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SERVICEMAN'S LOG Front little acorns, giant oak trees growThis month's story is something like the acorn that grew. It grew into several stories within the main story, each important in it's own right. How best to present these without losing the main thread presented something of a problem, so bear with me if I digress along the way. It all started with a routine phone call from a customer, complaining that his TV set was "on the blink". In more detail, the set involved was a National Panasonic TC-2690, a 63cm model using an M14 chassis. By "on the blink" he meant that it was completely dead, so I simply said "OK, bring it in". I left the delivery job to him for a good reason. This set is housed in a large, double-ended, lowboy cabinet and, since he has a utility, it was no problem for him to transport it. The M14 chassis is one of a whole series of"M" chassis; M9, MlD, Mll, etc, up to at least M16. I have dealt with several of these in the past but FRACARRO SAVES.•• ... more than $1 o. from every UHF television antenna you buy. Fracarro's 4PUN for better performance from weak UHF signals. Peter C. Lacey Services Pty. Ltd. P.O. Box 678 (74Fulton Rd.) Mount Eliza 3930 Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438 40 SILICON CHIP this was my first time for an M14. This chassis was produced between 1986 and 1988 and, apparently, was fitted to only a few models. Anyway, the set was duly delivered. When I switched it on, I realised that the description "completely dead" was not strictly true. It sounded more as if it was dying, or being tortured to death, judging by the way the power supply was screaming. Clearly, there was a serious overload of some kind, somewhere. It didn't take long.to confirm my initial suspicion; the horizontal output transistor, a 2SD1632, was shot. No problem; I had one in stock. This transistor comes in a plastic flat pack (TOP-66) encapsulation and is mounted on a heatsink on the side of the chassis. I fitted the replacement, taking care to use plenty of heatsink compound; an important precaution with such devices in heavy duty roles like this. Another precaution was to make a routine check of two potential problem areas , based on past experience of the "M" chassis in general. One is the soldered joints to the horizontal output transformer pins, while the other concerns the horizontal driver transformer pins. Faulty joints in these areas are a frequent cause of failure of the horizontal output transistor. But there were no problems here, so the set was plugged in and fired up. And everything worked fine. The only sour note - and it was nothing to do with the original failure -was that the picture tube seemed to be in rather poor condition; certainly much poorer than I would have expected for a set only about five years old. But beyond making a note to draw the owner's attention to this - for my own protection as much as anything - there was little I could do at this stage. I refitted the set in its cabinet and ran it in the workshop for a couple of days. I then attempted to contact the owner but his work takes him out of town from time to time. I eventually contacted his wife, who said she would pass the message on. In the end, it was something like 10 days before he eventually turned up, during which time I had let the set run all day and every day in the workshop. So that should have been that. But it wasn't, of course. Two days later he was on the phone complaining that the set - which he described in most colourful terms - was " ... on the blink again". There was no point in trying to determine whether it was displaying the same symptoms; it was "on the blink" and that was that. So I said "bring it in and I'll have another look at it". In fact, it was quite some time before he brought it back; close to a month, in fact. Yes, I know that sounds strange but one encounters some strange types in this game. I know he has another set and apparently pressure of other things had intervened. Anyway the set eventually turned up in the back of the ute and we manhandled it into the shop. And I realised that the lapse of time had not improved his opinion of the set, though I suspect that many of his thoughts were really meant for me, even though he stopped short of directing them at me. In short, he was not a very happy customer. Well, I wasn't very happy myself. A bouncing set is never a cause for celebration, if only because it usually .3'.:>" 0"3CIIV ~ VT o~oil-\.OIJGH l 'SUS'P~T "tr{A, MANY OF H\S TH0UGH"'f'S W~Re: 'REAt-L'-f tll\E"N'T" FOR ~e:. means more time spent on the job; time for which one can seldom charge. And when the customer is clearly in no mood to accept any explanation, the situation is doubly worrying. Anyway, at the first opportunity, I pulled the back off, plugged the set in and switched on. There were more tortured screams from the power supply and another shot horizontal output transistor. If I was not happy before, I was even less happy now. There was obviously no point in simply replacing the transistor. There was a "nasty" lurking somewhere in set, just waiting to pounce. Unless I could find it - and be sure I had found it - I dare not return the set to the customer. At a servicing level, I obviously had to restore the set to working order before I could start searching for the fault. So a new transistor was fitted and the set came good, just as it had done before. I let it run but not before I had set up the CRO to monitor the collector and base waveforms of the transistor, both of which appeared to be quite normal at this stage (CRO points 17 and 16 respectively in the manual). A diversion Having set things up as above, I had the urge to monitor everything for a while, in the hope that some clue might emerge. I knew it was long shot, because the set had previously behaved faultlessly for 10 days in the workshop. But I wanted some reason to hang around for an hour or so . Looking at the picture on the screen, I was reminded of the poor picture tube performance. But was it the picture tube or could it be a circuit fault? And if it was a circuit fault, might it possibly have some bearing on the main problem. This was long shot but stranger things have happened. A far more likely fault was low heater voltage, so that was the first thing to check. The heater is fed from a winding on the horizontal output transformer, via a 2.7Q 2W resistor. And being AC at 15,625Hz, it cannot be measured with a conventional meter. I use one of the Fluke 87 series meters, which will measure this quite accurately. And yes, there was something not quite right, But it wasn't a low voltage; instead, it was on the high side at 6.7V. Why was this so? And to what extent was this responsible for the premature aging of the tube? More on this latter point in a moment. I backtracked to the 2. 7Q resistor (R523) and checked it. It was spot on and that left me in something of a quandary. I had no idea why the voltage was high, only that it was wrong. But what could I do? Increasing the resistor to reduce the voltage would only make the performance worse. On the other hand, I could reduce the resistance and restore the tube performance but that would · further shorten its life. Finally, I decided to take the easy way out and leave the setup as it was. So just how significant was that over-voltage in the life of the tube? It doesn't sound much - a mere 6%, in fact - but all the indications are that it is quite serious. For a start, it must be remembered that the power dissipated by the heater increases at a greater rate than the voltage increase. If the heater resistance was constant, it would increase as the square of the heater voltage. The heater resistance is not constant, however, as it increases with temperature, but the tendency towards the square law remains. At a more practical level, there is plenty of workshop evidence to sup- VIDEO & T.V. SERVICE PERSONNEL TV & VIDEO FAULT LIBRARIES AVAILABLE AS PRINTED MANUALS $85 +P/H BOTH MANUALS T.V. & VIDEO $145 +P/H OR AS A PROGRAM FOR IBM COMPATIBLES OR AS AN APPLICATION PROGRAM FORD-BASE 111 PLUS FOR MORE INFO. CONTACT TECHNICAL APPLICATIONS PO BOX 137 KENMORE 4069 OR FAX/PHONE (07) 3781064 Now, I'm going to digress here, because this is what actually happened. MAY 1993 41 I 2SD1632? There wasn 't much in the driver circuit - just the driver transistor (Q500), the driver transformer (T500), two resistors (R515 and R526), and two capacitors (C525 and C527). SERVICEMAN'S LOG - CTD The penny drops r---1 - r--··- · 7 r-C-Vw~ - - · ---r-VV\, l. - - I ...J Q501 Q551 ZSM51 8Vp -p 20,,sec . Fig.1: this is the horizontal output stage of the M11B chassis in the National TC2656. The waveforms are similar to those of the M14 chassis as observed on the CRO but the indicated time scale could be misleading. port the claim that it is serious. I have another set on the bench at the moment with a sick tube, which is also running with 6.7V on the heater. The set is still within its 3-year warranty, with about six months to go. It will probably be replaced but two and a half years is a ridiculously short life for any tube. All of which has started me on a witch hunt. I have found at least half a dozen sets with varying degrees of over-voltage, ranging from 6.4V to the 6. 7V already quoted. That raises the question as to what kind of tolerance is acceptable. The best figure I can recall - and this is from the monochrome days was issued by a local tube manufacturer and was to the effect that increasing the heater voltage from 6.3V to 6.5V could reduce the tube life to repeat "to" - something like three or four years. If I remember correctly, this warn- ing was prompted by a particular model TV set which was gobbling up picture tubes like a kid let loose in a chocolate factory. Again this was due to a modest over-voltage but it was just too much for the heaters. So what has this to do with the original fault? Well, it enabled me to rule out any connection between it and the poor tube performance and that was important. It also enabled me to hang around the set long enough for something to show, which it did. I suddenly realised that the picture was shrinking horizontally and that the transistor base waveform, as shown on the CRO, was changing. The alarm bells started to ring. I'd seen that shrinking effect before somewhere and knew it meant that the transistor was about to be destroyed. I pulled the power plug and saved the transistor. More importantly, I now had a clue. What would cause a change in the waveform at the base of the Fig.2: the horizontal output stage of the M14CM chassis (National TC-2698) is similar to the M11B circuit but uses different transistors. The resistors and capacitors were easily checked and were spot on. That left only the transistor and the transformer. And this was when the penny - which must have been teetering on the edge - suddenly dropped. In my October 1990 notes, I described a similar horizontal shrinking problem in a National TC-2656 fitted with an Ml lB chassis. This chassis uses a similar driver circuit to the M14 and an identical driver transformer (designated TLH6476). After much travail, the fault was eventually traced to this transformer. Well, that was good enough for me. In a toss-up between the transformer and the transistor, history favoured the transformer. And, since I had one in stock and they are worth only a few dollars, I lost no time in fitting it. I set things up again, with the CRO as before, and let the set run. And I let it run all day and every day for the next week. Then, with no sign of trouble, I fitted it back in the cabinet and let it run for several more days. When it still didn't misbehave, I was cautiously optimistic that I had found the fault. I hadn 't forgotten the last episode, when it ran for more than 10 days before it failed but I had to call a halt somewhere. It was time to contact the owner. In fact there was a further delay; he didn't respond for some three weeks, during which time I flogged the set every minute that I could. By the time he did collect it, I was convinced that the fault really had been found. The only charge I made was for the parts replaced , with no charge for labour. And I emphasised that he should contact me immediately if there was further trouble. All that was several months ago and, at a recent check, all is well. So I consider the point proven. What's more, I sense that I am once again in his good books. He was quite lavish in his praise for the trouble I had taken, so all ended well. Another case But there is a good deal more than that to this story. I have had another 42 SILICON CHIP model TC-2698 with the same fault since then, this time with an M12 chassis. With the benefit of experience, this was an easy one but the nature of the transformer failure remains a mystery. I have taken the matter up with National's service department but they are equally mystified; my three cases are the only ones that have been reported. Even so, I find it hard to believe that my cases are the only ones that have occurred. I suspect that there may have been others but that the true cause has not been diagnosed. Just how many of these sets have bounced and how the situation has been handled, we shall probably never know. More to the point, perhaps, is the exact nature of the failure and how it damages the output transistor. What happens internally may well remain a mystery but I have been trying to make some waveform observations in an effort to explain why the transistor is destroyed. Unfortunately, these observations have been only fleeting. As I have learned from bitter experience, when the circuit goes into its horizontal Fig.3: these waveforms are from the M14 output stage, as displayed on a Hewlett-Packard dual-channel CRO. The upper waveform is from the base of Q501 (10V/div & 20µs/div), while the lower waveform is from the collector (500V/div). The arrows on the extreme right indicate zero reference. shrink mode, it's only seconds away from destroying the horizontal output transistor. And at around $20 a pop, I am not keen to sacrifice any more than I can help. So I have not been able to do more than glance at the CRO and register that the waveform has changed. There AUSTRALIAN MADE TV TEST EQUIPMENT 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 is no time to make any measurements or records. A normal waveform is shown on the accompanying Ml 1 circuit, which is essentially the same for the M14 circuit, and an actual CRO pattern printout from an M14 chassis is also shown. This was made with a Hewlett- ANTRIM TOROIDAL TRANSFORMERS 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 1· 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 ¼ .. '"".' :~ ~~--tf) ;' ,. From $19 exchange plus post & pack Cheque, Money Order, Visa, Bankcard or Mastercard Tli v~.•• TUN~~s J Phone (02) 774 1154 ···< 216 Canterbury Rd, Reve~bJ~.~:i~. 2212, Fax (02) 774 1154 • Now made in Australia. • Standard range 15VA co 2kVA. • Large ex-stock quantities available. • Competitive pricing available for OEMs & resellers. • Simply the best transformers all round. HARBUCH ELECTRONICS PTY LID 90 George Street, Hornsby NSW 2077. Telephone (02) 476 5854. Fax (02) 476 3231. MAY 1993 43 SERVICEMAN'S LOG - CTD Packard 2-channel digital CRO, model HP54600A, which has a facility (called ScopeLink) to feed the pattern into a computer and store it on disc. It can then be displayed on the computer screen or printed out. This is an extremely valuable facility which makes it possible to build up a store of waveforms that are not shown in a manual. In fact, the dual scan CRO pattern is more meaningful than the separate waveforms on the circuit, because it presents both waveforms on the same timebase, making it much easier to relate the instantaneous behaviour of one with the other. This pattern shows that the output transistor has a small positive voltage applied to it for most of the 64µs line period but is turned off for about 12µs at the beginning of the scan. This triggers the accompanying 900V collector pulse. The shape and amplitude of this pulse would be governed largely by the resonant characteristics of the horizontal output transformer. This background information may help us answer the inevitable questions at some time in the future. In the . meantime, we can only speculate as to why the transistor fails and a number of theories have been suggested by various colleagues. Most have subsequently been discarded but there is one that seems reasonably creditable. This puts the blame on the spikes on the base waveform, which accompany the 12µs turnoff period, particularly the longer one of the two. This has an amplitude of around 32V in the CRO waveform shown. This value should be within the base/emitter ratings of the output transistor but if the faulty transformer is generating much larger spikes, they may well exceed this rating. Significantly, the faulty transistors were broken down completely; base to emitter, base to collector and collector to emitter (the latter breakdown via the base). And that's about all the comment I can offer at the moment. I realise that it is not a complete answer. And not every output transistor failure in this circuit will be due to this cause; most will be normal internal failures. But, if you encounter two failures in rapid succession, be warned. End of story? Not quite. My discussion with the National service department raised another matter of concern regarding the 2SD1632 transistor. When I first de- ,~ v." 0'AtRS \ ""- . . \~ \ \ . ~------ ~ \; ;;,_ 11,ur.,~ ,~pv-,•c,~-- '---- - :Y\JST HOW W\AN.'( OF 1'-\E:.SE. SE,S 14A\JE: 8001'\CEC> ANt> \-\OW 1'\-\e:. SlTUAT\ONa \-\A~ ~E.E.N H~N'O\..E.D ~HA\-.\- ?ROBASL-'< NE.VER \<).\OW we:. 44 SILICON CHIP . scribed these transistor failures, the first question the technician asked was whether I had replaced the faulty unit with the original type. I assured him that I had used a type 2SD1632 but that didn't satisfy him. "Are you sure it's an original replacement?" I couldn't get the drift of his question immediately and asked him to clarify it. But I added that I would have bought it from either his own spare parts department or from another spare parts dealer whom we both know to be thoroughly reliable. Bodgie transistor It was only then that he realised that I wasn't as up to date as he thought I was. And he explained that there was a bodgie version of this transistor going around, apparently from an Asian manufacturer. The original 2SD1632 is made by Motorola and is stamped with its type number and the registered Motorola logo; ie, a stylised letter "M" in a rectangular frame . The bodgie device also carries the type number and th_e Motorola logo and appears identical to the genuine device. On that basis there is no way to pick the bodgie from the real thing. Fortunately, there is a way to pick it. There is a slight difference in the plastic moulding. With the package lying flat on the bench, check the space between the terminals. The bodgie device has a number of vertical ridges in this area, whereas the genuine device is quite smooth. A quick check of my own stocks revealed that I have only the genuine version. In fact, I have not seen a bodgie one but I am assured they do exist. In fact they have turned up in small pockets in Queensland and Victoria, as well as NSW. It is unlikely that a bodgie unit will be found in a set, unless the set has already been serviced in that section. But check existing stocks, or stocks being purchased. It is possible that some suppliers may be stocking them quite innocently. · And that really is the end of my trials and tribulations for this month. Unfortunately, I am not exactly the bearer of goodj:idmgs (I'm the kind who would have come to a sticky end in the good old "shoot the messenger" days) but at least readers should now be better informed about these chassis and the pitfalls to watch for. SC