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SERVICEMAN'S LOG The dingiest corner of a dingy room I have a story about another long-in-thetooth set this month – one with a nasty sting in its tail. And the situation wasn’t helped by having to work in an anything-but comfortable environment. The set in question was a Pye model 48SL1, a 48cm set, fitted with what Pye called a T38 chassis. It was, in fact, a Philips KT3A-1 chassis, one of several Philips KT3A chassis, some of which were live. The KT3A-1 had an earthed chassis, however, and this particular model would be about 13 years old. It was owned by a lady and the complaint, as nearly as I could determine from her description, was a virtually 54  Silicon Chip complete failure which possibly involved a hiccuping condition. Well, that was fair enough and I didn’t anticipate that it would be a particularly difficult job. But there was one snag – the lady insisted that the job be done in her home; she didn’t want the set to leave the house. Don’t ask me why but she is not the first person I have struck who had a thing about not letting a set out of their sight. And, as I was to discover, the lady was rather eccentric in other ways as well. I try to avoid house calls if possible. It is impossible to take everything one is likely to need for the job and it invariably tran­spires that the one thing you do need is back at the shop. But the lady was insistent and, since she was willing to pay any additional costs, I agreed. The lady’s house turned out to be what was once undoubtedly a Victorian-style luxury home but which had seen better days. But what really struck me, even before I pulled the bell knob, was the modern security fittings. The door was fitted with heavy security bars, was obviously fitted with more than one lock, and every window was fitted with heavy security shutters. All of which should not have worried me except that, when I moved inside, I realised that the security Fig.1: the vertical output stage of the Pye 48SL1. The blanking pulse is derived from the junction of resistors R531 and R532 and is fed to line A51. Note the waveform at this point. to tackle a fault like that anywhere away from the shop, let alone in this Victor­ian chamber of horrors. Naturally, the lady protested at this suggestion but I explained, as politely as I could, that there was no alternative; I needed equipment and facilities which I simply could not provide in her lounge room. So, finally, she agreed, albeit reluctantly. Back at the ranch shutters not only kept out the burglars but kept out the light as well. It would not have been so bad if the rooms were reasonably well lit. However, I doubt that any of the light fittings boasted a globe larger than 40 watts. Again, I cannot explain why. I can only assume that it was an attempt to recreate what she imagined would have been the dingy atmosphere of the house in its heyday. It was a weird setup; the only thing that seemed to be missing was a black cat named Salem! But speculation aside, the result was that I found myself down behind the set, in the dingiest corner of a dingy room, hoping that I could manage to see what I was doing. In fact, when my eyes became dark adjusted, and with the aid of a hand lamp, I was able to find my way around without too much difficulty. These chassis are well laid out and this, cou­ pled with the fact that I am reasonably familiar with them, also helped. The hiccups And so to the problem itself. My original assumption was correct; the set was hiccuping madly, which invariably means an overload on the power supply due to a breakdown of some kind. But the question was, where? My first checkpoint was the main electrolytic capacitor, C298, off the bridge rectifier. This can produce symptoms like this if it dries out and, with a set of this age, it was a prime suspect. But no; it checked OK and there was about 350V across it, which was normal. I also made a routine check for dry joints but, as far as I could see, there was nothing obvious. The next step was to isolate the horizontal output stage and the quickest way to do that was to pull the deflection yoke plug, which carries a protective link. That cured the hiccups and allowed the main HT rail to come up to a steady 131V. So, the fault was somewhere in the output stage. I narrowed this a little, after replacing the yoke plug, by short­ ing the base and emitter of the output stage transistor, Q562. This also cured the hiccups. I spent some time checking various possibilities. I discon­nected the tripler and, in turn, the various auxiliary voltage rails off the output transformer secondary. And I went over the transistor stage itself, checking all the components around it. I even checked the output transformer for shorted turns but to no avail. And, remember, all this was done in the confined space and poor lighting I have previously described. I sat for a few moments and had a bit of a think. Somehow, my thoughts came back to the transistor itself (Q562). Perhaps it had a weird fault in it. I decided to pull it out and check it or, if necessary, replace it. I didn’t get that far. As I removed the transistor there was the fault staring me in the face; a black spot on the insu­lating washer, where the voltage had punched through. Fancy being caught with that old chestnut. I fitted a new washer, the hiccups vanished, and I had a picture on the screen. But it was a hollow victory; the top half of the picture was riddled with horizontal retrace lines. I baulked at that. No way was I going When I got back to the ranch, I hoisted the monster onto my workbench and set to work. Since it was obviously a vertical blanking problem, I went over the circuit to familiarise myself with the blanking circuitry. It is fairly straightforward really. A deflection pulse is taken off the vertical deflection output stage (Q530 & Q532) and goes to a pulse processing stage (Q535). This stage is biased so that it conducts only during the vertical flyback period and delivers a series of square pulses of about 1V amplitude to the blanking section (pin 9) of the chomi­ nance/luminance IC (IC192). At least, that is the theory of the circuit. And as far as I could determine, this was what appeared to be happening. There was an appropriate waveform at the vertical output stage and a replica of it, somewhat attenuated, at the base of the processing stage (Q535). And there were pulses out of Q535 being applied to pin 9. So why wasn’t the system blanking? The only clue I had – if it could be called that – was the discovery that the problem varied with the height control setting; reducing the height would eliminate the lines, as would increasing it beyond a normal setting. And that, if it suggested anything, point­ed to the vertical stage. As a result, I made a whole swag of checks around this stage, including changing transistors, likely electro­ lytics and any resistors which were marginally high. It was all to no avail. Next, I went back to the shaping stage and, in spite of what the CRO had told me, I changed transistor Q535. It wouldn’t have been the first time that such a trick had paid off, contrary to all the tests. But not this time. Nor did a detailed check of all the associated components. In a fit of desperation, I hooked up the CRO again and made another check of the waveforms around this February 1996  55 not contain much detail. It simply indicated a square pulse with an amplitude of 0.9V and this amplitude appeared to be correct. But the manual gave no indication of the pulse width and this was what I was now querying. It wasn’t an easy point to check. Apart from the lack of detail in the manual, the CRO wasn’t too happy trying to resolve the pattern. Pin 9 takes in both vertical and horizontal pulses and, while in theory one can resolve either one, Fig.2: the vertical blanking pulse from the according to the selectvertical output stage in the Pye 48SL1 comes ed timebase, this is not in on line A51 (bottom, centre) and is fed to always so in practice the base of Q535 via R540. The base bias on this transistor is set by R529 and R534. The and there was some processed blanking pulse at the collec­tor is difficulty locking the fed via D467 to pin 9 of the chrominance + image. luminance IC (IC192). Nevertheless, now that my suspicion was aroused, the CRO patstage. There didn’t seem to be any tern seemed to confirm it. And from doubt about the waveform into Q535 this observation came the thought that but closer examination of the pulses Q535 was not being turned fully on coming out made me suspicious. I during this portion of the waveform. could­n’t be sure they were exactly as That, in turn, directed my attention to they should be. The waveform given resistors R529 and R534, both 6.8kΩ. in the manual – waveform 32 – did These set the bias for this stage; 15V at the base – from the 30V rail – against 13V at the emitter. Suppose I reduced that 15V bias on the base? Suiting the action to the thought, I unsoldered one end of R534 and substi­tuted the nearest appropriate value to hand, which happened to be 10kΩ. And, presto! – the lines vanished. Problem solved? Well, fault cured, which is not exactly the same thing. Naturally, there was a temptation to leave the circuit like that, since it obviously worked. But I’m never happy with such situations. Was I simply curing the fault by brute force without actually finding it? While trying to decide how to resolve this question, the answer was almost literally served up to me on a plate – well, on the shop counter to be correct. It was another Philips set, this time with a KT3A-2 chassis, which is virtually identical. Its fault was simple enough and I soon had it up and running, which provided an excellent opportunity to make comparative voltage and waveform measurements. In fact, I didn’t need to go that far. As soon as I moved to this part of the set the answer was plain to see; R534 in this set was 8.2kΩ. And it was obviously the original component, which I subsequently confirmed by reference to the KT3A-2 circuit. I fitted an 8.2kΩ in place of my 10kΩ and it worked just 20 Electronic Projects For Cars Yes! Please send me ___ copies of 20 Electronic Projects For Cars Enclosed is my cheque/money order for $­________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ Master Card Card No. Price: $8.95 plus $3 for postage. Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail the coupon to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 56  Silicon Chip Signature­­­­­­­­­­­­________________________ Card expiry date_____/______ Name _______________________Phone No (_____)____________ Street PLEASE PRINT _________________________________________________ Suburb/town _____________________________ Postcode_________ Fig.3: the power supply circuitry for the NEC N-3540. Note the “HOT” and “COLD” designations and the COLD secondary of T601. IC651 is at top left and portion of IC1001 at right. as well. Problem solved. Obviously, the fault I’d been chasing was not the first in this type of set. There must have been previous cases which had prompted this modification. Nor does it answer all the questions. Why did this set suddenly develop the fault when it had obviously performed satisfactorily for all those years? I can only assume that the original design was a bit mar­ginal, so that minor changes in components as the set aged were enough to tip the balance. Anyway, that was the end of the story. All that remained was to return the set to the dingy recesses of the customer’s abode. I hope I don’t have to go back, although it was a valuable lesson learned. Hot & cold NEC My next story is from the much more convenient and familiar atmosphere of my own workshop. It concerns an NEC colour set, model N-3450, the 34 indicating 34cm. It was fitted with a typi­cal infrared remote control system. According to the owner, the set was completely dead but he didn’t think there was much wrong with it, because the stand-by LED was on. He also indicated that he didn’t want to spend a great deal on a repair. Apart from the fault itself, the interest in the set con­cerns a rather unusual circuit arrangement. It is a live chassis arrangement but with considerably more of it being live than in most cases. And the circuit is clearly marked “HOT” and “COLD”, as appropriate – see Fig.3. Well, at least one is warned. As is usual, the mains connects straight to a bridge recti­ f ier and thence to a switchmode power supply, involving a trans­ former T601 plus a switching transistor and error amplifier in one package (IC601). The primary winding of T601 and one of two secondary windings is on the HOT side, but the other secondary is COLD. This provides a 20V rail via diode D650. Back on the HOT side, the output from IC601 is the main HT rail at 115V. This supplies the horizontal output section, con­sisting of horizontal driver transistor Q501, horizontal output stage Q502, and the primary of the output transformer, all still on the HOT side. The input to Q501 is from the main IC (IC701) via transformer T503, which has a COLD primary and a HOT second­ary. All the output transformer secondaries are COLD. Having digested all that, I turned my attention to the problem itself. The fact that the stand-by LED was on suggested that at least some part of the power supply was working. And, in fact, checks confirmed that the previously mentioned 20V and 115V rails were functioning. On the other hand, there was no horizontal waveform on any part of the horizontal system. This made me suspect that the fault could be in the remote control system; either the remote control receiver (PWC 3607C) or the microprocessor (IC1001) which it controls. In other words, the set was simply not being switched on. With the aid of the CRO, I established that the remote control receiver was working and delivering a signal to pin 14 of IC1001 (not the easiest path to trace on the circuit). However, there was no signal coming out on pin 33 of February 1996  57 YOU CAN AFFORD AN INTERNATIONAL SATELLITE TV SYSTEM SERVICEMAN’S LOG – CTD SATELLITE ENTHUSIASTS STARTER KIT YOUR OWN INTERNATIONAL SYSTEM FROM ONLY: FREE RECEPTION FROM Asiasat II, Gorizont, Palapa, Panamsat, Intelsat HERE'S WHAT YOU GET: ● ● ● ● ● ● 400 channel dual input receiver preprogrammed for all viewable satellites 1.8m solid ground mount dish 20°K LNBF 25m coaxial cable easy set up instructions regular customer newsletters BEWARE OF IMITATORS Direct Importer: AV-COMM PTY. LTD. PO BOX 225, Balgowlah NSW 2093 Tel: (02) 9949 7417 / 9948 2667 Fax: (02) 9949 7095 VISIT OUR INTERNET SITE http://www.avcomm.com.au YES GARRY, please send me more information on international band satellite systems. Name: __________________________________ Address: ________________________________ ____________________P'code: __________ Phone: (_______) ________________________ ACN 002 174 478 58  Silicon Chip IC1001 to switch the set on. The signal from pin 33 is applied to transistor Q1072, then to Q1071 to turn it on – see Fig.3. Q1071 functions as a voltage regula­tor, generating a 12V rail from the 20V rail. This 12V rail powers IC701 which contains the horizontal oscillator and this feeds horizontal driver transistor Q501. And that is how the set is turned on and off – by switching this 12V rail. With no 12V rail, there is no signal to drive the output stage or, in fact, any other function depending on IC701. Voltage checks OK, so why no signal on pin 33? It could be a fault in IC1001 of course but I wanted to check everything else before I pulled that out. And the first and obvious check was the voltage supplying this IC. It is a 5V supply, derived from a 3-terminal voltage regulator (IC651) operating from the 20V rail. Well, it was delivering voltage all right – too much vol­tage; it was closer to 8V than 5V. At the same time, I was prompted to look more closely at the 20V rail. In fact, that 20V figure is a nominal one. According to the circuit, it can vary from 22.4V on stand-by to 18.7V when the set is running. This is why I was deceived when I first confirmed that this part of the set was working. With the set switched off, that rail should have been at 22.4V, whereas it was slightly less than 20V, a value which had appeared to be close enough at first glance. But it was the 8V at the regulator output which was the real clue. I pulled IC651 out and replaced it. And that was it – there were now normal input and output voltages and the set was up and running. IC651 had broken down and was acting more like a resistor than a regulator, thereby placing a heavier load on the 20V rail and applying excessive voltage to the microprocessor. And it would appear that it was that excessive voltage which upset the microprocessor. The 5V rail feeds several pins on this IC and it is not surprising that the excessive voltage upset some of the internal logic functions. After all, it was not without good reason that the rail was regulated in SC the first place.