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SERVICEMAN'S LOG Faults that don’t obey the rules Frustration is the theme of this month’s notes. It’s nice to restore a device to full working order but still very frustrating when it is not clear why it failed, or why it behaved as it did when it failed. The first frustrating story concerns an NEC colour TV set, model N2092. It belongs to a local motel – a new customer – and it turned out to be one of those frustrating jobs which, while satisfactorily concluded at customer level, leaves a legacy of doubts and queries as to just why it behaved as it did. It started with a 9 o’clock phone call from the motel proprietor, asking me to come and have a look at a TV set which, to use his own words, “wasn’t going”. That expression prompted me to ask whether it was completely dead – an unfortunate phrase perhaps – to which he replied that, yes, it was completely dead. And he wanted me to service the set in the motel, because the set was bolted to a shelf in the motel room. But I had to explain that I did not make house calls, that service was seldom practical away from the workshop, and that, in any case, the set would have to be unbolted before I could work on it. For once, I had struck a customer who was quite reasonable about such matters. He appreciated the problems and agreed to bring the set to the shop. However, he did stress that he would like it back that day, if that would be possible. Naturally, I couldn’t make such a promise. But I did say I would look Fig.1: the horizontal output transformer circuitry in the NEC N2092. Pin 2 is at bottom right and feeds diode D503 via fusible resistor R522. Note the waveform at pin 2. 40  Silicon Chip at the set immediately and do what I could, depending on the fault. All of which doesn’t have much to do with technicalities, but is simply an example of the various matters which have to be sorted out before a set is even sighted. Anyway, the customer turned up a little later with the set in the back of a ute. I set it up on the bench and turned it on while he was there. And that was the first setback. Far from being completely dead, the set was very much alive with a full raster on the screen and a healthy hiss from the speaker. Grant­ed, there was no picture or sound and I suppose, to the customer, it might just as well have been completely dead. Oh well, my fault for not being more specific. But it did change the situation somewhat. On the positive side it appeared that the rear end was working, particularly the horizontal de­ flection circuit and all that goes with it. And that, in turn, suggested a front-end fault. Unfortunately, it also ruled out the chances of a clear-cut fault, as in a completely dead set. And that, in turn, meant that there was less chance of a quick fix and I advised the customer accordingly. Again he was quite understanding and so he left me to it. When I turned the set on again some time later, it came up as before. Then the phone rang and I turned the sound down to remove the hiss while I dealt with the call. When I eventually returned to the set, it was displaying a first class picture. What’s more, the sound had also returned to normal, as I quickly found when I advanced the volume control. So the fault was intermittent; the last thing I needed when the customer was hoping for a same-day job. What I did need was a service manual – or at least a cir­cuit. But I had neither. The best I could dig up was a circuit for a similar model, which I felt might be sufficient for the job. In fact it served very well, its main shortcoming being that it lacked any waveforms. Later – much later – I found a colleague with the correct version, complete with waveforms. The fault returns The set had been turned off while I was searching for the circuit and, when I turned it on again, it came up in the fault condition. In fact, this was to be the pattern; switch it on from cold and the fault would appear. Then, after anything from a few minutes to half an hour, it would come good. Similarly, once up and running, it would need to be turned off for up to half an hour for the fault to reappear. This was something of a mixed blessing. It was helpful to be able to create the fault, almost at will, but the half-hour wait each time was highly inconvenient and time-consuming. I left the set for half an hour or so, while I attended to another, more routine job, then switched it on again. It came up faulty and I quickly switched it off. I then took the back off, pulled the works out, and began finding my way around the boards with the aid of the circuit. And, since it appeared to be a front-end fault, I concentrated on the tuner and IF sections. Next, I switched the set on again and made some quick vol­tage checks before it came good. And I hit it almost in one; both the tuner and the IF section are fed from a 12V rail – which didn’t have 12V on it. So that was it –all I had to do was find out why there was no 12V. And I was silly enough to imagine that this would be quite straight­forward. It was no problem to trace out the 12V rail on the circuit. It was a conventional arrangement, derived from a tapping (pin 2) on the horizontal output transformer (T502). From this point, there was a 2.2Ω fusible resistor (R522); a diode (D503); a 4700pF capacitor (C523) in parallel with the diode; and the 2200µF main filter capacitor (C524) 2200µF. In short, it was perfectly conventional and it looked like a snack. I checked the 2.2Ω resistor and the diode but could find nothing wrong with these parts. But I did suspect that there might be a dry joint to one diode lead, so I resoldered these and those of the 2.2Ω resistor. By this time, the set should have cooled into its fault condition but, when I switched it on, it came good immediately. This seemed like a good omen but I have been caught before in this situation. I turned it off for another half hour to let it really cool down. And, incidentally, these half hour periods were adding up; the day was slipping away and there wasn’t much time left if this didn’t fix it. Unfortunately, it didn’t. The set came up faulty as before. So what next? The diode seemed the best bet and, to save time, I simply tacked another diode in parallel with it on the copper side of the board, crossed my fingers and switched on. The result was completely unexpected. The set really was completely dead now; no raster, nor sound hiss, no sign of life at all. After the first shock, I did some probing with the meter and eventually realised that there now virtually a dead short on the 12V rail, with only a couple of ohms to chassis. And so began the laborious task of tracing the 12V rail and isolating various sections in an effort to pinpoint this fault. Naturally, as readers can imagine, tracing this rail on the circuit is one thing; tracing it in reality is something quite different. It weaved and wandered all over the place and was almost impossible to follow in places. The only good point was that it used a number of links and these proved valuable in isolating various sections. I think I lifted about five links altogether and, including inevitable interruptions, spent about two hours tracking it down. The faulty parts The faulty components were associated with pin 38 of the jungle chip, uPC1420CA. This pin is fed from the 12V rail via isolating diode D504. Also connected to it is zener diode ZD501 and resistor R514 (12kΩ, 2W). The other end of this resistor connects to the 120V rail. This is the kick-start network, which is needed to start the June 1995  41 SERVICEMAN’S LOG – CTD horizontal oscillator at switch on. Both D504 and ZD501 were shot (dead short) and this was what was loading the 12V rail. Why had this happened? I have absolutely no idea. Naturally, I checked the substitute diode and anything else that I might have done wrong. I drew a blank on all counts. So all I could do was replace these two components and try again. I replaced the main diode (D503) and removed the substi­tute diode I had shunted across it, then I switched it on again. Well, at least the set was “alive” (raster and hiss) but there was still no 12V. Then I realised that the 2.2Ω fusible resistor had done its job and fused. I fitted a new resistor and tried again – still no 12V rail. I went over everything again, checking and double checking, but could find nothing wrong. But I did realise that something else had happened; no practical warmup period would now cure the fault and it appeared to be permanent. Well, that could be all to the good. And, having checked everything else, the main suspect now was the horizontal output transformer, unpleasant though this thought was. With the frequency and waveform involved here, the only practical way to check this is with a CRO. But even here I had a problem. As I mentioned earlier, I was working from a circuit which had no waveforms. So I had only a very general idea of what I would find on pin 2. In fact, there was a waveform there and its shape was not unreasonable. But I had no clue as to what the amplitude should be and it was rather beyond my grocery bill mathematics to work out what it should be to deliver the required 12V. But I did suspect that it was rather low, which only sup­ported my impression that there was some kind of weird fault in the transformer. I finished up disconnecting it entirely and making resistance checks on all the tappings. They all showed continuity and appeared to make reasonable sense, at least as far as I could tell without any precise reference. 42  Silicon Chip Finally, I pulled the transformer out and checked it on the shorted turns tester. Again I drew a blank. Nevertheless, I had now convinced myself that the transformer had to be the culprit. Good news & bad On that basis, the next step was to check availability and replacement cost. A call to the NEC service department produced a good-news-bad-news reply. The good news was that replacements were available and the bad news was the retail price of $166. For most sets, the cost would range from about $60 to $100, so this was a real shocker, particularly as there was still a niggling doubt as to whether I was really on the right track. But I had more or less committed myself now, so it was up the motel proprietor. I rang him, explained that the job was going to take longer than we had hoped and that it was going to be quite expensive. By the time the transformer price, labour and other costs were added in, the bill would be over $250. Did he want to go ahead? He thought about it briefly, then said, “yes, go ahead”. As he explained it, there were a couple of factors involved. One was the alternative cost –it would cost a good deal more to replace the set and it was an essential item. The other reason was more unu- sual. When the motel had been fitted out, the cabinet colours had been specially chosen and supplied to suit the decor (it was a light cream colour that was not normally available). This could be difficult and expensive to replace. So I ordered the transformer, which arrived in a couple of days, and cost another $8 freight. And from there it was someth­ ing of an anticlimax; I fitted it, switched on, and the set snapped into life with perfect picture and sound. Of course, I gave it a thorough workout, with a routine of on-off cycles over the next couple of days. But it never missed a beat and hasn’t missed one since. Unanswered questions So that was it; a faulty transformer. The set went back to the motel and I had a happy customer, in spite of the cost. But, as readers will agree, it leaves a lot of questions unanswered. For a start, what kind of fault was it? Remember, it pro­duced what appeared to be a typical waveform at pin 2, even though there was no DC after the rectifier. The best suggestion I can make is that it was some form of high internal resistance, intermittent, and probably non-linear in some way. In other words, it was incapable of supplying any useful current to the load but could still produce a waveform of sorts on a sensitive CRO. Further to that last thought, it was only when the job was finished and the set back in the motel that I found a colleague with the correct circuit. And it is the appropriate portion of that circuit which is reproduced here. The waveform shown for pin 2 is essentially the same shape as that which I observed for the faulty transformer. But the amplitude is another matter. I didn’t take as much notice of it as I should have but, as I recall, it was nothing like the 120V p-p as on the circuit. And what about the destruction of the diode and the zener diode? This is an even greater mystery. My best suggestion here is that the substitute diode I shunted across the original was faulty and was breaking down at high voltage. OK, so it’s a long shot. But I am sure of one thing – if one such faulty diode existed in a batch of ten million, it would finish up in my spare parts stock. The microwave oven And now for the second spot of frustration. This involves a complete change of scene; from a colour TV set to a microwave oven, and an intermittent one to boot. This was a first for me. Until now, I’ve had intermittent faults in every device I can think of except a microwave oven. It started with a phone call from a regular customer and concerns a Panasonic model NN-9859. This is a combination mi­crowave and convection heating type and, in order to appreciate the problem, it may help to describe the operating procedure, particularly for the convection mode. Having turned the oven on, the required temperature is selected by pressing an appropriate key, which increments the temperature indicator in 10°C steps. When the oven reaches the preset temperature, the system beeps and flashes the temperature indicator. The oven is then held at that temperature. The customer’s complaint was that, having gone through this procedure in the convection mode, the oven would behave normally for about five minutes and then shut down. If the start button was then pressed, it would run for another few minutes, then shut down again. This procedure might need to be repeated several times but, eventually, the oven would come good and run as long as needed. I immediately enquired as to whether this also happened in the microwave mode, thus suggesting a common fault area. But he couldn’t say; they seldom used the microwave mode, only the convection mode. The microwave mode was used on the odd occasion to reheat a cold meal but then the time needed was probably too short to create the problem. So I said, “bring it in and we’ll have a look at it”. And so it finished up on the bench. I deliberately avoided removing the covers, so as not to disturb anything, but simply switched it on, set it up for a couple of hundred degrees, and let it run. And it ran perfectly; not a sign of trouble. I switched to microwave mode, added a jug of water as a dummy load, and tried that. Again, it ran perfectly. Fig.2: this drawing from the service manual shows the top of the Panasonic NN-9859 with the cover removed. Note the temperature sensor below the circulation fan pulley. I turned it off, let it cool for a couple of hours, then tried the convection mode again. And this time it did misbehave; it ran for a couple of minutes and then shut down. The tempera­ture display was still showing the correct value and pressing the start button set it off again. And, just as the customer had said, I had to do this two or three times. Then it came good and ran up to the selected temper­ature. I repeated the test in the microwave mode and it behaved perfectly. I let it cool overnight and repeated the tests the next day. The result was exactly the same as before; intermittent on convection, perfect on microwave. On the face of it, it looked like a nasty problem. And it could have been, had I not serviced this model and earlier models before. Which is not to say that I had seen this problem before – I hadn’t. But I had encountered a fairly common fault whereby the display panel would exhibit a string of eights, which meant that the oven could not be programmed for either mode. And the reason? An open circuit oven temperature sensor. So, while the symptoms differed, I went straight to this sensor. This looks like a ceramic encased resistor and is mounted on a ceramic strip. This in turn mounts over an opening in the top of the oven, with the sensor below it. The sensor connections consist of two flat metal lugs, to which are con- nected leads which run back to the microprocessor. In all the units I had seen before, these lugs were about 75mm long and were encased in insulating sleeving which extended back over the connecting leads. They were also bent parallel with the top of the oven. In this oven, however, the lugs were only a few millimetres long and the connections to the leads were plain­ ly visible. And so was the fault. Instead of the usual welded or staked connections, these looked as though they had been soldered. But there was little solder to be seen now. The lugs were blackened and the tinned leads simply wrapped around them. The wonder was that the thing worked at all. For a start, soft soldered connections on those lugs simply do not make sense. The oven is programmed up to 250°C and would commonly run at up to 200°C, so the sensor and its lugs would also be heated to that level. Against this, the melting point of 60/40 solder is around 190°C or even a little less, creating a complete­ly incompatible situation. My bet is that it was bodgie repair. The original sensor probably failed and some smart type salvaged a sensor from a ditched oven, clipping the lugs short in the process. He then attempted to solder the un­solder­able, creating the ultimate in dry joints. I can’t prove it of course but it’s the best theory I can come up with. SC June 1995  43