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SERVICEMAN'S LOG When I switch it on, nothing happens The old gag about the power switch being at fault be­cause the set won’t work when you turn it on is taking on a new twist these days. And it’s no longer a gag – these days, when the remote control system fails, the set won’t work. That was the situation I faced recently, involving a Super­star brand remote control colour TV set – 34cm model 1401R made in China. It was another repair for a colleague, so I had only a secondhand version of the fault. But the complaint was straight­forward enough; the set was completely dead. This is one of those sets which can only be switched on or off by the remote control and that was the first problem. The set came in with the remote control but this was in a rather grotty state. It had obviously had a hard life, judging by its external appearance, and was even worse inside. For starters, the batteries were flat. And although they hadn’t leaked, a previous set of batteries had, as was all too obvious from the badly corroded contacts. In fact, the corrosion was so bad that one of the contacts broke off as I was removing the dead batteries. The next problem was that I didn’t have a circuit or manu­al and so I had to track down the agents to get one. And when I did finally get a manual, the circuit turned out to about the worst quality copy I have ever encountered. I would defy anyone to decipher any of the values at anything more than a guesstima­tion level – and then only by cross referencing to the set it­self. This not an unusual state of affairs these days, unfor­tunately. I don’t know who is to blame but I do know that the service industry is being given a pretty raw deal. Anyway, back to the set itself. One of the first things to determine in situations like this is whether there is a fault in the set itself or a fault in the remote control system. Initial­ly, I set about familiarising myself with the layout and making some preliminary checks which might suggest where the fault lay. And, in order that the reader can follow the story, it is neces­sary to convey some idea of the circuitry – something which is made all the more difficult by reason of the poor circuit quality which I’ve already mentioned (the only other justification for reprinting it would be as an ‘orrible example). Voltage checks My first step was to identify and check the main voltage rails. As far as I could see, there were three: a 120V rail and two 12V rails, which I will call “A” and “B”. The 120V rail is derived from a switchmode power supply. This involves the usual bridge rectifier (D121) across the mains, a chopper transformer (EM110), and IC104, which provides the oscillator and Fig.1: the power supply circuitry in the Superstar 1401R TV receiver. The bridge rectifier is at lower left, the chopper components (EM110 & IC104) centre and right, and transformer EM112 above the bridge rectifier. Connector CN201 is at top left. 38  Silicon Chip K ALEX The UV People ETCH TANKS ● Bubble Etch ● Circulating LIGHT BOXES ● Portuvee 4 ● Portuvee 6 ● Dual Level TRIMMER ● Ideal PCB DRILL ● Toyo HiSpeed MATERIALS ● PC Board: Riston, Dynachem ● 3M Label/Panel Stock ● Dynamark: Metal, Plastic ✸ AUSTRALIA’S NO.1 STOCKIST ✸ K ALEX 40 Wallis Ave, East Ivanhoe 3079. Phone (03) 9497 3422, Fax (03) 9499 2381 TRANSFORMERS control functions for the transformer. The output from D121 is around 340V and this is smoothed by a 100µF 400V capacitor (C101). This is applied to pin 1 of IC104 via the primary winding of EM110 (pins 2 & 4). The 120V rail comes off pin 9. The 12V “A” rail is derived from another winding on trans­former EM110 (pins 12 & 13), via diode D108 and voltage regulator IC103. The 12V “B” rail, on the other hand, comes from a small 50Hz power transformer (EM112), via diode D107 and two filter capacitors (C127 & C128). It is used to power the remote control receiver and its associated circuitry, ensuring that this is functional at all times, even when the main part of the set is shut down. In general terms, this is all fairly conventional. More importantly, it enabled me to make the first assessment as to the broad nature of the fault. At first switch-on, there was no 120V rail and no 12V “A” rail. However, there was output from the bridge rectifier and there was 12V on the “B” rail. In other words, the switchmode supply wasn’t working. The switchmode supply is turned on and off – from the remote control board – via a chain of three transistors: Q116, Q117 and Q118. In simple terms, to turn the set on, a positive voltage is applied to Q116’s base from the remote control board (via pin 4 of plug/socket CN201). This turns on Q116 which then turns off Q117 and Q118. Q118 is connected between pins 2 & 4 of IC104. From this, it appears that the set is held off by connecting pins 2 & 4 together via Q118, when this is turned on. Conversely, when this transistor turns off, the set turns on. And since there was no positive voltage applied to Q116’s base when the Power button on the remote control was pressed, it • TOROIDAL • CONVENTIONAL • POWER • OUTPUT • CURRENT • INVERTER • PLUGPACKS • CHOKES STOCK RANGE TOROIDALS BEST PRICES APPROVED TO AS 3108-1990 SPECIALS DESIGNED & MADE 15VA to 7.5kVA Tortech Pty Ltd 24/31 Wentworth St, Greenacre 2190 Phone (02) 642 6003 Fax (02) 642 6127 April 1996  39 Serviceman’s Log – continued was obvious that the set could not turn on – quite apart from any other reason why it may not work. I pulled a swifty here – I set the analog multimeter switch to the low ohms range and connected the positive probe to the chassis and the negative one to Q116’s base. Like most such meters, mine applies reverse voltages to the probes when in the ohms range, which meant that I was applying a positive voltage to the base of the transistor. And it worked; the set burst into life. Well, that was a major step forward. The fault was quite clearly in some part of the remote control system. I still had to find out where but the search had been narrowed considerably. Remote control section The remote control section consists of a photo receiver module, two ICs (IC1 & IC3), a few transistors, and the 40  Silicon Chip usual array of switches, diodes and pots in the channel selection network. Fig.2 shows part of this circuit. At this point I had to get the remote control unit itself working. Apart from its grotty external appearance and the broken battery contact, there wasn’t a great deal wrong with it and I was able to get it working on a temporary basis. More permanent repairs could come later. The next thing was to determine whether the photo receiver was functioning. When a valid signal is received, this should deliver pulses to transistor Q4, which in turn drives pin 13 of IC3. In fact, the CRO confirmed that all this was happening. However, there was no positive voltage produced at pin 6 of IC3, which ultimately connects to the base of Q116. And that seemed to throw suspicion on either IC3 itself or its associated circuitry. I checked that 12V was being applied to pin 12 and that the clock crystal (Z1) was functioning (the frequency meter confirmed that this was oscillating at 455kHz). I made a few more checks of the other associated parts but could find nothing wrong. In short, it all came back to the IC. I didn’t have a replacement, so I ordered a new one from the agents (price $30 trade). And while I waited for it, I tried something else; I fitted a socket in place of IC3. Now I know that sockets have not enjoyed a very good reputation in the past and with good reason. Some of the early attempts were pretty woeful. Fig.2: part of the remote control receiver in the Superstar 1401R. Q4 buffers signals from the photo receiver module and drives pin 13 of IC3. The output from this IC appears at pin 6 and goes to pin 4 of connector CN201. But the scene has changed for the better and there are now some very good quality units available. And there is no doubt that a socket makes things a lot easier where there may be some doubt about the fault. On the other hand, space around or above the site often makes such a modification impossible. But there was room in this case, so I went ahead. And as if to justify what I had done, I suddenly found a spare IC that I’d had all the time. It wasn’t a new unit, having been removed some time previously from another set. Nevertheless, I pushed it straight into the socket, switched on and everything came good, with all remote control functions fully operative. This not only confirmed that it was the IC at fault but, in the process, cleared everything else, including the remote con­trol unit itself. I let the set run for the rest of the day and all next day and it never missed a beat. But on the third day it died. I wasn’t particularly worried; the IC was suspect, so I simply assumed that it had failed and waited for the new one to arrive. When it did a couple of days later, I pushed it in and the set came good again. I let it run as before but took the oppor­tunity to go over the various adjustments and make sure that everything was up to scratch. So the job was virtually finished, or so I thought until, a couple of days later, the set suddenly died again. Can something “die again”? Well this set did and it came as a rude shock. I had a horrible feeling that there was a “nasty” lurking in there somewhere, causing the set to fail every few days. A simple explanation In fact, it was a simpler explanation than that. A few meter checks revealed that the 12V “B” rail had failed and that this was due, in turn, to the failure of the EM112 transformer. In fact, its primary winding measured open circuit. And that created a difficult situation. While I hadn’t quoted for the job, I had given an estimate. A new transformer would be expensive and, when added to what had already been chalked up, it wasn’t going to make a very nice figure. Then I had an idea – many of these transformers feature an internal thermal fuse and I was prepared to bet long odds that this was what had failed (it wouldn’t be the first time). So was it worth trying to fix? Well, I didn’t have much to loose. The winding was wrapped in yellow plastic tape and, armed with a razor blade, I very carefully cut through it near the winding terminals, where I judged it was clear of the winding. In fact it was and, working very carefully, I was able to peel back the tape to give a good view of the winding. That was fine but Murphy had seen to it that the thermal fuse was on the opposite side to where I had started. When I finally did reach it, a quick check revealed that it was open circuit. The failure was not due to any normal fuse action; rather it appeared to be a simple structural failure. More to the point, what should I do about it? In theory, I suppose, I should have aimed to replace it. However, I didn’t fancy the time and trauma that would be involved in getting a replacement. Nor could I see the justification for it in the first place. The set is adequately fused in the mains lead, which should surely take care of any fault which could occur anywhere in the set. Why pick on this component? I simply bridged it, then rewrapped the winding in new tape, refitted the transformer and gave the set another soak test. This lasted several days and passed without further incid­ent. I handed the set back to my colleague, filled him in on the thermal fuse situation, and left him to deal with his customer. By all accounts, everyone was satisfied. Postscript: having done all the above and written about it, I suddenly acquired another version of the circuit. It is a quite different drawing but exactly the same circuit and, while not perfect, a far better quality print (most of it is readable). This is the one used to illustrate this article. The distorted Toshiba My next story is about a Toshiba 48cm colour set, model 207E9A, April 1996  41 electronic design, and applications. The sixth edition has been expanded to include chapters on surface mount technology, hardware & software design, semicustom electronics & data communications. 63 chapters, in hard cover at $120.00. Silicon Chip Bookshop Radio Frequency Transistors Newnes Guide to Satellite TV Installation, Recept­ion & Repair. By Derek J. Stephen­son. First published 1991, reprinted 1994 (3rd edition). This is a practical guide on the installation and servicing of satellite television equipment. The coverage of the subject is extensive, without excessive theory or mathematics. 371 pages, in hard cover at $55.95. Guide to TV & Video Technology By Eugene Trundle. First pub­lish-­ ed 1988. Second edition 1996. Eugene Trundle has written for many years in Television magazine and his latest book is right up date on TV and video technology. 382 pages, in paperback, at $39.95. Servicing Personal Computers By Michael Tooley. First published 1985. 4th edition 1994. Computers are prone to failure from a number of common causes & some that are not so common. This book sets out the principles & practice of computer servicing (including disc drives, printers & monitors), describes some of the latest software diagnostic routines & includes program listings. 387 pages in hard cover at $59.95. format and R-DAT. If you want to understand digital audio, you need this reference book. 305 pages, in paperback at $55.95. The Art of Linear Electronics By John Linsley Hood. Pub­lished 1993. This is a practical handbook from one of the world’s most prolific audio designers, with many of his designs having been published in English technical magazines over the years. A great many practical circuits are featured – a must for anyone inter­ested in audio design. 336 pages, in paperback at $49.95. Components, Circuits & Applica­ tions, by F. F. Mazda. Published 1990. Previously a neglected field, power electronics has come into its own, particularly in the areas of traction and electric vehicles. F. F. Mazda is an acknowledged authority on the subject and he writes mainly on the many uses of thyristors & Triacs in single and three phase circuits. 417 pages, in soft cover at $59.95. Digital Audio & Compact Disc Technology Electronics Engineer’s Reference Book Hard cove Produced by the Sony Service Centre (Europe). 3rd edition, published 1995. Prepared by Sony’s technical staff, this is the best book on compact disc technology that we have ever come across. It covers digital audio in depth, including PCM adapters, the Video8 PCM Power Electronics Handbook Your Name__________________________________________________ PLEASE PRINT Address____________________________________________________ _____________________________________Postcode_____________ Daytime Phone No.______________________Total Price $A _________ ❏ Cheque/Money Order r Edited by F. F. Mazda. version now available First published 1989. 6th edition. This just has to be the best refer­ ence book available for electronics engineers. Provides expert coverage of all aspects of electronics in five parts: techniques, physical phenomena, material & components, ❏ Bankcard ❏ Visa Card ❏ MasterCard Card No. Signature_________________________ Card expiry date_____/______ Principles & Practical Applications. By Norm Dye & Helge Granberg. Published 1993. This book strips away the mysteries of RF circuit design. Written by two Motorola engineers, it looks at RF transistor fundamentals before moving on to specific design examples; eg, amplifiers, oscillators and pulsed power systems. Also included are chapters on filtering, impedance matching & CAD. 235 pages, in hard cover at $85.00. Surface Mount Technology By Rudolph Strauss. First pub­ lished 1994. This book will provide informative reading for anyone considering the assembly of PC boards with surface mounted devices. Includes chapters on wave soldering, reflow­ soldering, component placement, cleaning & quality control. 361 pages, in hard cover at $99.00. Audio Electronics By John Linsley Hood. Pub­lished 1995. This book is for anyone involved in designing, adapting and using analog and digital audio equipment. Covers tape recording, tuners & radio receivers, preamplifiers, voltage amplifiers, power amplifiers, the compact disc & digital audio, test & measurement, loudspeaker crossover systems and power supplies. 351 pages, in soft cover at $52.95.   Title  Newnes Guide to Satellite TV  Guide to TV & Video Technology  Servicing Personal Computers  The Art Of Linear Electronics  Digital Audio & Compact Disc Technology  Power Electronics Handbook  Electronic Engineer's Reference Book  Radio Frequency Transistors  Surface Mount Technology  Audio Electronics Postage: add $5.00 per book. Orders over $100 are post free within Australia. NZ & PNG add $10.00 per book, elsewhere add $15 per book. TOTAL $A Price $55.95 $39.95 $59.95 $49.95 $55.95 $59.95 $120.00 $85.00 $99.00 $52.95 Fig.3: the vertical output stage of the Toshiba 147R9E. IC303 at left provides the vertical output signal to the deflection yoke (note the input and output waveforms). Capacitor C317 is at centre, while the deflection coils (L462) are at the extreme right. made in Singapore, vintage 1989. The complaint was gross vertical scan distortion. Only the top half of the screen had any recognisable image, while the bottom half was compressed in the centre. A colleague has a theory about vertical distortion. His rule of thumb is that if the problem is at the top of the screen, it is a power supply problem; if it is at the bottom, it is a feedback problem. Frankly, I’m always rather suspicious about general state­ments of that nature but I have to agree that it has some merit. Did it apply in this case? I leave the reader to judge for him­self. The relevant sections of the circuit involve two ICs: IC501 and IC303. IC501 is a TA8718N, a 30-pin multi-purpose chip which provides most of the front-end processing. This includes colour decoding and the derivation of the vertical and horizontal sign­als. The vertical signal comes out on pin 11 and goes to pin 4 of IC303 (AN5515). This is the vertical output stage and the signal from pin 11 goes into it on pin 4, comes out on pin 2, and goes to terminal 7 of the vertical deflection yoke. My first step was to check the voltages on IC303 and they came up virtually spot on. Next, assuming that it was a signal path fault, possibly in the feedback network, I decided to check out the various electrolytic capacitors, particularly the lower value ones, which are notorious for poor reliability. And no sooner had I made that decision, than I found one staring me in the face. It was a red Elna 2.2µF unit (C317) in what appeared to be part of the feedback path from terminal 8 of the yoke. It had leaked its inside outside, all over the board around it. Bingo, I thought. Picked it in one; I’ll knock this one over in no time. Alas it was not to be. I removed the sick unit, cleaned up the board, fitted a new one, and switched on. Result: exactly as before. Circuit waveforms So it wasn’t going to be easy after all; I would have to tackle it stage by stage. The circuit shows two waveforms; the input to IC303 on pin 4 and its output on pin 2 – see Fig.3. I reached for the CRO leads and checked pin 4. It was virtually spot on, its amplitude and shape exactly as shown. But pin 2 was a different story. The waveform was nothing like that on the circuit. I followed the signal through to the yoke (terminal 7) and then to the other side of the yoke (termi­nal 8), speculating on the remote possibility of shorted turns in the yoke. This check didn’t tell me much. For some strange reason, the waveform on terminal 8 was more like the circuit pattern than the one direct from IC303 at terminal 7. If it meant anything at all, it seemed to rule out the shorted turns theory. And that, in turn, put suspicion back on IC303 and its sur­ rounding components. With one crook electro already encountered, I first proceeded to check all the electros around the IC. And by checking, I really mean replacing, because I felt this was the only sure test when chasing a weird fault like this one. That achieved nothing. To cut a long story short, I fin­ished up checking or replacing every component around that IC – even the diodes. Nothing made any difference, which left the IC itself. It is a common type and I had stock on hand so I changed it. Again I drew a blank. I was feeling pretty desperate by April 1996  43 ning from terminal 8 of the yoke to pin 14 of this IC (via R304). And the circuit indicates 6.7V on pin 14, which was exactly what it measured. Was the fault in IC501? I didn’t fancy the time and expense involved in changing this – I would have had to order one – and looked around desperately in this part of the circuit for further inspiration. And I found it in the most unexpected place. Connected to the adjacent pin 13 of IC501 is the height control (R351), a 50kΩ pot to chassis. Now I probably would never have suspected this part of the circuit in a month of Sundays but what caught my eye was a bypass capacitor, C303, from pin 13 to chassis – it was a red Elna 2.2µF electrolytic, identical to the one I had already replaced in the yoke circuit. I should have spotted it sooner; it was the only other red electro on the board. But having spotted it, I didn’t stop to ponder the technical implications – I reefed it out and replaced it. And that was it; problem solved. Still a mystery now and came back to the idea of a fault in the yoke winding. Not surprisingly, I didn’t have another yoke of that type on hand but I did have a somewhat similar one from another set. I decided to temporarily substitute that, at least electrically, and note whether it made any drastic difference to the faulty waveform at pin 2. It didn’t, so I finally ruled out that theory. So what was there left to check? At this stage, I remem­bered my colleague’s theory about the feedback circuit. I hadn’t consciously checked this, as such, assuming that checking all obvious components would include it. But it hadn’t. The feedback circuit also involves IC501, with a line run- I’m still at a complete loss to explain just how the height control came to be involved in this particular fault. But then, without knowing the exact circuit details within the IC by which the height is controlled, who can say. Is the height control part of the feed­ back circuit? And what is the function of the 2.2µF ca­pacitor which caused the fault? But those questions aside, the story reinforces what I’ve said so many times before and with which all my colleagues agree; never trust a low SC value electrolytic capacitor. 20 Electronic Projects For Cars Available only from Silicon Chip 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 your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 44  Silicon Chip