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SERVICEMAN'S LOG Same set, same symptoms, but. .. Usually, we welcome a set of symptoms we've seen before; it can mean a quick cure for what, originally, might have been a sticky one. It's what we call experience and it's what the customer pays us for. But sometimes two different faults can produce identical symptoms. My stories this month demonstrate the confusion and red herrings this possibility can sometimes cause. The first one involved an AWA Mitsubishi 34cm colour TV set, model C-3423. It was a new set, only about nine months old and, of course, still under warranty. I was .making a house call on a different matter when the teenage daughter asked me if I would look at her TV set. According to her, it had simply failed in the middle of a program and was completely dead. Could I fix it? I assured her that I should be able to and, if she had the necessary documents to cover the warranty, it should noi cost her anything. In fact all these were in order, so I loaded the set into 105 the van and headed for the shop. When I set it up on the bench and switched it on, the result, superficially, was as she had described it: no picture, no raster and no sound. But it was not quite dead. At the moment of switch-on, I had heard a faint "boing" as the de-gauss circuit activated. This meant that the mains fuse was intact, eliminating one initial check. I pulled the back off and switched on again, looking for any obvious damage. I found none but I was able to observe that the picture tube heaters were up and running, which ruled out a lot more possibilities. Delving a little deeper, I established that the main HT rail was spot on at 103V. I pushed the EHT probe in under the ultor flap and came up with a 1N•9'& T 40 2 F BT C421 IOOOfliF 2~V Fig.1: the horizontal output transformer circuitry in the Mitsubishi Model C3423, The 16.5V rail is derived from pin 5 ofT402 via resistor R435, diode D406 and capacitors C422 & C421. similar answer - EHT spot on. So where to now? I began imagining all kinds of weird and subtle faults, involving a faulty IC or something similar. Fortunately, calmer thoughts prevailed. There are always other voltage rails and it is essential to check all of these before suspecting individual components. And so I very soon discovered that what should have been a 16.5V rail was actually sitting at only 3V. I'm not sure of all its functions but one is to provide another rail - a 12V rail via a voltage regulator, 1502. In any case, it was the most likely cause of the failure and had to be fixed. There is very little to this circuit (Fig. l). It is derived from a tapping, pin 5, on the horizontal output transformer (T402) and consists of a lQ fusible safety resistor (R435), a diode (D406), a 1500pF capacitor (C422) across the diode, and a lO00µF 25VW electrolytic capacitor (C421) as the filter. Well, . with only four components involved, it didn't take long to find the culprit. It was C421, the lO00µF filter capacitor, which was virtually open circuit. In fact, it couldn't even struggle up to lµF on the capacitance meter. Anyway, it was easy enough to fix; a new electro and the set was up and running like new. I rang the house, spoke to the girl's mother and told her that the set was ready. She said she would be in to pick it up the next day. In the meantime, I left it running. And so the next day the lady collected the set and I completed the necessary warranty claim forms and sent them off. · All of which added up to a perfectly routine exercise involving a relatively simple fault. In fact, the reader is probably wondering why I bothered to relate the story. Mum returns There is a very good reason; the set 34 SILICON CHIP bounced - and bounced in a particularly nasty way. Mum was back the next day with the set and rather testily complained that it had run for only 15 minutes, than failed with exactly the same fault as before. (It's always the "same" fault when a set bounces). Naturally, I apologised, tried to explain that these things do happen occasionally, and assured her that the warranty conditions would still apply. This mollified her somewhat but she wasn't laughing when she left. Why did that crucial 15 minutes have to happen in the customer's home rather than on my bench? And so I started all over again. I switched the set on and there was the reassuring "boing" from the degauss circuit. And with the back off I could see the picture tube heaters were alight. And the main HT rail was spot on, as was the EHT. Of course, it couldn't be the 16.5V rail again but it was. It was down to 3V, exactly as before. I was starting to pinch myself now, expecting to wake up and find it all a horrible dream. (I do have such dreams!) But it was real enough. And, with Fig.2: power supply circuitry in the Mitsubishi model CT-1447AM. The bridge rectifier is at centre and the voltage regulator, IC901, at lower right. Note the voltage on pin 4 and the incorrect voltages shown for Q902. identical symptoms, I was totally confused. On the one hand, it appeared that the brand new capacitor I had fitted had lasted only a few hours. On the other hand, the odds on such a coincidence seemed too long to consider. Nevertheless, I felt that it had to be the capacitor, so I pulled it out and checked it on the tester. But I felt a bit foolish when the tester showed that it had the full lOOOµF plus; there was nothing wrong with it. So what was wrong? With the capacitor cleared, the wasn't much left. The lQ safety resistor? A likely culprit - only it wasn't. That really left only the diode. The only snag was that an in-situ check with the meter showed it also was OK. I was getting the horrors now. Was it a faulty tapping on the transformer? I put that idea to one side and decided to try a long shot. In spite of the test, I was having second thoughts about the diode. I grabbed another diode, tacked it across the existing one and tried again. Bingo! That was it. The 16.5V reappeared, and the set began to play. I pulled the original diode out and checked it again, but couldn't fault it. Yet it wouldn't work; it obviously could not handle the voltage and/or -~---·· ij~t<t )I. . . <at>?)o~=--· cs~-r :BOUNC..E:..D 11-\e:. NE:.X, PAY WHF;.N ~UM "RA~E.~ 1"E.S1iLY COM?LA\Nl::P.u .. 11-\~ JANUARY 1992 35 SERVICEMAN'S.LOG - CTD current conditions off the circuit. I fitted the new diode properly, ran the set for a couple of days, crossed my fingers, and advised the owner that it was ready. That was several weeks ago and all is well so far. But why did those two components fail in quick succession? I've tried to think of some reason why the capacitor failure may have contributed to the diode failure but without success. I have finally concluded that it was sheer coincidence - but I wish that the diode had chosen -to fail in the workshop rather than in the customer's home. So that was the first example of two faults with identical symptoms. It was another case of a totally dead set. As before, preliminary observa1 .---t-lJol-, tions, indicated that the mains fuse .,.bi ~ was intact but there the similarity ~--~ I ® !l!l!il ended. There was no HT rail voltage and, not surprisingly, no EHT. I went back to the bridge rectifier (Fig.2), which is directly across the mains, and measured its output across the main filter capacitor, C906 (l00µF). The reading was around 300V, which was a little on the high side and suggested a lack of load. From there, I went to pin 4 ofIC901, a voltage regulator package which provides the main HT rail at 115V. (Incidentally, some of the voltages quoted around this part of the circuit are quite strange. Pin 4 of IC901, . More Mitsubishis c________ , j shown as 115V, connects directly to The second example involved two the collector of Q902 where, miracuQ more 34cm Mitsubishi colour sets, lously, it becomes 165V. The other models CT-1447AM and CT-1440A. voltages on this transistor are equally Fig.3: horizontal output transformer And perhaps I should add at this point strange.) But there was no voltage at pin 4. circuitry in the Mitsubishi Model CTthat any apparent emphasis on Mitsubishi by these stories is purely So, was it a regulator failure or was 1447 AM. The collector of the horizontal output transistor, Q503, coincidental. In no sense do I intend something loading the circuit? In fact, connects to pin 1 of the primary to imply that they are more prone to it appeared to be the latter because a winding, while the 115V rail goes to faults than any other set. It is just that resistance check from pin 4 to chassis pin 2. · the need to present these stories in showed only 3.5Q. Aha!, I thought, sequence may give a wrong impres- I'll bet that's the horizontal output sion. transistor broken down - this being mary, pin 1, while the 115V rail conThe first set appeared a couple of one of the more common causes ofHT nects to the other end, pin 2. weeks after the incident described rail loading. So it was off down to the other end A wobbly diagnosis above. The owner was going on holidays for several weeks and the set had of the circuit (Fig.3) to Q551, the tranAnd this was where my snap diagnosis began to look a bit wobbly. A failed a couple of days earlier, so they sistor in question. In typical fashion, decided to leave it with me while the collector connects to one end of resistance measurement from the colthey were away. That suited me fine. the horizontal output transformer pri- lector ofQ551 to chassis gave a virtually identical reading to that at pin 4 of the regulator IC. I tended to over11ao• look the full significance of this iniHK(lll 112W tially, regarding it as no more than a confirmation of the load on the HT rail. But it suddenly struck me that there was the resistance of the transformer C802 primary winding in between these two oaoe Rf-I 6 points. Granted, it is quite low - only AIOI$ 11011 about JQ - but it is measurable and HO should have shown up as a difference between the short at one end of the winding and that at the other. So how M:.2,r.'.., JVOI come they both read the same? How come both ends of the transformer PI04 OICIIUI- coi. primary were showing the same resistance to chassis? Fig.4: power supply circuitry in the Mitsubishi Model C-3423. The power supply If the fault was in Q551, it was a bridge rectifier is at left, the chopper transformer (T801) at centre, and the voltage regulator (Q801) at right. The 103V HT rail is derived from pin 4. very funny one. Either that or we had 36 SILICON CHIP OS!5 more than one fault; or the fault was in the horizontal output transformer. That was a nasty thought because there are some 10 pins connecting the transformer into the PC board and, together with the physical layout, this could make getting it out rather tedious. On the other hand, it was a simple job to lift Q551 to clarify the position. And as far as I could determine from a rough check, the transistor was OK. However, it is important to realise that these transistors have an internal resistance of about 400 between base and emitter, plus a diode between collector and emitter. This can sometimes confuse simple tests. But the more important check was from pin 1 of the output transformer to chassis and this still showed 3.50. So it wasn't Q551. It had to be the transformer- or was it a "funny" somewhere else in the set? I felt that the latter possibility was a long shot and, in any case, tracking it down (or disproving it) could be a long and involved business. Among other things, it would probably involve cutting various copper tracks. This is not always as easy as it sounds if the tracks are crowded to- gether and is something I try to avoid if possible. So before pulling the transformer, I decided on one more check. One of my more valuable pieces of test gear is a shorted turns tester. I wasn't sure how well it would work in this situation because the transformer second.ary winding was pretty well loaded with, among other things, the picture tube heater winding. Nevertheless, I connected it across pins 1 and 2 and made the test. And the indication was that there were no shorted turns in the transformer. Well, that was a useful piece of information but not really conclusive. So where to now? After some cogitation, I decided that in spite of the work involved, the quickest and most satisfactory approach was to remove the transformer. Granted, if it wasn't the culprit, the work of pulling it out and putting it back might appear to be wasted but at least it would settle the question once and for all. I set to work with the iron and solderwick and, after about 15 minutes, was able to lift the transformer clear of the board. This finally gave me a clear go. A quick check at the pin AUSTRALIAN MADE TV TEST EQUIPMENT Happy New Year Specials. 10% discount on all test equipment. 15% discount on two or more items SHORTED TURNS TESTER Built-in meter to check EHT transformers including split diode type, yokes and drive transformers . $78.00 + $4.00 p&p TETIA TV TIP Princess 14CT8 (PC-02X chassis) Symptom: weak vertical hold. Video input to the sync separator (IC301) is normal but the vertical oscillator is inside the same chip so it is not practical to see if the sync pulses are normal. Cure: the clue is that the voltage at the input to the sync separator is lower than specified. The cause is that the first video amplifier, (0201 2SC1815) is slightly leaky collector to emitter. This does not seem to affect its gain but does put a wrong bias on the sync separator. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute ofAustralia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. 1 and pin 2 positions on the board showed that there was no short to chassis, which meant that it had to be PRESTON ELECTRONICS 217 High Street, Preston, Melbourne Victoria 3072 (Cnr of Bell & High Street) TEEPROM PC Eprom Programmer Card, Cable + Software $250.00 HIGH-VOLTAGE PROBE Built-in meter reads positive or negative voltages from 0-50kV. For checking EHT and other HT voltages. $98.00 + $5.00 p&p 1991 European/Jap Transistor Equivalent Book, 790 Pages $39.95 DEGAUSSING WAND Strong magnetic field. Double insulated for your safety with momentary switch operation. 240V AC 2.2 amps. As important as a soldering iron! $75.00 + $10.00 p&p REMOTE CONTROL TESTER Designed to test infrared or ultrasonic control units. Supplied with extension infrared detector lead. Output is via a LED and piezo speaker. $84.00 + $4.00 p&p ~~ ~ From$19exchange plus post & pack Cheque, Money Order, Visa, Bankcard or Mastercard 216 Canterbury Rd, Revesby, NSW 2212, Australia. Phone (02) 774 1154 Fax (02) 7741154 Economy Soldering Iron - 240V Fine Point Tip $19.95 Video Recorder Replacement Belt Kits (Specify Make & Model} $12.95 CB Radio Power Amp, 35W AM, 70W SSB Output $199 Fujitsu 12V 1A PC Relays $1.00 ea. Mail Order Hot-Line 8 /Card-Mast/Card-Visa-Cheque Phone (03) 484 0191 JANUARY 1992 37 SERVICEMAN'S LOG - CTD the transformer. Fairly obviously, there had to be a short between the primary winding and one of the secondary tappings. A resistance measurement between pins 1 & 2 and pins 9 & 10 (the picture tube heater winding) gave the all clear. But from primary pin 2 to pins 3, 4, 5 & 7 (which are all part of one winding), there were very low resistance readings. And the lowest was 3.5Q to pin 7, which connects to chassis. Also significant was the fact that the readings from pin 1 were almost identical. Conclusion: the breakdown between primary and secondary must have been at, or very close to, the centre of the primary winding. And that makes it one of the dirtiest tricks that Murphy has pulled for a long time. And the shorted turns test? No, it didn't fail; the fault was not a shorted turn. On the other hand, it was areasonable try as faults in output transformers commonly result in shorted turns. Anyway, these thoughts aside, I had finally tracked down the fault. A call to AWA for a replacement transformer was handled with commendable speed and it was on its way in few hours. And that, apart from the routine fitting, was the end of the story. I ran it regularly each day until the customer returned from holidays and have heard nothing from him since. The twist to the story But now comes the twist to the story. While that set was still in the workshop waiting to be collected, another 34cm Mitsubishi arrived, this time a model CT-1440A. Although a different model, with some superficial differences, it uses essentially the same circuit and chassis as the CT1447 AM. The complaint was the same; completely dead. I started at the bridge rectifier as before and found a similar situation; a slightly higher than normal voltage of about 300V. I then moved to pin 4 of the regulator, IC901, only to find no voltage. Almost automatically I made a resistance check to chassis. And what did I find? Yes, you've guessed it; 3.5Q I could hardly believe it. More to the point, I hardly knew what to do. Remembering how I had been caught in the first story, I had a horrible feel- ~MONG 011-1~ 11-lt~G'E> rf WOUl.--D PROBABL-V \NVOL-VE'. CV"'\I\NG VA'R\OUS COP'P6:.'R -.1RAC.KSon 38 SILICON CHIP ing that Murphy was having a lend of me. On the one hand, the symptoms seemed so obvious - perhaps too obvious - while, on the other hand, the odds against two such identical faults landing on my bench within a week or so of each other seemed pretty long. I couldn't escape the feeling that which ever way I jumped, it would be wrong. IfI pulled the transformer out, there would be nothing wrong with it. If I didn't, I would spend hours looking for an elusive fault with the same symptoms, while it was the transformer all the time. Either way, I couldn't win. But, of course, I had to start somewhere. I went over the circuit in fine detail, repeating every measurement I had made on the first set, looking for something that didn't match and which might provide a clue. It was a vain effort; as far as I could determine everything was exactly as it had been in the first set. So, finally, there was nothing for it but to pull the transformer. And it really was the same fault. So I'd been caught out by my own caution. But I still reckon it was long odds for those two faults to turn up in such quick succession. Or have I latched on to an inherent fault in a batch of these transformers. Well, only time will tell but, if I encounter these symptoms again, I reckon I'll take a punt on the transformer. Postscript Well, that was supposed to be the end of this month's contribution. However, shortly after I had finished bashing it out on the computer, another dead Mitsubishi turned up. It was a model C-3423, the same as the first one with the failed 16.5V rail. And, like that one, it was a new set, only about one month old and still under warranty. In view of the previous stories, my first reaction was to suspect that 16.5V rail. But then another thought crossed my mind; I realised that this set uses the same horizontal output transformer as the models CT-144 7AM and CT-1440A. So I couldn't overlook the possibility that this was going to be number three with the primary/secondary breakdown. I checked the 16.5V rail first. It was completely dead whereas previously there had still been about 3V. Next, I checked the 103V HT rail and this You can now afford a sate II ite TV system For many years you have probably looked at satellite TV systems and thought "one day". You can now purchase the fol lowing K-band system for only: $995.00 Here's what you get: * A 1.8-metre prime focus dish antenna, complete with all the mounting hardware. *or better). One super low-noise LNB (1.4dB *magnetic One Ku-band feedhorn and a signal polariser. * 30 metres of low-loss coaxial cable with a single pair control line. * A 99-channel infrared control This horizontal output transformer is from the CT-1447AM (second story), although it is also used in the C-3423. It had shorted from the centre of the primary winding to one of the secondary windings. was also dead, a symptom more like that of the output transformer fault. It looked even more like it when I checked around the voltage regulator package, Q801 (Fig.4). Voltage is applied to pin 1 of this from a chopper transformer, T801, and was sitting at around 300V. But at pin 4, which delivers the 103V rail, there was nothing which indicated that the voltage regulator wasn't working. A resistance check from pin 4 to chassis showed only 2.6Q. And hard alongside the voltage regulator on the circuit were two likely culprits: a lO0µF electrolytic capacitor (C807) and a 160V zener diode (D807) which protects against excessive HT voltage. It was easy enough to disconnect both of these but I drew a blank; the 2.6Q remained. Another possibility was a breakdown in the voltage regulator. But while not ruling it out, I felt it was most unlikely considering the symptoms. A breakdown of the transistor in the regulator would most likely put 300V on pin 4. So was it the previous transformer fault? This still seemed like a possibility until I took a closer look at the circuit. Between the HT rail and pin 2 of the output transformer primary (Fig.1) is a diode, D408. It seemed to me that it would be most unlikely that I would be able to get a reading of 2.6Q through this diode, considering the low voltage of the meter. Nevertheless, I made some readings at the transformer and this quickly ruled it out. So it was back to the circuitry around pin 4 of Q801. The HT rail also connects to a tap on the lower winding of the chopper transformer. So was it a fault in the chopper transformer? I had a lucky break here; using solderwick, I was able to soak up the solder around the tapping pin until it was isolated from the copper pattern. This cleared the fault from the HT rail, which was my first real breakthrough. So it could be the chopper transformer. But then I spotted something else; the left hand end of this winding connects to chassis via diode D805 and choke L802 in series, and also via a parallel 2200pF capacitor, C808. So it could be either the diode or the capacitor. I pulled the capacitor out first and, of course, drew a blank; nothing wrong with it. But the diode was a different story; it had broken down and the 2.6Q was mostly from the choke (L802) and the transformer winding. So I finally cracked it, in spite of the red herrings. But that's all part of the game. SC satellite receiver with adjustable IF and audio bandwidth, polarity and dual digital readout. Your receiver is pre-programmed to the popular AUSSAT transponders via the internal EEPROM memory. This unit is also suitable for C-band applications. Call, fax or write to: AV-COMM PTY LTD PO BOX 386, NORTHBRIDGE NSW 2063. Phone (02) 949 7417 Fax (02) 949 7095 All items are available separately. Ask about our C-band LNBs, NTSCto-PAL converters, video time date generators, FM 2 and EPAL. I I ----------YES GARRY, please send me more information on K-band satellite systems. I I : Name ... ... .... .......... .. .. ................ ...... : I Address.. ... ... .. ... .. .. ....... .. ............. .... I I ...........................P/code ..... .. .. ..... .. II Phone ..... ...... .. ............. ............. ..... .. I I I II . ___________ .I I AC N 002174478 01 /92 JANUARY 1992 39