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SERVICEMAN'S LOG Doing the rounds with remote control This month’s notes have turned out to be a continuation of last month’s. It wasn’t planned that way; it just happened. As readers will recall, they were about remote control units & this month’s notes describe two more faults. One of last month’s stories was about some funny goings on with the infrared LEDs in a particular model remote control unit (NEC RD-309E). They would work when the unit was upside down but not when it was right way up. And it wasn’t just a one-off; I had two with identical symptoms, which was enough to suggest that it involved an inher­ ent weakness in the LEDs themselves. Replacing them was all that was required to cure the fault but the exact failure mechanism remained a mystery. Which was where we left things last month. But hardly had the presses begun to roll, than there was another episode. It was the same model unit and it came in with a familiar complaint: “it doesn’t work.” And with very good reason, as I found when I opened the case. It was another case of a broken crystal lead, only this time the break was so close to the case that there was no chance of salvaging it. Fortunately, a scrabble through the junk box produced another such unit from which I was able to retrieve a perfectly good crystal. So it all looked like plain sailing. I fitted the substi­ tute crystal, put everything back together, and gave it a try. No joy. For a moment I wondered whether I had tricked myself and fitted another dud crystal. But then I remembered the upside down behaviour. Surely not another one? But it was; I turned it over and it worked, and when I turned it back again it was dead. I could hardly believe it. I fitted another LED and that was it; its behaviour was back to normal. So there it is – mystery fault number three. And that means there must be some inherent fault in those LEDs. I had hoped to make some attempt to find out what it is but, as I mentioned last month, the LEDs involved are coated with an infrared filter which excludes visible light and makes them appear black. My idea was to try to break one open, with a minimum of force, in an effort to preserve the electrodes and, hopefully, reveal the fault. No such luck. These things are not hollow, as I had thought, but solid plastic. But one final thought. With two Fig.1: Der Fernbedienungstester RCT 5502 (ie, the remote control tester). It carries a microphone (marked “US”), an in­frared sensor (marked “IR”), two LEDs (shown on the top of the case), & a 3.5mm socket on one side. 46  Silicon Chip faults in this last unit, which came first; which one prompted the customer to call me? We shall never know. The main event So much for that little preliminary bout. The main event this month concerns a Philips colour TV set, a 63cm model using the KL9A chassis. This chassis first appeared about 1012 years ago and was used in a whole range of sets. In many sets, it was used in its basic form, without any frills, but in this case it came with the works: remote control, stereo sound and Teletext. The customer’s complaint came in the form of a phone call along the now familiar lines, “the remote control doesn’t work.” So I said, “bring it in and we’ll test it”. At this stage, it may help the reader to follow the story if I describe the test unit I use for situations like this. It is a commercial unit of German manufacture and carries the Konig brand name (type number RCT 5502). As a matter of interest, the German term for remote control transmitter appears to be “Fernbe­dienung”, so the name of this device becomes “Fernbedienungs­ tester” (I wonder if they play scrabble in Germany!). I understand that there is also at least one locally made unit available. This is carried by J. V. Tuners, 216 Canter­bury Rd, Revesby, NSW 2212. Phone (02) 774 1154. The unit I have is basically a remote control receiver, similar to that used in TV sets but, for reasons which will become apparent, is a good deal simpler. It is designed for use with both infrared transmitters and the older ultrasonic transmitters, being fitted with both an IR photocell and a small microphone. It is housed in a small plastic case about 35mm wide, 25mm thick and 120mm long. There is an on/ off switch on one side of the case, two LEDs (one red & one green) on the top, and a 3.5mm socket on the other side. The red LED indicates that the power is on, while the green LED indicates when pulses are being received from the remote control. The 3.5mm socket may be used to bring the pulses out for checking on a frequency counter or CRO. Power is supplied by a 9V alkaline battery, while the internal circuit consists of just four transistors and a few minor components. It’s all quite simple really but it works very well. Howev­er, it is not infallible. Anyway, the customer brought in his control unit and I put it through its paces on the tester. This initially involves setting up the remote control transmitter and the tester so that they are about 150mm apart on a flat surface. The tester is then switched on and each of the transmitter buttons pressed in turn. It is important to test every button because only one or two may be faulty. Many customers don’t bother with such subtle points; to them it is all summed up in the phrase, “it doesn’t work.” There were no such problems in this case. Each button pro­duced a response from the green LED and I pronounced the unit OK. Unfortunately, this wasn’t the good news one might imagine because it meant that the fault was in the TV set. This would now have to be brought in for service. Fortunately, the customer had a second set and he duly organised delivery of the Philips set to the workshop. So, at the first opportunity, I put it up on the bench for a preliminary check. And this produced a surprise; there were no channels programmed into it. This was rather strange, particularly as the customer had not mentioned it, but I considered that it might be a byproduct of whatever fault there was in the remote control section. Anyway, I programmed the local channels into it, just to get it working, and this caused no problems. Nor did there appear to be any problems with the set’s overall behaviour; it was first class. But, as the customer had indicated, it would not respond to the remote control. No circuit At this point, I fished out my circuit of the KL9A but very quickly realised that it was for the basic chassis only; there was no remote control circuitry in it. And that was about it for then; there was little point in wasting time working blind and so I rang Philips and placed a manual on order. But I left the set running on the bench for the rest of the day, until I shut down and pulled the main switch for the night. Next morning, when I pushed in the main switch, everything came up as normal except for the Philips set. All it produced was snow and noise. It didn’t take long to confirm that all the channels I had programmed into its memory the day before had been lost. Fortunately, the reason wasn’t hard to find. This set uses a nicad battery backup for the memory, designated as part No. 1675. And since it was the original, it was not surprising that it had failed after about 12 years. I rang the customer and explained that I would have to wait on a manual before I could fix the remote control problem. At the same time, I took the opportunity to point out the additional problem with the channel memory system. He was quite understanding about any delay caused by the manual. And when I mentioned the memory loss his reaction was immediate. “Oh yes, I forgot to mention that – it’s all right as long as I leave the power point on but it loses it if I turn it off”. Well, that figured; the channels had been lost when he unplugged the set to bring it in. But at least I could go ahead with this problem. In greater detail, the battery is a 2.4V 110mAh type, about 7mm in diameter and 30mm long. It was readily available from one of my regular parts suppliers and, after fit­ting it, we had no more memory problems. With luck, it might last another 12 years. I now had to solve the problem with the remote control. Eventually, the manual arrived and, after some confusion due to the fact that it contains two different versions of the cir­cuit, I was finally able to tackle the job. The relevant part of the circuit is reproduced here – see Fig.2. It shows the IR receiver (part 1725) and a couple of vol­tages and waveforms. I decided to start by checking the voltages. The two points involved were 3C1 March 1995  47 Fig.2: the IR receiver circuitry in the Philips KL9A. The incom­ing pulses from the transmitter are processed by the IR receiver at extreme left & then fed to the base of a BC548 transistor via a 1µF capacitor. The resulting signal on the emitter of this transistor in then fed via a 10kΩ resistor to pin 13 of the data processor IC at right. and 2C1 on the IR receiv­er. And, in a moment of carelessness, I neglected to observe the polarity signs at these two points, assuming instead that the 5V marking indicated two separate rails, each at 5V with respect to chassis. The habit of measuring all voltages to chassis is strongly ingrained but it is not always the right thing to do. I woke up to this very quickly but, by a strange twist of fate, both points measured very close to 5V with respect to chassis. In practice, of course, the 5V is supposed to be read between 2C1 and 3C1, with 2C1 being at 5V with respect to chassis and 3C1 being at 10V with respect to chassis. Or that was how it was supposed to be. But 3C1 was not at 10V; instead, it was almost exactly at 5V, so there was virtually no voltage between the two points. By now, having re­alised my mistake and analysed the circuit correctly, I realised that there was something amiss around 3C1. The easiest thing to do was to pull the 3-pin plug to the IR receiver, whereupon the 3C1 supply line from the main part of the circuit jumped to 10V. Pushing the plug back in again Especially For Model Railway Enthusiasts Order direct from SILICON CHIP Price: $7.95 plus $3 for postage. Order today by phoning (02) 979 5644 & quoting your credit card number; or fax the details to (02) 979 6503; or send a cheque, money order or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 48  Silicon Chip pulled it back down to around 5V, which suggested a fault in the IR receiver module. The IR receiver I pulled the entire IR receiver out for a closer look. It is housed in a small but substantial aluminium box and consists of a PC board carrying a 16-pin IC, an IR photodiode, a couple of coils, and a few other components. I quickly concluded that there was little point in thinking about repairs. There was no circuit available and, as far as I could determine, the IC wasn’t This photo shows the IR receiver board after it has been slid out of its metal case. The IR photodiode is on the board at right, while the 3-pin socket is at left. Note the IR lens on the end of the case. avail­able as a separate item. Trying to troubleshoot a problem in these circumstances can be very risky. One can waste hours, only to finish up being unable to repair it anyway. The only logical answer was a new receiver. And that posed a whole new set of questions. Was a replace­ment readily available? What would it cost? And, most important­ly, would the customer want to incur such cost? While I was fairly confident that a replacement would be available, the cost was another matter. Receivers for other brands retail from $25 to $50 and I had a gut feeling that the higher figure would be the place to start from. I rang the customer and explained the situation. Naturally, he wanted some idea as to what it was all going to cost. I went over the cost of the work already done, added my estimate of the receiver price plus labour, and we came up with a guesstimate of between $150 and $200. Did he consider it worthwhile to go this far? Yes, he did – I should go ahead. And so I contacted Philips. Yes, the receiver was avail­ able and my gut feeling was not far out; the retail price I should charge my customer was $75. Since this kept the overall cost within my guesstimate, I went ahead and ordered it. It duly arrived and, at the first opportunity, I set about fitting it. This took no more than a few minutes work but when I gave it a trial run, it simply would not respond to the remote unit. So much for my optimistic “she’ll be right now mate” attitude! She wasn’t right at all. All kinds of horrible possibilities raced through my mind. Had I fouled up the receiver installation, which seemed so straightforward? Was it a modified receiver design, unsuitable for a set of this age? Was it a much more subtle fault, somewhere in the bowels of the set itself? Was the fault really in the transmitter, in spite of my previous tests? And had I invested unnecessarily in a replacement receiver, which would sit in my stock for years to come? When the panic subsided, I decided that the first two thoughts were the least likely, so I concentrated on the possibility of a fault in the set. The first step was to check the rail voltages at 2C1 and 3C1. These now measured 5V and 10V respectively, exactly as marked on the circuit. Next I turned to the CRO. The circuit shows a waveform coming out of the receiver at terminal 1C1. The circuit depicts square wave pulses with an amplitude of 5V but, unfortunately, there is no indication as to the frequency of these pulses, nor is there any other data on the coding used. Anyway, this was the first check point. And on the basis of the limited information in the manual, everything appeared to be OK at this point. From there, the signal goes via a 1µF electro­ lytic capacitor to the base of a BC548 transistor. The resulting signal on the emitter of this transistor is then fed via a 10kΩ resistor to pin 13 of the data processor IC, where another wave­form is shown. This is similar to the first but with a slightly lower (4V) amplitude. I traced the signal along this path and finished up with the correct waveform at pin 13, as shown on the circuit. So pulses were coming out of the remote transmitter, being picked March 1995  49 up by the receiver, processed, and passed to the data processor IC. So why wouldn’t it work? The most logical suggestion seemed to be a fault in the data processor IC, which wasn’t a very happy thought. From previous experience, I tipped that it would be quite expensive and that, in turn, meant that the cost situation would be getting out of hand. But there were other factors to be considered. How could I be absolutely sure it was that IC? Granted, the evidence was strong but if I was wrong, I would be down the drain for an expensive IC. What else could it be? One slim possibility was the trans­ mitter, in spite of the tests I’d already made. Remember that I said earlier that the transmitter tester was not infallible. Its weakness is that it can only confirm that pulses are being trans­mitted; it has no way of confirming that they have the correct coding sequence for a particular set. No gambling So although the risk appeared to be slight, I wasn’t pre­pared to gamble the cost of an IC until I was absolutely sure that the transmitter was clean. Ideally, this could be confirmed by acquiring another transmitter, perhaps borrowed from a col­league if I was lucky enough. But first I decided to have a look inside the transmitter for any clues or obvious faults. A general once over didn’t show up anything obvious, such as dry joints or obviously faulty components, and the voltages seemed to be at least sensible, which was the best I could do. Next, I connected the CRO across the crystal oscillator circuit and confirmed that this was working correctly. Its fre­quency was around 4MHz, which is similar to many other systems. So it looked as though I had drawn a blank. And then the system suddenly came good. Acting on an im­pulse, I pressed one of the control buttons, whereupon the re­ceiver immediately responded. I went through the whole range of control functions and they all worked perfectly. The trouble was, I hadn’t a clue as to why this had hap­pened. I could only assume that there was faulty connection in the transmitter somewhere (perhaps the battery contacts), which had come good as a result of my prodding and probing. But, try as I might, I couldn’t recreate the fault. So I simply went over the board with a hot iron and resol­dered all the joints, with particular attention to those around the crystal oscillator circuit. I didn’t find anything suspicious in the process and the unit still functioned in all modes after­wards. By now, I imagine, some readers are querying whether I goofed over the receiver; was it really faulty, or had it been a transmitter fault all the time? Well, the same thought occurred to me and I couldn’t rest until I had plugged the old receiver back in. No question; it pulled the 10V rail down as before and simply wouldn’t work, so that was the end of that theory. Finally, after putting the system through a week’s hard yakka, I returned the set to the customer with a warning to contact me immediately if the fault recurred. That all happened nearly 12 months ago and the system hasn’t missed a beat since. And what did it cost? It all added up to $149, so my initial estimate of $150-200 wasn’t SC too far out. March 1995  51