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SERVICEMAN'S LOG They came in two by two Some of my jobs arrived in “twos” this month or at least that seemed to be the pattern. There was also a reminder of the problems than can be left by someone who has gone ahead. And of course, there always seems to be at least one unhappy customer. I was trapped into fixing Mrs Cartland’s Philips CR635 TV set. This set is over 10 years old and of course I am familiar with the old Australian-made KL9-A3 (and KT3A-3) chassis which, in my opinion, were very well built. And I say “trapped” because I try to steer clear of such old sets but she laid on the flat­tery and praise – tactics to which I’m quite susceptible. There was no sound or picture, although it was hiccuping or motor­ boating, the sound coming from the speaker. However, it didn’t seem to be quite the same kind of motorboating sound that comes from a faulty audio amplifier circuit. Instead, I felt that it was more likely to be a faulty tripler or horizon­tal output transformer and so I agreed to have a look. First, I disconnected the tripler (1570) but it made no difference. I then removed the horizontal deflection plug, which also links the 140V rail to the horizontal output stage (plug/ socket 4M5/2M5) and measured test point M2 (marked on the board but not the circuit) to check the voltage there. This voltage had previously been pulsating but was now steady. Next, I shorted the base and emitter leads of the horizontal output transistor (7562). There was still voltage (335V no load condition) but again, the pulsating stopped. Most of the noise was coming from the loudspeaker and by turning down the bass and treble controls this could be stopped, but the horizontal output stage could still be heard hiccuping or pulsating. All I had proved was that there were no DC shorts on the main 18  Silicon Chip supply rail but there could be a problem within the inductive load of the horizontal output transformer circuit. It was at this stage I realised that the set had to go to the workshop. This was a nuisance but Eileen’s praise for my techni­cal skills somehow made it all worthwhile. To settle any lingering doubts, I fitted a new horizontal output transformer (5564) which made no difference. I also dis­ connected the deflection yoke without removing the voltage rail links and ran it very quickly to see if there was any change (I didn’t want to incur screen burn from the dot) but there wasn’t. Having eliminated these two major items, I also unplugged the CRT socket in case there were any shorts inside the tube. In order to check the main HT rail, I connected a dummy load (consisting of a 100W globe) from the collector of the horizontal output transistor to chassis and again shorted its base to emitter. This time the voltage read a steady 141V on M2, which was what I was hoping for. I then checked R3561, the limit­ing resistor to the base of the horizontal output transistor, as well as all the components in that circuit. I was beginning to think that there was a problem on the secondary side Sets Covered This Month •  Philips CR635 TV set •  Mitsubishi HS621 VCR •  Mitsubishi HS-M60 VCR •  National M15L TV set of the horizontal output transformer (5564) and so I decided to disconnect each pin, one by one, and check the effect. Disconnecting pin 18 restored the HT without it pulsat­ ing. This pin supplies three 32V rails and the significant one was 32b, to pin 10 of the sync IC (7375, TDA2577), which should be at 12.3V. By disconnecting this one leg I could get sound and hori­zontal deflection but no vertical deflection. I now felt sure that this was where the trouble was (ie, around IC7375) and I spent a lot of time trying to find a fault in this IC or the circuits connected to it. As far as I could work out, its pin 10 was a secondary supply that was switched on following the start-up voltage (10.6V) being applied to pin 16. But I could not see what effect it could have had on the vertical timebase. Eventually, I forced myself to stop thinking about IC7375. I had already disconnected all of its pins to no avail. So the basic fault was that the set was unable to deliver a stable 140V rail and this in turn affected the 26V rail to the audio amplifier, which oscillated with the tone controls turned up. I followed the waveforms from pin 11 through to the chopper transistor 7463. I also disconnected D6317 and R3384D – a current limiter – to prevent any red herrings coming from that quarter (this power supply runs at 15.625kHz). It was then that I noticed that the CRO waveform on the base of transistor 7322 (ie, out of IC7375) wasn’t exactly as shown on the circuit, in diagram 19. And in the process, I also found that with the dummy load connected, the 140V rail could be adjusted with R3325, which further implied that all this circui­ try was working. I checked transistor 7322 and D6322, as well as D6323, D6325, R3317 and a host of other parts but could find nothing wrong. To cut a long story short, it is always the last component checked that is the culprit! I just wish there wasn’t quite such a long queue. Anyway, the culprit was C2317, a 330pf ceramic decoupling capacitor to the base of transistor 7322. It had gone leaky and a new one restored everything. Apparently, this leaky capacitor was applying forward bias to transistor 7322, switching it hard on. The exact sequence of events following that is quite complicated. Suffice it to say, that was it. And I think it is extremely unlikely I will ever see this particular fault again. Eileen, although happy to have her set back, really had no idea of the angst it had caused me. I think I deserve whatever praise she gave me! Two Mitsubishis I recently had two Mitsubishi video recorders arrive in the workshop, both of which came via other service departments. The first was a 1996 HS621 which employs a U deck. Mr Ford was fairly annoyed; he insisted that he had hardly used it since it was new. His complaint was that there was only a blue screen on playback. Perhaps I should explain what the blue screen condition means. This is a purely cosmetic function; a visual and audible muting system. In older sets, a blank channel produces a bright screen made up of multiple white dots (snow) and a blast of noise from the speaker. This can be objectionable. To overcome this, modern sets are normally programmed to present a blue screen when they encounter a blank channel or a very weak or varying one. But the control system can be used to turn the blue off, if the viewer elects to persist with a poor picture. In addition, a blue screen can also indicate a fault in the signal chain. In this case, I thought the fault could be a simple case of dirty heads muting the picture and so I used the remote control to turn off the blue screen, using its menu system. That done, I could now see what was really happening. The picture was flicker­ ing, with tracking bars moving fast down the screen. The tracking control didn’t work. What’s more, when I put the tape in, I noticed that I had some difficulty in making it go all the way into the machine. With the cover removed, I could see the cassette as it was low­ ered onto the deck. Unfortunately, the take-up arm did not always pick up the tape and wrap it around the ACE head, resulting in no sound, no control pulses and a jumping picture. Because it was intermittent, it was difficult to understand why the takeup arm was missing the tape. Initially, I suspected that the loading gears were out of alignment but I was also becoming more aware of the symptom the owner hadn’t mentioned – the difficulty the machine had in accepting tapes, especially on the lefthand side where it often jammed completely. On the earlier F decks, the cassette tray holder didn’t always hold the tape cassette firmly, resulting in the tape not always going in and staying in. However, because I’m not at all familiar with the mechanics of this particular machine, I eventu­ally had to seek help from the Technical Support people at Mitsu­bishi, who put me on the right track immediately. And “track” was the operative word. The tray slides in along moulded plastic rails on either side but somehow, over the years, these had fractured, causing the runners to run slow over the bumps – despite being lubricated with a pink grease. The result was that the tray with the tape cassette arrived too late at the bottom of the action and the take-up arm had January 2000  19 Serviceman’s Log – continued to pay for my work but I decided it wasn’t worth pushing my luck. The Mitsubishi HS-M60 already left its station! Unfortunately, there was no easy solution to this problem. The plastic cannot be replaced and the two side rails are an integral part of the U-deck assembly. The only approach is to change the deck main plate assembly. The good thing is that the chassis is quite cheap (about $20) but the labour involved in fitting it would be prohibitive and I certainly didn’t have all the jigs and adjusting gauges required to set it up. Mr Ford was furious. He claimed that he had played only about five tapes since he’d bought the machine and he had chosen Mitsubishi because he thought it was a reliable brand. I hastened to assure him that the latter is true but I couldn’t help him with his assertion that he had played only five tapes – a claim which I felt was rather far-fetched. In an attempt to find someone responsible for his predica­ment, he then suggested that a previous repair, which was done under warranty, had not been performed properly. I invited 20  Silicon Chip him to fax me the account and I would chase it up. The copy of the account duly arrived and this made it easy to work out what had happened. He had forced a tape in the wrong way around and broken the bottom cassette housing unit, which had nothing to do with the rails. The repair had been carried out two years ago and could not have been responsible the present fault. My theory is that the VCR had probably been kept near a window where it was directly exposed to the hot summer sun and temperature variations has caused the plastic to crack. In short, his problems were all of his own making. However, Mr Ford was in no mood to accept any culpability or explanation and stamped out of the shop cursing everything in sight. I had suggested that he take it to Mitsubishi who would obviously be the best and most experienced people to swap the decks but somehow I think he will just leave it for the council clean-up. I was somewhat miffed that he didn’t bother The second Mitsubishi was an even earlier model, an HS-M60 J deck from about 1994. This machine had also been to another service centre, who regularly serviced a local club’s Mit­ subishi video recorders. They had given up on this particular machine and returned it still faulty. It too was producing only a blue screen but in this case, if one wiring harness plug (GR/MR) was unplugged from the head amplifier, the picture would be restored, without colour. This was an intriguing fault which, once again, I thought would have a simple solution. But I should have been cautioned by the fact that someone else had had a go and abandoned it – and I didn’t even know what the original complaint was. I decided to tackle the no colour problem first. And almost immediately I noticed that the 4.433619MHz crystal on IC2AO had long pigtails and had obviously been replaced. This was my first suspect but equally obviously, someone had already tried that. Anyway, I measured the waveform on pin 18 of IC2AO on the YC/CG module and found that I had plenty of reference oscillation at what looked like the correct frequency. I also checked all the other waveforms and voltages marked on the circuit dia­ gram. Almost all were spot on, the exceptions being pins 5 & 15 on record. These were so weak as to be almost non-existent. I then spent a lot of time checking this board and replaced all the ICs, to no avail. I felt a bit stupid about this later when I found out that the whole board was available for not much more cost and in the end that is what I did. This new board fixed the colour problem and then, because the board doesn’t look very complicated, I thought that I should be able to fix the old board by comparing the two. Fruitless and totally uneconomic exercises like this are one reason why I seem to be perpetually poor but I really wanted to know why one board worked and the other didn’t. This indulgent luxury was eventually satisfied when, more out of desperation than anything alse, I swapped the crystals over and transferred the fault. Well, obviously both crystals work­ed so what was the dif­ference? Elementary, my dear Watson; one was nearly spot on fre­quency but the other was on 4.432185MHz, which was 1.434kHz out. This represents an error of only 0.032% but is marginal for the capture range of the APC discriminator circuit, to enable it to pull in. In fact, some authorities quote 1kHz as the limit. Because there is no tuning capacitor, if the crystal is not within tolerance, one has to choose one that is, so that the discriminator will lock. In this case, I had to try two or three before I found one that would work. I kept the old crystals; they may work in other circuits. Next, I concentrated on the video muting problem. I disen­ gaged the blue screen via the remote control on-screen menu so that (hopefully) I could look at the real situation. Alas, the picture was still blank but now black. I traced the video mute circuit and could find no fault with it. There are two wires running from IC301 on the head amplifi­er board to the motherboard, via connector GR/ MR. One wire, from pin 8 to GR pin 3, also connects to chassis via RJ306 (a wire link). When this was removed, the picture was fine. Or rather, the PC board shows a position where RJ306 could be added but RJ306 is not actually shown on this particular circuit. If it was fitted, pin 8 and the connection on the moth­ erboard would connect to chassis. If it isn’t fitted, the mother­board termination would be above chassis and connected directly to pin 8 on IC301. So what did all this mean? I wasted more time chasing this and trying to comprehend the circuit and how it was supposed to work. Finally, I gave up and contacted the long suffering techni­cal staff at Mit­subishi. If I had looked more carefully at the circuit I would have noticed a small table alongside each one, showing the differences between each model. It was now fairly obvious that the deck had been swapped with that from an HS-M50. And the HS-M50 chassis uses an RJ306 but the HS-60 does not. All I had to do was convert it back to an HS-M60 to make it all work properly again. It is comforting to know that some organisations, such as Mitsubishi, still have professionals who know their product well and can offer advice when needed. TO GR3 A couple of Panasonics The 1990-1992 M15 series of Panasonic TV sets have been excellent for reliability. I had only seen a few of these sets until quite recently, when suddenly I had quite a few to repair. Perhaps it’s because they are beginning to reach their use-by dates or perhaps it’s just fate – who knows? Anyway, the M15 series comes in two basic packages: (1) the M15L for sets up to 51cm and (2) the M15D for larger screens. Both are an improvement on the M14 series except for access, the M15D in particular being quite difficult. Usually this chassis has a tall vertical small signal board on the lefthand side which, along with the short lead wiring harness, prevents access to the power and deflection board. Because of the size and weight of the larger screen sets, I have been forced to repair them on site. Most have had short circuit horizontal output transistors but I cannot be sure what causes this, as I have never had a recall after changing the 2SC1175 (Q501). I always try, where possible, to resolder any suspect joints and replace C816, a 47µF 50V electrolytic, as it has often dried out from the heat. I also had one such house call on a TC2670V, where D620 had gone short circuit, causing the same symptoms. However, the main dramas have all concerned the M15L chassis, which fortunately are easily manhandled into the workshop. The easy ones involved replacing the regulator IC (IC801, STR50213) and the horizontal output transistor (2SD154LB), plus the fuse or R841 (a 4.7Ω resistor) where necessary. It was Mr Rodrigue’s set that gave the most grief. It was not quite dead in that it was making a screaming noise but there was no picture. The main HT rail, normally at 113V, was down to only a few volts at TPE1, out of the power supply. I replaced the horizontal output transistor Q501 (2SD­1541­LB) and IC801 in the power supply but this made no difference. It was very hard to determine where the noise was coming from but I assumed it was T801, the chopper transformer. This made me think it was an overload problem on the power supply. I disconnected Q834 and hung additional electros across C808 and Fig.1: this circuit section from the Mitsubishi HS-M60 J VCR shows IC301 on the head-amp/audio board. Note the terminals for component RJ306 (off pin 8). C847 before eventually realising that I was looking in the wrong area. The jungle IC (IC601, AN5601K) is fed from the 113V rail via R519, a 6.8kΩ 5W resistor supplying 8.5V to pin 42. Without this voltage, the set closes down and there is no horizontal oscillator or indeed anything. Also connected to pin 42, via R536, is a safety shutdown circuit consisting of Q451 (which monitors the vertical output deflection), Q504 and Q503 (which monitors the beam current and secondaries of the horizontal output transformer and vertical deflection). This shutdown circuit was obviously denying voltage to IC601. To test this theory, I desoldered R536 and isolated the shutdown circuit and suddenly things started to happen. We had sound but no vertical deflection. The vertical output IC (IC401) was the likely suspect and a new IC fixed every­thing – until I resoldered R536 that is, after which the set started screaming again. So the fault was in the shutdown circuit but where? Voltage measurements around the shutdown circuit, starting at Q503 and back-tracking from there, indicated that all was not well around Q451, which monitors the vertical January 2000  21 IC401 Fig.2: the safety shutdown circuit in the Panasonic M15L TV receiver is based on transistors Q503, Q504 & Q451. Q451 monitors the vertical deflect­ ion output via resistor R411. output. This transistor moni­tors the current flow in the 24V rail to pin 7 (Vcc) of the vertical output IC (IC401). It does this by means of a 1.2Ω resistor (R411), inserted in this rail. The base/ emitter junction of Q451 is connected across this resistor, with the base to the negative side. This establishes the operating conditions for Q451 which, under normal conditions, would be turned off. Only when the voltage across R411 rises above normal would Q451 turn on and initiate a shutdown sequence. However, IC401 appeared to be functioning normally, in that the set was working perfectly OK without the shutdown circuit. So either IC401 was drawing excess current – which seem­ed to be ruled out – or R411 had gone high. And the latter assumption proved to be correct; R411 had been damaged by the previous faulty IC401. The only flaw in this explanation is 22  Silicon Chip to query why, if R411 in the 24V rail was high, was IC401 still functioning normally? The answer is that it had gone only a little bit high, as they say in the classics – high enough to upset the shutdown system but not high enough to upset IC401. A new R411 allowed R536 to be reconnected and the set to remain fully operational. I fitted it back into the cabinet, replaced the back, put it on the soak bench and switched on. To my extreme annoyance, the shutdown circuit appeared to be falsely activating again. Feeling rather miffed, I put it back on the workbench and took the back off again, whereupon the set resumed working. A faulty back? I hoped so but it was more serious than that. It only required the chassis to be tapped for it to switch itself off and gradually I found the sensitive spot to be around IC401. I checked whether I had sol- dered the IC correctly and it was OK but there were suspect joints on the heatsink, which is also the chassis return for the safety circuit. I resoldered those but it was still intermittent when tapped. Again, this spot was still around IC401. I examined the area very carefully and eventually found that the chassis end of the copper track to R531 (22kΩ) from Q504’s base was fractured. I repaired this and put it back onto the soak bench where it worked until the next day before failing yet again. Once again I delved into it and went through the previous procedure, including disconnecting R536 and measuring each tran­sistor to find which was switching what on. The problem this time was that it would take 10 minutes or so before it occurred, suggesting a heat-sensitive component. To accelerate this, I used a hairdryer to make the fault happen and freezer to stop it. This time, it was Q503 that was being switched on falsely but not via Q504’s base. Possible suspects were Q504 being leaky, D520, D502, D522, R529 and the horizontal output transformer. The picture was still perfect and there was no sign of distress anywhere else. Gradually, I pinpointed it to D502 (MA4360), a zener diode to the base of Q503. Heating and freezing this would switch on Q503. Now I am not averse to ordering the correct zener from the Panasonic spare parts agents but this time I didn’t want to wait. So what value is an MA4360? Fortunately, I have some notes on other Panasonic nomenclature and the code for this series of zener diodes works like this: the first significant digit is the power rating – 2 = 1W; 3 = 150mW; 4 = 400mW; and 5 = 500mW. So in this case, it is a 400mW zener diode. The last three digits represent the operating voltage, with the decimal point going between the last two digits. This gives a value of 36.0V for D502. A new one allowed me to leave the set on for more than 10 minutes. In fact, two weeks later, Mr Rodrigue finally had his set back. These have been just two of several stories out this ser­ ies. Tomorrow I have to go to see a little old lady’s TC1400A set. She is complaining there is “an echo in the sound”! I do hope there isn’t another similar problem SC involved.