right side up sequence, without my realising it. All of which was food for thought but not of much help with the immediate problem. But I was determined to track it down now. I pulled the board out and went over it with a glass, look­ing for dry joints. 64  Silicon Chip The only suspects were the two for a 47µF electrolytic capacitor, which looked a trifle dodgy. I un­ soldered the joints, pulled the capacitor out, checked it (it was spot on) and soldered it back in. This made no difference to the be­haviour. I tried bashing and prodding to try to make it stop when it was in the working position, or to try to make it work when it was the other way up. There was no response either way. What about the ceramic resonator? Was this operating in both positions? With some delicate fiddling I attached the CRO in a manner which I hoped would hold when I turned the device over. Thankfully it did and this confirmed that the ceramic resonator operated in both positions. So what else was left? Not much, it seemed. It had to be a mechanical fault of some kind, but where? I thought about the IC but, without ruling it out completely, put it at the bottom of the list. For one thing, it contains the oscillator circuitry and I knew that this was working. Then I had a wild idea. It wouldn’t be the first time that an IR LED had given trouble, although only as a total failure. But what if...? Well, it was a long shot but it was easy to try; I had spares on hand and it involved only two soldered joints. And believe it or not, that was it. The new LED cured the problem and the unit has now been back with the customer for several weeks, with no sign of trouble. Naturally, as soon as I had proved the point, I could hardly wait to fish out the junked unit and confirm my suspi­cions. And I did; it was an upside down/right side up fault and when I fitted a new LED it came good, just as the other one had. So I now have a spare unit, which will come in handy for testing. But what could possibly be wrong with the LEDs? I dunno please – as they say in the classics. Visual inspection is point­less; both units are totally opaque to visible light and appear black. One of the good ones I fitted is clear but there is little to be seen that would provide a clue. Electrically, the faulty LEDs measure exactly as one would expect them to; ie, like a diode. Nor is there any indication of position sensitivity. Which is about all I can say about it. Not only is the fault almost unbelievable but it has turned up in two units. Well, two that I know of. I wonder if this story rings a bell with any readers. It would be easy to be deceived the first time, just as I was. Food for thought My next story is in a quite different vein. In fact, it is not particularly profound technically but the symptoms, and their possible effect on how the job might have been tackled, provide some food for thought. In particular, the sequence of events demonstrates just how easy it is for there to be a breakdown in communications between customer and serviceman. And while it turned out to be unimport­ant in this case, it emphasises that the risk is always there. And it can prove costly, both in terms of money and reputation. The device was a colour TV set, Palsonic model 345, now about 12 years old. And the same chassis was sold under the Princess label. It belonged to one of my lady customers and was brought in by a friend, who was simply acting as a courier. When I asked if he knew the nature of the complaint, he answered simply, “No picture”. Well, as we all know, that can have a couple of interpreta­tions. If there is a raster on the screen but no image, then there is a fault somewhere in the signal chain, anywhere from the tuner right through to the video amplifier system. If, on the other hand, there is no raster, it is a fair bet that the trouble is somewhere in the horizontal deflection system. So I normally try to clarify this point, using terminology appropriate to the customer (I avoid the word “raster” –it only produces a blank stare). But there was no point in trying to delve further in this case; the good samaritan courier was in no position to help. Oh well, no worries; I’d know as soon as I turned it on. Or so I thought. But when I did turn it on, the result did not really slot into either category, although I had to admit that the customer’s description was not strictly wrong. There was a raster on the screen, and there was evidence of video on the screen too, but there wasn’t a picture because the horizontal scan was out of lock. So much for trying to pick the faulty area from a customer’s description. But that was a minor hiccough; the real question was why was it out of lock. The horizontal hold control (R451, 10kΩ) on these sets is not on the rear apron of the chassis, as in most sets, but towards the front of the chassis. This means that the chassis has to be pulled in order to adjust it but it also means that it is unlikely that anyone has fiddled with it. So I pulled the chassis, located the pot, and gave it tweak. The picture locked up immediately, even though it was only a very small tweak. It gave me the distinct impression that it could have been due to drift in any of the associated resistors, particularly considering the age of the set. And, of course, it was a situation where one might be tempted to make a good show by returning the set to the customer within a couple of hours. But I’m too old to be caught that easily. I let the set run for the rest of the day and planned to do the same the next day too. The set was still running when I pulled the master switch that night and I naturally expected it to come on when I turned the switch back on next morning. But it didn’t, it was dead; no sound, no picture and no raster. My reaction was to wonder whether this was the condition the customer had experienced, when she nominated the fault as “no picture’. Either way, it seemed likely that the set had two faults; the one I had just fixed and the one I was now facing. At first, I suspected a power supply fault but a few quick meter checks tended to rule this out – the HT rail was normal at 112V. However, it was clear that there was no horizontal circuit activity. This prompted a voltage check on the collector of the horizontal February 1995  65 Fig.2: the horizontal & vertical oscillator circuit in the Palso­nic 345 is based on IC301 (top, left). The horizontal output transistor, Q404, is at bottom right, while the driver transis­tor, Q402, is immediately to its left. The horizontal hold con­trol, R451, is below pin 1 of the IC. output transistor (Q404), which was correct at 112V, and also on the collector of the horizontal driver transistor (Q402), which was correct at 42V. Similarly, there was correct voltage on pin 11 of IC301, the horizontal and vertical oscilla­tor generator. Those points cleared, it was necessary to delve a little deeper. I reached for the CRO leads and checked for any horizon­tal signal coming out of the IC at pin 4. There was none. A faulty IC? It could be, of course, but there were other possibilities. One was that some other circuit fault could shut down the horizontal oscillator, particularly involving the x-ray protection (over-voltage) circuit which connects to pin 3. So, before rushing in to replace the IC, I made some more checks. One of these involved the x-ray protection circuit but, as far as I could tell, it had not activated. Another check was at pin 10, which provides the output from the vertical oscillator. And this was quite revealing because it appeared that the vertical oscillator was dead also. And that immediately threw suspicion back on the IC. I made a few more voltage checks around the IC and finally decided that the only logical step was to replace it. They are not particularly expensive and it is no big deal to make the change, the only snag being that I didn’t have one in stock. So it had to be ordered and I put the set aside for a couple of days until it turned up. When it did, I lost no time in fitting it, whereupon the set leapt into life. But, interestingly, the horizontal system was now out of lock again and I had to reset it to what was virtually its original position to restore the lock. And that was it. After a couple of days running on the bench it went back to the customer and it hasn’t missed a beat since. But I did wonder about the faulty IC. Was it two separate faults or two different degrees of the one fault? We can never be sure, of course, but my tip is that it was one fault in some part of the internal circuitry common to both oscillators. At first, its effect was quite mild, being sufficient only to upset the vertical oscillator frequency slightly. Subsequently, it went all the way and shut down both oscillators. It’s all rather academic, anyway. What was more important was that I could easily have been caught out by it. After all, just what did the customer mean when she said “no SC picture”? 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