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Sound reasons for confusion Last month I commented on the rarity of sound faults in TV equipment, and compared them with the old fashioned radio sound faults. Well, hardly was the ink dry on the papyrus - and before I'd had time to cut myself a new quill pen - than another one landed on the bench. And it was as tricky as they come. The device involved was a General colour TV set, model GC187 and the complaint was simple enough; no sound of any kind. I thought the solution would be equally simple; which just goes to show how naive one can be. The set belonged to a local motel for which I have the contract for the whole TV installation. This particular set was one of several devices which had failed after a particularly violent thunderstorm. During the storm there was a lightning strike which probably hit the antenna directly. Anyway, it knocked out the distribution amplifier, put several other TV sets out of action, and cooked a microwave oven. All in all, quite a mess. Most of the repairs were quite straightforward, though somewhat ,s rr .... SAt>? WEL-l-, "flie:. FLOAAL- AR~ANGEME.tJ1""'S HAD 1-r. .. expensive and not particularly newsworthy. As things worked out, the GC187 was last on the list. Knowing the capriciousness of lightning strikes I was quite prepared to find the fault in almost any part involved with the sound channel; ie, it could be anywhere between the antenna and the speaker. At a practical level I started at the input to the audio amplifier. This uses three transistors; QB0l, Q802 and Q803. Q802 and 803 form a complementary symmetry output pair, driven by QB0l; a perfectly conventional set-up. Feeding a suitable level of audio signal into the base of Q801 produced a healthy output from the speaker; nothing wrong there. Back-tracking from this point brings us to pin 2 of IC2 (uPC1382C), the sound IF and audio detector system. This amplifies and demodulates the 5.5MHz sound carrier applied to pin 14. This signal comes from pin 12 of ICl (T A7607 AP); the video IF amplifier and detector. And signal into this is via pin 1 from the first IF amplifier, Ql and the SAW filter, FLl. Considering that the set was producing a first class picture, I felt that the fault was most likely to be closer to IC2 than the front end. Colour bar signal ''"--t nus PA~1"1CUl-AR S6,,.. w~s ONE. ~•c.~ HAO FAll..EO APfER A PA~i,cu1..~Rl.'t V\Ot..EWr' 'O'\UN~S1'"01'M ••• 28 SILICON CHIP I fed a colour bar signal into the set and checked the output at pin 12 of ICl. As far as I could determine all was well at this point, so I moved on to IC2 and gave it a full voltage check. Again I found nothing suspicious, all values being acceptably within tolerances. Next I used the CRO in an effort to check the 5.5MHz level at pin 14 of IC2. Results were inconclusive except that the level seemed to be well down on what I expected. So what could cause this? ICl was obviously delivering adequate video I UNtK .,. VHF/UHF CA-335S TP15 IF A [IBJ JJK TA 760 VR1 SK(B l VB UI vsw E TU + 15Y ... ,,.. :HAN NEL ;ELECTOR !OARD R672 100K ED C673 o.o,, l CUI 2200P I IC670 uPC574J ca10 1200P .. 05 y IF SOUND .,- - BOARD ~ •uo10 OUT - + ~AIN BOARD - ISV CEC:ill 0670 EQAOI -15Rl ~ 'f '" C661 _ 0.01 - C672 -1 J,. ,oisev1F rl,- R667 1a1< ( 1/2) Relevant portion of the GC187 circuit. The audio stage is at bottom left, fed from IC2 to its right, which in turn is fed from IC1 at top right. The video line goes off to the right from pin 12 of IC1. and chrominance (colour) signal to the rest of the set. Only the 5.5MHz component seeming to be missing. So was something in ICl, or even earlier, upsetting the response curve to the point where the sound plateau had been lost? I didn't think this was very likely since the chrominance carrier was obviously intact. Still it had to be considered. I had a spare IC2 on hand, and it was an easy job to fit it, so I went ahead. Well, as Thomas Edison would have said, I now knew one more thing that wasn't wrong, because it made no difference. So where to now? Still thinking in terms of front-end distortion of the response curve, my thoughts turned to the SAW (surface acoustic wave) filter. It would not be the first time I had encountered a SAW filter which upset the response curve, though the effect is usually less subtle than this. Still, there is always a first time. Again it wasn't a difficult point to prove. I had a spare filter on hand and it took only a few moments to fit it. And that was another victory for the Edison philosophy; I now knew of two things that weren't wrong. More importantly, it now seemed obvious that the fault was not at the front end or in ICl, but somewhere from pin 12 of this IC onwards. I made some attempt to observe the 5.5MHz level at pin 12, in order to compare it with that at pin 14 of IC2, but results were inconclusive. There was some loss between the two but this could have been normal, considering that there were several components in the signal path. On the other hand, one of these could be faulty. Fortunately, most were easy enough to test and a lk0 resistor, a 39pF capacitor and an 18µH choke all proved to be OK. That left only the 5.5MHz filter, CF2. This presented a problem, in that I had no spare on hand to substitute. I could have ordered one but that would mean a few days' delay, and I was anxious to get on with the job. This prompted a rather wild idea. There was another filter, CF3, in this part of the circuit (pins 9 and 10) and by all that I could determine the two were identical. What would happen if I swapped them over? Granted, I could hardly expect it to solve the problem. If CF2 was faulty I would be simply shifting the fault from one place to another. But I reasoned that it would at least change the symptoms and help confirm whether or not CF2 was faulty. It was a simple operation and in a few minutes I was ready for the big test. But I was hardly ready for the result. The set came on with a perfect picture as before but also with full sound; not a thing wrong with it. But what did it mean? Frankly, I wasn't sure. The most likely possibility seemed to be that (a) CF2 was faulty and (b) the filter in the CF3 position was more of a technical nicety than a necessity, such that a faulty filter in this position wasn't all that serious. But even as I formed the theory I had AUGUST 1988 29 SERVICEMAN'S LOG doubts. I couldn't shake off the feeling that it was just too glib an explanation. Dead silence I left the set running while I pondered over all this, and tried to decide whether I should order up a new filter anyway. This went on for about half an hour, while I pottered around tidying up the bench and attending to routine matters, then suddenly there was dead silence; the fault was back just as it was before. So what did this mean? In one sense I was almost relieved. It meant that the rather nebulous and unsatisfactory situation I had been trying to resolve was a furphy. There was now a chance that the fault could be nailed more positively, and with less chance of a rebound. But I still had to nail it; and that meant starting all over again. Considering what had already been done, I felt that about all that was left was a more or less brute force approach; to check every component around IC2, for I was convinc- ed that it was in this area that the trouble lay. In fact, there weren't all that many components involved, so it wasn't such a big deal. The few resistors were easily checked, in situ, and all were within specs. Then there were the capacitors. Prime suspects were three low-value electrolytics; C23, C24 and C27. These were lifted at one end and measured for capacitance and leakage but again, all appeared well. That left only a few ceramic types and statistically, these seemed unlikely suspects; ceramic capacitors in Japanese sets are extremely reliable. I started by removing each capacitor and checking it on the capacitance meter. But then, since the fault was obviously inter- r SPURIOUS IY\A&NE-,l'SIV\ ~ SHAD0WMA59 30 SILICON CHIP · mittent, I decided to replace each one anyway, regardless of how it tested, just to make sure. This achieved nothing initially, the set remaining dead as each one was replaced. Finally, there was only one left - C30, a .OlµF from pin 13 to chassis. It tested OK along with all the rest but when I fitted a new one in this position, the sound came forth at full voloume. Had I really found it? I re-fitted the original capacitor and again I had full sound, so I simply let the set run. This time I was lucky. After about 15 minutes the set went dead again. I had hoped for such situation and was ready for it. I bridged C30 with the other .OlµF whereupon the sound came good. More importantly, when I took it away the sound vanished. So that was it. I don't think there is anything more satisfying than pinning down a fault in circumstances like this. If any more proof was needed it emerged when I pulled the capacitor out and put it on the checker again; this time it showed zero capacitance. Which was the end of the story from a practical point of view; the set went back to the customer and a recent check confirmed that it had not missed a beat since. But a couple of questions remained unanswered. For one thing, I'm not clear as to the role of C30, or why it is so critical, except that it is associated with the limiter circuit. Which only emphasises how difficult it is to keep pace with all the circuit design theory these days. There is also the question as to why the set failed in the first place. Was it really the result of the lightning strike, or was the timing purely coincidental? We will never know but I'm inclined to support the lightning strike theory, with the rider that the capacitor was probably a bit dodgy in the first place. Lightning strikes I suppose this raises the old query as to just why lightning strikes behave the way they do. Why do they destroy one component in the middle of a circuit, often far removed from the antenna and front-end circuitry, while leaving the rest of the equipment intact? While there is no simple answer to this question, we do know a lot more about the effects of lightning strikes than we used to. There are really two major mechanisms by which lightning causes damage. The first is the obvious one where the lightning actually strikes some object, such as an antenna, and causes quite catastrophic damage, often melting the antenna elements and associated cables, coils and other components. The second effect is more subtle and is due to the intense magnetic field created by the massive current flow which accompanies such a strike. This magnetic field can induce damaging currents in electronic components, solid state devices being particularly susceptible. f was actually reminded of this phenomenon, and just how intense the magnetic fields can be, by another case which came up shortly after the aforementioned lightning strike. The customer lives about TETIA TV TIPS Philips CA826 (Toshiba chassis) Symptom: Raster narrowed unevenly but more so at top of screen. Picture hiccups and set occasionally cuts out. 11 2V rail down to only 90 volts. Cure: 0801 (2SC1195) open circuit between base and emitter. This transistor is the power series regulator, in parallel with 25W resistors R81 0/R811 . When it fails, the resistors will pass sufficient current to keep the set going, though obviously not very well. 50 metres from the motel and he called me a couple of days after the storm with a complaint about his AWA Mitsubishi TV set. As he described it, the colour had "gone funny". It was a large set (63cm) so I made it a house call. And when the owner ushered me into the lounge room where the set was running, I needed only one glance to know what was wrong. It was one of the worst cases of purity error I had seen for a long time. Turning the colour off left the screen displaying all the colours of the rainbow in a psychedelic pattern. But before I had time to ask the obvious question the owner volunteered the information that this fault had appeared immediately after the lightning strike. Since several days had passed since the strike, and the set had been in constant use, I was a little surprised that the normal degaussing system had not corrected the condition, and even considered the possibility that this had been knocked out by the strike. But a quick check confirmed that it was still intact, meaning that the spurious magnetism in the shadowmask was far too strong for it to correct. Fortunately, a couple of passes with the old faithful degaussing wand wiped out this latest example of modern art, and made the set suitable for the more mundane art form for which it had been designed. But the real point of the story is to emphasise the strength of the magnetic field which must have ac- companied the strike, remembering that there was a good 50 metres between the point of strike and this TV set. So don't be surprised at anything that happens when lightning strikes. One day at a time And here's another contribution from my colleague, J.L., in Tasmania. He calls it "One Day at a Time" and, as this implies, it describes a typical day in the life of a serviceman - or as near to typical as one can get. This is how he tells it. Began the day by collecting an old Akai video recorder from a customer's home, then made some money disappear into my bank account. I hate those state and federal bank taxes. Sales and income tax I have learned to live with - well, more less - but not FID, SDT and BAD! Then it was across the river and up a steep hill to a house with the best view in Australia. I was hoping for a lengthy repair so I could spend some time admiring the scene. Unfortunately, I had only to put a new fuse in an old Rank colour TV set. Ten minutes and I was on my way, back across the river to the workshop. On the bench was a G.E. colour TV set awaiting attention. It was a fairly common problem - R731 burnt up. This is a fusible resistor in the 120V line to the horizontal driver stage. A new resistor lasted 10 minutes, so I assumed it was probably a protection circuit fault. This protection circuit, TR56 and its drivers TR57 and TR58, cause more trouble than enough by going over-sensitive. They tend to trip for no reason. The easy solution is to remove TR56, which is the crowbar switch. This invariably cures the problem and R731 never fails again. Nor does anything else that I have ever encountered. But first it is necessary to check that the 120V rail and the EHT are not running high. They never are but they must be checked before it is safe to remove the protection. (Some Rank Arena sets also have one of these "protector" circuits that often fail for no reason at all. AUGUST 1988 31 SERVICEMAN'S LOG Whether they are ever called on to actually protect something is a moot point)! The next job was an old National colour TV set and a new National VCR, both from the same customer. The TV set was supposed to spit and splutter whenever a commercial came on the air. It was reported to be OK on ordinary program material, and ABC and SBS programs. All of which only goes to demonstrate its good taste. I know a lot of viewers who behave in much the same way - including spitting and spluttering at some of the tasteless commercials they try to bash our ears with. The VCR was brand new and had never been tuned to the local channels. Nor could the owner find any combination of buttons that would allow the machine to play a prerecorded tape. I have often complained about incomprehensible service manuals and now I will add user manuals. This National manual was written in tolerable English but omitted 50% of the info needed to enable the user to set up the tuner and timer. I was able to get things working because I have done it before but a non-technical owner might have fiddled for years and got nowhere. The number of buttons and the permutations of them is staggering. (In recent months I have had dozens of calls from people prepared to pay me to come to their homes to teach them how to use their new "'what's it". Often I have nearly as much trouble as they do, sorting out the user manual). In this case the TV set was badly off tune on all channels and this was easily fixed. The VCR was not so easy to tune but eventually responded to some gentle prodding and twiddling. After this·, the picture was perfect on all channels and also off tape. I let the set run all day but it showed no signs of any other fault, so the owner can collect it tomorrow. Thorn 3500 The next set in line was an old Thorn 3500 series. It had no blue in the picture, due to total lack of voltage on G2 in the blue gun. This can be caused by an open circuit or high resistance feed resistor, a faulty adjustment pot, or a faulty beam switch. The last is the least common, but also the hardest to find as the switch tests perfect in or out of circuit. In this set it was switch trouble. It seems that the switch develops a leakage path at the high voltages at which it normally operates but tests OK at the modest voltages normally used for measuring. Replacement switches are now unobtainable and, as the switch is only needed for occasional purity adjustments, I prefer to remove it entirely and fit a jumper in its place. Philips K11 The next problem concerned a Philips Kl 1 with intermittent pincushion distortion. The problem came and went at odd times; from seconds to minutes to nearly an hour. When the fault was present no amount of thumping or bumping would alter it, so there seemed to be little chance that it was a dry joint. I tried freezing the E-W correction transistor, TS421, when the problem showed and again when it vanished, but results were inconclusive. The spray did seem to have some effect on what little .°c,~ "o/. "f' ,,. ~ o"• C) --f" I' tt q p ~~": : r..: . .. -y~~~~-·:·-...__, :,.,.. , ___,, - -- -- . . ----- ' · ~ -------~-~-~ffw: -· So l>Orv'-r a£ SUR?R\SEJ:> A"T' AN"<-r\-\\tvG ~ ~ . ~-,, 'THAT HA'PP&NS WHE.N l-\GHTtJIN6 ST'R\~~4S. 32 SILICON CHIP ------------ ' I wasted some time on the Akai VCR because it was unfamiliar gear. The time I lost on this one will be made up the next time I see one of these machines - it will go straight to my colleague and no messing about. Bread and dripping --o~ L t(f._ ·--.. 1\-l&: TV S£.T WAS SOPf>OS~D iO SPI, IN 'DISGUST WHENE.VE.R A COMMERCIAL.CAME. -ro A\R ..... behaviour pattern there was, so I changed the transistor just to see what would happen. In a sense nothing happened, because there was no distortion when I switched back on, and there has been no sign of it since. After several hours the set was still working perfectly, so I felt it was safe to consider the fault cured. So, another one down and one to go. Akai video recorder The Akai video recorder mentioned at the beginning was an old toploading machine. All I had been told was that it didn't work. It was left to me to find out in what way it didn't work. I fired it up and was rewarded with all the expected lights on the front panel. I pressed the eject lever and received an unenthusiastic response from the cassette carrier. It came up about one centimetre and then stopped. It took some gentle pressure to release it from whatever was holding it. I put in my test cassette and closed the machine, whereupon the drum motor started and nothing would stop it. Normally the drum does not start until the play lever is pressed, so this looked like some sort of clue. Unfortunately, this was of little value because the cassette carrier was locked down (or jammed) and I could find no way to get the cover off to release it. Without a manual and little detailed knowledge of this particular model, I was out of my depth. It will go into a specialist colleague in the morning. By this time it was too late to start another job so I spent what was left of the day doing the books, trying to make the income bigger than the outgo. This has been an easy day; Apart from the VCR, each job was done fairly quickly because past experience has taught me where to look for these faults. Well, that's J.L.'s story of a typical day, and it emphasises a point that I have made in these notes many times; elusive intermittent faults and similar nasties may make interesting reading, and even boost one's ego - at least in retrospect - but they don't put the bread and dripping on the table (Oh dear, how sad, never mind. Ed.). That comes from the routine, day to day faults , which one takes in one's stride on the basis of previous experience. And there is another point in J.L.'s story worthy of comment; his complaint about the inadequacy of user manuals. I couldn't agree more. At one time customers were advised: "If in doubt, read the instructions." Now, the more appropriate advice would be: "Find someone to show you how to operate it, then read the instructions to see if they make sense". That may sound cynical, but it's true. It's not that the manuals are inaccurate but simply that they are inadequate. They are apparently usually written by people who are very familiar with the device and its workings - people who simply cannot understand the confusion it is likely to cause the consumer the first time he encounters it. I have no doubt that their defence - if ever one should be fortunate enough to confront them concerning any inadequately explained point - would be, "Oh everyone knows that". Carried to its logical conclusion, that means that we don't need a user manual at all; which is close to what is happening, in effect. So it is not until the user has found someone who has already been through the exercise the hard way, and is able to instruct him, that the manual begins to make sense. Which makes it all a bit pointless, really. See you next month. 1§::l AUGUST 1988 33