Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG Sound reasons for confusion Yes, audio problems are the order this month. And before you sniff disdainfully, let me assure you that audio problems in TV sets “ain’t what they used to be”. Like everything else, they’ve gone high-tech. The first story is about an NEC colour TV set, a model N-4830 using a C500 chassis and made by Daewoo. The fault was straightforward enough; it was completely dead. More than that, the reason was obvious, at least to anyone who had seen it be­fore. This set has an inherent weakness. The power supply uses a 10-pin IC switching regulator (I801), a type STK-73410II. This IC overheats and fails, taking a lot of components with it. And that is what had happened in this case. But there is more to the fault than that. It appears that the failure is not the fault of the IC itself. Rather it is in the associated circuitry and the makers have issued a modification instruction to cover this. Attention to this is most important because if I801 is simply replaced, the set will back again in a few months with exactly the same fault. The modifications are quite extensive and I don’t propose to set them out here. Quite apart from the space needed, I under­ stand that since I did this job, further modifications have been issued which I have yet to receive. My original modification sheet came from NEC Spares, 23-25 Coombes Drive, Penrith, NSW 2750 – phone (047) 21 7655. It would be wise to contact them for the latest information. Suffice it to say, for the purpose of this story, that I carried out all the modifications prescribed at that time, re­placed the IC, then switched on with my fingers crossed. And, initially, all seemed well. There was no smoke, 48  Silicon Chip fire or confusion and the picture tube warmed up normally and presented a first class picture. There was only one thing wrong; there was no sound. In the good old days, audio faults were relatively rare and, when they did occur, they were usually quite easily found and fixed. It’s not quite the same these days, at least on some of the more elaborate sets such as this one. Signal tracing I commenced searching with an audio probe attached to a test amplifier, starting where the audio comes out of the IF module, at pin 9 (AUDIO OUT). Well, there was audio there all right, which was encouraging. All I had to do now was trace it through to the output stage and speaker. And that, as they say in the classics, was the hard part. The audio path is a real aroundthe-world-for-sixpence arrangement. From pin 9 it goes to pin 7 of I701 (on a separate page), comes out of I701 on pin 5, goes back to the IF module on pin 8 (ATT IN), comes out again on pin 10 (ATT OUT), and is finally fed to the audio output stage, I602. Tracing all this out on the circuit – with much turning of pages – was an exercise in itself. Then I had to relate this to the set itself, which involved a similar order of complexity. But laborious though it was, it paid off. Audio was being applied to pin 7 of I701 but there nothing coming out on pin 5. At this stage, I hadn’t paid a great deal of attention to the role of I701 but now I needed to know. It wasn’t hard to work out. This set Fig.1: the terminal connections for the IF module in the NEC N4830. Audio comes out on pin 9, goes back in on pin 8, and comes out on pin 10 to be fed to I602 (the power output IC). features audio and video input sockets (J202, J703) on the rear of the chassis, which allow external video and audio signals (eg, from video recorders and cameras) to be fed directly to the appropriate parts of the set. This avoids the losses involved in modulating a carrier, feeding it through the front end, then demodulating it to recover the original signals. And I701 is a switching module which feeds the audio and video sections of the set from either the front end or from the external audio/video inputs. Why no audio? So why wasn’t audio coming out of I701? In an effort to clarify the situation, I fed an audio signal into the audio input socket, J702, and, lo and behold, the signal went straight through to the speaker. What’s more, this situation remained, regardless of the switching signal instruction to pin 11. In short, the audio signal path through I701 was jammed in the external position. The most likely possibility was that the IC itself was faulty. The only other one was that power supply or switching voltages to it were at fault but I tended to rule this out on the grounds that the video switching function was normal. Nevertheless, I went through the motions. The IC is powered from the 12V rail at pin 6 and this was quickly cleared. The switching voltage is applied at pin 11 and this was turning on or off according to instructions from the control unit. That didn’t really surprise me. After all, the device was switching the video signal, so it should have been switching the audio signal as well. More to the point, it pointed the finger fairly and squarely at the IC. So I pulled it out, fitted a new one, and that was it. And as is often the case, it all seemed so simple and obvious once I’d found it. Why did the IC fail the way it did? Who knows? It could have been a simple random failure which just happened to occur at this time, or it could have been a byproduct of the power supply failure. But it’s worth keeping in mind. The set involved was a Philips type 02CR035, model 20CT6750/75Z, using a CTO-S chassis. It is a 48cm set, about 10 years old, with remote control and options built in for Teletext. More about the type number and options later. I didn’t deal directly with the customer – a hazard in itself, as it turned out – but with a colleague who was short of time and needed some help. The complaint was intermittent loss of picture, with a bright raster. He had tried to find it but the intermittent nature had made it difficult. He suspected a dry joint and nomi­ nated “the board on the left” as his prime suspect. The board on the left was the VST decoder board and I had my reservations about this diagnosis. It seemed unlikely that a fault here would produce the symptoms described and my experi­ence with this board is that Another sound failure My next story is also about a sound failure but that is the only similarity. By comparison, the previous fault was not unduly difficult to find, whereas this fault was one of the most frus­ trating I have encountered for a long time. And the irony was that it didn’t even start out as a sound problem. Fig.2: the audio from the IF module in the NEC N4830 is fed to pin 7 of I701 and comes out on pin 5. Note the external audio/video jacks, J702 and J703. March 1996  49 Fig.3: the relevant sound circuitry in the Philips 02CR034/35. Audio comes out of IC7664 (left) at pin 8 and goes to board 1070 via plug 3M12. It is subsequently applied to pin 7 of IC7681 via plug 2M12. it is remarkably free from dry joint problems. As it turned out, all this speculation was wasted. When I switched the set on, the fault appeared immediately – a brilliant white raster with no picture and no sound. And, as it turned out, this condition was permanent. I immediately suspected a voltage error around the picture tube; most likely a loss of cathode voltage, which is effectively the bias between the cathode and G1. And since the raster was white, rather than coloured, it suggested a fault common to all three guns. The cathodes are coupled to the RGB drive transistor collectors, so this voltage supply was the prime suspect. And so it proved to be. The collectors are fed from a 180V rail but there was no 180V. It didn’t take much tracking to find out why. A 4.7Ω resistor, R3583, in the power supply was open circuit. Apparently it had been intermittent but had finally failed completely. I fitted a new resistor and we had our 180V and a first class picture. So far so good but there was still no sound. I wasn’t particularly worried about this, as I imagined a spot of audio signal tracing, as described earlier, would quickly solve the problem. Before doing this, however, I went over the motherboard, checking it for 50  Silicon Chip dry joints. In particular, I checked the horizon­tal output transformer connections and remade several which look­ed suspicious. Having done this, I came back to the sound problem. And, in order to follow what happened next, it is necessary to describe the relevant part of the circuit – see Fig.3. The section shown is from the motherboard. The audio comes out on pin 8 of the demodulator module (IC7664 – left) and goes via plug 3M12 to an auxiliary board designated 1070 (Tone Con­trol/Headphone Panel). It is then supposed to come back to pin 7 of IC7681, the power output IC, via plug 2M12. I say “supposed” because it didn’t. Using the audio signal tracer, I detected audio at 3M12 but there was nothing at 2M12. Fun & games So the fault was on board 1070 which, as its name implies, provides remote control of the volume, treble, and bass levels. And this was where the fun and games started. I didn’t have a manual for the 02CR035; the closest I had was one for an 02CR034 which I believed used essentially the same circuit. And it did, at least for the main circuit, and I had used it to track down the original power supply fault. But board 1070 was another matter. There was a general similarity in that the same IC, a TDA1524 (IC7001), was used for the level control functions. However, there were significant differences between my circuit and the board in this set, which used additional circuitry and components. This didn’t worry me unduly at first. I confirmed that audio was coming into the board (connection 4C6) and to pin 15 of the IC but there was nothing at the IC’s output (pin 11). Next, I checked the supply voltage to the IC (pin 9) and all the asso­ciated components. I could find nothing wrong and that put suspi­cion squarely on the IC itself. I ordered one and fitted it but no joy; everything was just as before. I was getting worried now. The IC and everything around it tested OK, yet I couldn’t get any audio through it. I needed to delve into things at greater depth and, clearly, I needed an updated circuit. Fortunately, I found a colleague with a CR035 manual but my complacency was short lived. Although this circuit accounted for some of the additional circuitry, it still wasn’t complete. Apparently the set on the bench was a different version again! Service department My next stop was the Philips service department. I set out the circuit problem in some detail and was assured that a manual was available –price $84. I did a mental double take on that; it was high, even by current standards. Still, I needed it, and could need it again. I placed an order. The manual duly arrived and I turned eagerly to the 1070 circuit. Imagine my frustration when I realised that it was no different from the one my colleague had loaned me. I went back to the Philips service department – they were just as puzzled as I was and could find no record of a circuit which fitted the de­tails I described. Granted, after 10 years, the trail would be cold. However, they agreed to take the manual back and credit me and, at the time of writing, the search is still on. In the meantime, the silly situation remained; I could find nothing wrong with the IC or its associated circuit but it would not process the signal. I sought the assistance of various col­ leagues but without any real success. Some had experienced a similar failure but it had been traced to a faulty IC. And they, too, had sought an updated circuit without success. In desperation I finally decided to trace out the extra circuitry on the 1070 board in the hope that it might provide a clue. And it did. The extra components were connected to pins 1 and 17 of the IC. There was a 10µF electrolytic capacitor (C2010) to chassis; a 6.8kΩ resistor (R3010) to plug 1C3; and a 100Ω resistor (R3000) to a single additional plug, C7. Hot trails & smelly rats This plug connection went to a yellow lead which ran to the VST board, 1220, via pin 1 of a 3-pin plug and a mating connector on that board, mark­ed V17. And this was where I began to smell a rat. Pin 1 on the VST board went straight to chassis which, even with the limited knowledge I had gleaned of the circuit, didn’t seem to make sense. Hot on the trail, I disconnected the yellow lead, most conveniently at the 1070 board end, whereupon I had sound – lots of it and quite uncontrollable. That was both gratifying and confusing but it clearly indicated that there was something amiss with that part of the circuit. And there was something else I noted at that time. In both the manuals, the board pattern showed V17 as a 7-pin arrangement, whereas the set actually used a 3-pin combination, as already mentioned. But, with all the other differences, I didn’t attach much importance to it. I also noted that the yellow lead was anchored at the corner of the VST board and that it was pulled quite tight Fig.4: the circuitry for board 1070 in the Philips 02CR035, as shown in the manual. The actual board used in the faulty set con­tained extra circuitry and components. Audio goes into the IC on pin 15 and comes out on pin 11. March 1996  51 Serviceman’s Log ­– continued The result was music to my ears and this time it was com­pletely controllable. But it was not without some mixed feelings. Predominant, of course, was the relief at having solved the problem. On the other hand, I muttered some rude words about those responsible for the debacle in the first place. Mental abuse against the anchor tie to reach V17. Again, I thought little of it. But another observation proved more productive. Apparently, this board was designed to permit retrofitting of Teletext cir­cuitry and there were several unused connectors on the board. And one of these, V19, was a 52  Silicon Chip 3-pin type, sited close to and at right angles to V17. Way ahead of me? I’ll bet you are! I pulled the plug off V17 and fitted it to V19, noting in the process that the yellow lead was no longer strained against its anchor point. Then I switched on, for the umpteenth time. My colleague copped some of this mental abuse as well but that wasn’t really fair. Most of it was directed at whoever the bright spark was who put two identical plugs almost side by side on the same board. It was a disaster waiting to happen. With the benefit of hindsight, it was easy to work out how it had happened. My colleague had told me that he had suspected dry joints on the VST board, which meant that he must have re­moved the board for an inspection and work over. And in putting it back in place . . . Talk about Murphy’s law. And I don’t mean the fictitious leprechaun-like character who lurks in the corner of the work­shop, wrecking everything we try to do. I mean the real Murphy. More correctly Lieutenant Murphy of the US armed forces (I’m not sure which branch). As I understand it, Lieutenant Murphy was in charge of a series of tests involving acceleration and impact effects on military equipment, using a rocket propelled sled. In one such test, in which the equipment was tested to destruction, the measurements which were supposed to be record­ed during the run and impact were lost. The reason turned to be – wait for it – that two intercon­necting leads, fitted with identical plug and socket connections, had been transposed. I’ve no doubt Lieutenant Murphy muttered some rude words of his own but, for the record, he offered the classic statement “if anything can go wrong, it will go wrong.” I couldn’t agree more. But, again with the benefit of hindsight, I suppose I should have woken up sooner, knowing that the board had been removed and replaced. I wonder if anyone else out there has been caught SC like that?