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Double or quits Yes, this situation could easily toss you. How many things can go wrong with what should be a relatively straightforward service job? If you think you've been there, and know the answer, this story might well make you think again. This is not my story, but is from one of my regular correspondents, J.L., of Tasmania. More precisely, it is really two stories about the same set and, to further confuse the issue, also contains a story from my own bench covering a virtually identical problem. This is how J.L. tells it. J.L.'s first story A customer recently brought in two identical AW A sets; a rather rare occurence, but it was too much to hope that they would both have the same fault. They didn't. They were models C608; 48cm sets fitted with the Mitsubishi "G" chassis. The fault in the first set was so . simple that I can't remember what it was. The second 24,0V, 56 A one was a different story. The set was shut down and the 103V rail was up to 120V plus. There was no voltage on the collector of the horizontal output transistor, nor were there any subsidiary rails operating. Quite obviously, the horizontal stage was not working. As I tried to trace the course of the main rail towards the horizontal output transistor, I wondered why these sets didn't fail more often. They are a maze of plug and socket connections, any one of which can cause trouble. From the regulator board the main HT (103V} rail passes through an unreliable edge connector into the main board. It then goes into and out of a link in the deflection "'"'1,,'J; ~OOt> CA~AC\T'OP-, ?V,- ACROSS SHOUL-'t> S-rAV a>OL At-l.- t>A.V••• SILICON CHIP toss ya! yoke plug (DY}, back into the main board, via plug MF to the horizontal output transformer (Pin 3}, from pin 2 to PCB-FR via plug EA, through fuse F591 (1.6A}, then back to the main board via plug MA. Finally, it goes via plug MB (pin 1} to the collector of the horizontal output transistor. (See accompanying circuit, with path picked out in colour}. I was able to trace the HT voltage up to the fuse F591, but no further. The fuse was open circuit, but did not appear to be blown, just broken. I hoped it might be a spontaneous breakdown - but I should have known better. I fitted a new fuse and switched on. There was a rush of sound and I thought I heard a burst of EHT. But only for a second. The new fuse failed with only the tiniest flicker. There was obviously an overload, but only a small one - and that meant it was going to be hard to find. I started at the horizontal output transistor. This item unplugs easily for testing, but in this case it was a waste of time; the transistor was perfect. But I changed it anyway. Rather than blow another fuse, I connected a 1.5A thermal cutout in its place and tried again. I had sound and a trace of EHT for a couple of seconds before the cutout tripped. Which confirmed that the horizontal output transistor was intact and trying to function. The next items on the "output" side of the fuse are four flyback tuning capacitors, C531 to 534 inclusive. Two are rated at 2kV each and the other two at 630V. It's hard to measure leakage on high voltage caps with an ordinary multimeter so my trick is to put them across 240V AC and see if they get warm. A good capacitor should stay cool all day and these four passed the test. The last item on this board is --------------· ----------· J405 ZSC9l6 81< 12sc 1 5tA V.OUT 'tI I s,oE·' I ,...!181 :g-L .l ~ ~i~ l ---Pr-<. . •-•;~;;, 1':f'"'"""" ,w ~ -:. ' r-'V'\/\r 0~9l "4VT • lOS 115U 200 .,. UNIT-HV-RECT Fig. 1: horizontal output stage for the Mitsubishi C608 "G" chassis. The orange overprint shows the tortuous path of the main HT rail from the regulator hoard to the horizontal output transistor. boost diode D531. I checked it for leakage using 30V from the bench power supply and could detect no sign of leakage. It's not a perfect test, considering the voltage they have to withstand, but at least it's a guide. Tracking back from this board, via plug MB, I came to diode D571, a high voltage type that supplies 800V to the picture tube screens. This rail is bypassed by three 1µF, 450V electros in series. Considering that low voltage electros are unreliable beasts, I was prepared to find one, or all, leaky. I also suspected the diode. These capacitors and diode were as far out of reach as any windscreen wiper motor you have ever seen. They were tucked away on the main board, underneath the "tower" that supports the entire horizontal output stage, board, transformer, tripler, etc. The diode and two capacitors were on top of the board and one capacitor underneath. To make matters worse the underside of the board had been covered with some kind of goo which resembled anti-vibration compound used in cars. Before I could unsolder any joints I had to peel, scrape, and brush this goo away using Shellite and a stiff bristle brush. I eventually unsoldered the underside capacitor and freed the other two and the diode, but I could not get these latter out from under the "tower". This is held to the chassis with only three screws but also involves plugs EA, ED, MA, MB, and MF, plus the focus and EHT leads to the picture tube. Of course, after all that, all four items checked out perfectly. And those were the last items on the HT rail. So what was loading this section of rail? The only answer seemed to be the horizontal output transformer shorted turns? - or a faulty tripler. I disconnected the tripler from the transformer, then switched on while watching the fuse. It didn't blow, and there was a very healthy corona around the bare lead on the transformer overwind. So it was a dodgy tripler. This presented several problems. First, it was going to be an expensive repair. Second, I had been told that these units were no longer available. And, third, the UNIT-HVRECT , as Mitsubishi calls this assembly, incorporates the focus resistor chain and a very large Z00MQ bleed resistor. This makes any idea of a tripler substitution a very dicey one. I asked around among my colleagues whether anyone had ever fitted a substitute tripler to a "G" chassis. No one had, but they couldn't think of any reason why it wouldn't work. So I resolved to give it a try. I separated the rectifier unit from the focus and bleeder resistor unit, and found that there was room to mount a common tripler on the plastic casing of the focus unit. Everything went together nicely, but I deliberately left the bleeder resistor disconnected at this stage. Time enough when I knew it was going to work. Fortunately, I didn't fit the ultor MAY 1988 57 SERVICEMAN'S LOG srr ..... ,,.. L.-OC>Ks t\S -m()C.)(p'"' 9™tc: w,u_ g e:G,.O N\& ~"10"n\E,1\. '"ru"'1"-\ £. ..... cap to the tube. I brought it outside the cabinet and poked the high voltage probe into it. Then I switched on. To say that it worked would be the understatement of the year. The meter needle slammed hard against the 30kV stop and a loud zzzzz came from the ultor cap. I switched off hurridly. At this point it looked as though the bleed resistor was an essential part of the system. I carefully exposed the wire in the EHT lead, attached the resistor to it, then wiped a generous layer of silastic over the joint and left it to cure. I added another layer the next day and by the third day I was ready for another test. This was just as dramatic as the first. A bit slower, perhaps, but just as violent. Then I did what I should have done in the first place - looked more carefully at the schematic diagram of the "tripler". Had I done so I would have seen that the HV-RECT contains only three diodes. And three diodes don't make a tripler. I had replaced a voltage doubler with a tripler. No wonder I had excess voltage. So, with no HV-RECT units available, it looks as though the set 58 SILICON CHIP will become another junkie, useful only for whatcan be salvaged from it. I have yet to tell the customer the bad news. My story Well, that's the first part of J.L.'s story, and this would seem to be an appropriate time to take a break and consider what Yours Truly was doing during this time. In fact, my story would have started rather earlier than J.L.'s because I encountered several long waiting periods but, unfortunately, neither of us knew what the other was doing. My story is also about a "G" chassis, but in my case it was a model C609, 33cm set. And the symptoms were almost identical with those observed by J.L. But I was lucky. According to Murphy's Law, whichever end of a chain one elects to start from, it will always be the one furthest from the fault. Murhpy knows this and I know this. But on this occasion Murphy reasoned that I would reason, that Murphy would reason ... well you know what I mean; I beat Murphy at his own game. I took a punt on the transformer/tripler end of the string and picked it in one. It was the tripler - or what I believed to be a tripler at the time. I was lucky in another way too; nobody had told me anything about any shortage of triplers and, since the Mitsubishi Electric AW A spare parts division was only a 20c untimed phone call away, I simply dialled the appropriate number. When I told the bloke at the other end I was interested in a tripler for a "G" chassis, he didn't hesitate. "Yes we can supply a replacement high voltage unit for a "G" chassis exstock. It'll cost you about $70 plus tax. I can give you the exact figure in a moment.' ' I thanked him and said that would be near enough for now. I still had to get the customer's OK. The $70, plus Mr Keating's cut, would put the price over the $100 mark which, with my service fee, would make it a fairly expensive repair . Naturally, I didn't expect the customer to be too happy about the cost, but I didn't expect he would knock it back. But he did. He reckoned it wasn't worth it and that he'd rather put the money towards a new set. I didn't argue, of course - it was his decision. I simply told him that he could pick the set up any time, no charge. I put the set in the store room and more or less forgot about it for several weeks. Then, meeting the customer in the street, I reminded him that I still had his set. "Yeah, I must collect it sometime.' ' He didn't sound very convincing and, in fact , this episode, with variations, was repeated several times over the next couple of months until, eventually, it became obvious that he had no intention of collecting the set. Well, that suited me fine ; I had my own plans for it. More specifically, I needed another workshop monitor for VCR servicing and, if I could salvage this set for $100, it would be ideal. It's not a tripler So I placed an order and the new unit arrived in a couple of days. And it was only then that I realised that it was a substitute device, needing a moderate amount of mechanical modification to the chassis to accommodate it. And ~ \UF\S A 1\\00SHT ~ also that it was a doubler rather than a tripler. Thus prompted I looked at the circuit again and it became obvious that it could not possibly be a tripler. Anyway, the new unit was duly fitted without any serious problems, the set switched on, and up came a first class picture. It was as simple as that. So it was with some surprise that I read J.L.'s story, set out above, which came to hand shortly after. In particular I was surprised to learn that there was a supposed shortage of doublers for the "G" chassis, since I had just bought one. I realised that it could be a different type, being for a different size picture tube and, at the first opportunity, I checked with the Mitsubishi spare parts division. In fact, it is different, but I was assured that there were ample stocks of this model. I was on the point of writing to J.L. - in fact I had drafted a letter - when a sequel to his first story came to hand. This is how he tells it. J.L.'s second story When told the bad news about his set [that it was unlikely that I 1WSH 0,: SOONt) ANO "! \EA~O P. ~ OF ~ ' - • • could obtain the necessary part), the owner's response was quite unexpected. He had a special reason for wanting the set repaired and was prepared to spend up to half the cost of a new set to this end. I was to make any reasonable effort to get a new part, or a second hand one as a last resort. The first thing I did was borrow the doubler from the customer's other set and try it in the dud set. It came good immediately, the picture tube was in excellent condition, and a new doubler would make the set as good as new. I put in a call to Mitsubishi AW A spare parts in Sydney and, to my surprise, was told that, "Yes, 'G' cha ssis high voltage units are available" . There would be no change out of $100, but it was available, contrary to what I had been told earlier. I checked with the customer, then placed the order. When it arrived I was shocked to find that it was the wrong type; it was for the later "K" chassis, not the "G" chassis. I was about to throw a tantrum when I picked up what I had thought was an invoice and found that it was an instruction sheet on how to fit this doubler to a "G" chassis. There were 11 separate instructions on the sheet, mainly involving drilling various mounting holes and re-arranging the focus and earth wiring. It is a rather awkward arrangement with the focus control tucked away inside the set, but it does fit. Volts but no picture At switch-on the new doubler worked like a charm. There was EHT and sound and I waited for a picture to appear. But after two minutes there was no more than a few coloured specks , and occasional picture highlights. From these prelimina ry, clues I deduced that the problem might be around the video output stages. The meter showed 140V on the picture tube cathodes, some 30V higher than the figure on the circuit diagram. This same voltage appeared on the output transistors ' collectors and was very close to the B3 rail voltage which supplies them, suggesting that they were not turned on. The voltages at the base and emitter of these three sta ges were also higher than normal but, significantly, were the same for base and emitter. The absence of M AY 1988 59 the normal 0.6V difference between of electrolytics and resistors which these elements supported the idea were marginally off, but achieved that the transistors were not turned nothing. In desparation I rang a colon. league who I knew had a good "G" The base voltages for these three chassis and arranged to borrow the stages are derived from the colour video board from it. difference stages - Q619, 620 & I was shocked to find that this 621 - on the chroma board. These board was only a little better. There are driven, in turn, via coupling was now a picture outline and betcapacitors C621, 622 & 623 from ter highlights, but it was still far pins 1, 2 & 4 of the chroma IC from right. (IC106). The three coupling capacitors were in good condition, so the wrong voltage was arising 411: after the IC. This left Q619, 620 & 621, plus the DC clamper as possible suspects. The emitter voltages for the three \ stages are applied via a direct coupled network that goes back ' almost to the video detector. Included in this network is the second ' video amplifier (Q202), the ~ . \ luminance delay line (D1202), the video buffer (Q206), and the third video amplifier (Q203). The signal then passes via switch S20~ to the ~•--'---"'emitters of the output transistors. ~ .. Also associated with this net- '----__,.._..._. ► work is an automatic beam limiter t W~ Pl\Q\M' ,O '™tM ~ stage, QZ04, which funtions by if:\tJThUM Wli\£1\J RXJt\lt) .... varying the bias on Q203, in response to a voltage derived from 'M& INVO\CE WAS ~N pin 6 of the horizontal output , NS°TRvC1'"cO~ SMEJ:..'"\ ••••· transformer (T591). This varies the amplification of Q204, the bias on the picture tube cathodes, and the Comparing both boards revealed diode and a larger capacitor. I beam current. only one significant difference: the altered my board accordingly and Strangely, it was only the emitter network R208 and C203, between was rewarded with a similar order voltages on the three video output of improvement. the video buff er and the third video transistors which were significantMore importantly, the modificaly wrong. The colour difference out- amp, had been changed in my colleague's board to include an extra tion had somehow changed the puts were correct, as were the voltage levels around the the beam voltages on the bases of the three limiter, Q204. This suggested that it output transistors. Again, in the was a beam limiter fault but, also, TETIA CORNER video chain, all voltages were close that it might not be on the video to correct and the oscilloscope Philips K9 chassis board. This would account for both showed that all seemed to be proSymptom: One colour missing boards seeming to be faulty when cessing video normally. completely. Output stage voltages they had worked previously. It was only when I checked the are normal, as are the grid and Control voltage for the beam waveforms more carefully that I cathode voltages on the picture limiter is generated across R272 realised there was something · tube base board . The G2 voltage and R274 in parallel, these being wrong. The level at the delay line for the missing colour will be low, connected to pin 6 of the horizontal input was about normal at 3V p-p, although there is no measureable output transformer and diode D595 but at the output of the third video leakage on the rail. and C594 between pin 6 and amplifier it was les.s than half the Cure: the beam switch on the conchassis. Diode D272 is suitably ..6.5V p-p shown in the diagrams. vergence board has developed a biased to prevent this voltage leakage path at high voltage. The reaching the limiter transistor until easiest repair is to remove the Lost gain a certain critical level is exceeded. switch and bridge the appropriate I went right over this network, but Clearly, the video amplifier had pads. The switch is not essential to found nothing wrong. lost a lot of gain. Yet I could find the operation of the set. In desperation I tried to brute nothing wrong. I changed a number ,·~ __ _ _L.J_\~-- ,·.. \ ' . ' - ni~T 60 SILICON CHIP r force a change in voltage levels from the limiter transistor onwards. To turn the third video amplifier on harder I patched a 22kf! resistor from base to chassis - and was rewarded with an immediate increase in screen brightness. But the picture was still so washed out as to be virtually useless. This agreed with the very low amplitude video signal that I had noted on the CRO earlier, and convinced me that it was the beam limiter system that was falsely biasing the video stage. There aren't many components in the beam limiter circuit. I checked them all and found nothing wrong. Finally, I came to a conclusion that I should have reached hours before. Near the beginning of this story I mentioned that the set had worked perfectly with a borrowed doubler. Nothing had changed since then, so it should still work perfectly with an original "G" chassis doubler. Therefore the problem must due to the new "K" chassis doubler. Perhaps this unit is not a direct substitute for the original. Perhaps there is something different about it which has to be allowed for. Granted, the modification notes are purely mechanical, with no mention of electrical changes. But this would not be the first time that essential information has been omitted from such notes. The service manual decription of the beam limiter action suggests that the key voltage is that at the junction of R272/274 and diode D272. The limiter bias on Q204 varies with this voltage, so I decided to change it and see what happened. The easiest way to do this was to remove one of the two resistors. This would increase the resistance and indicate in which direction the change should go. With R272 out of circuit the picture became much darker, so this was the wrong way. R272 is 180kf! so I replaced it with lO0kf! and achieved an immediate improvement. I continued reducing R272, each time getting a brighter picture until finally, at 33kf!, I had a perfect picture with a small reserve in the brightness control range. It is not my usual practice to re- design a set while I am repairing it. I normally acknowledge that the original designer knew better than I what value should be assigned to each component. Yet there are times, like this, when what looks like a communications breakdown leaves me wondering just who is responsible for advising of changes in circuit values. In this case it appears that the replacement doubler has significantly different characteristics, requiring a change in external circuit values to achieve correct operation. The trouble is, the hours it took me to solve this problem are not chargeable to anyone. I have to bear the cost and hope that a similar job comes to light in the future to help me recover some of the losses. Conclusion Well, that's what happened to J.L., and it's not a pretty story. Not to put too fine a point on it, somebody goofed and, in my book, it wasn't J.L. Remember that J.L. had been able to test the chassis with a known good doubler and establish- ed that there was nothing else at fault. So what went wrong? J.L. suggests that Mitsubishi neglected to include all the information covering the substitution; ie, the information was available, but not enclosed. My bet is that the information was not enclosed because it didn't exist. Could it be that someone had established that the "K" type doubler could be used with the "G" chassis, but only on the basis of a trial in one, or perhaps two, "G" chassis types? While all " G" chassis are basically the same there are minor differences according to the size and type of picture tube used. As well as whole range of picture tube sizes, some sets use the delta gun tubes, others the in-line gun version, and EHT and other element voltages are provided to suit. So was the model C608 the odd one out, unable to accommodate a slightly different EHT voltage and/or loading created by the substitute doubler? I don't know, but I'd like to find someone who does. ·~ MAY 1988 61