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Knowing when to draw a line Maybe I'm cynical or superstitious but it seems that whenever I mentally register a fault as "routine" it turns out to be anything but. Instead, Murphy is usually waiting to pounce - to teach me a lesson for daring to tempt fate. Thus it was with this month's story. The customer rang to say that his TV set was playing up. The picture had vanished and all he had was a bright line across the middle of the screen - in other words, frame collapse. Now frame collapse is not usually a difficult problem; in fact, it's typically routine. I suggested that he bring the set in and that repairs should be quite straightforward. When he arrived, it was with a Sanyo model CTP8631, a 53cm model between 10 and 12 years old. I turned it on briefly while he was there, just to confirm his description. Sure enough, it was a classic example of frame collapse. I was rather busy at the time and because the customer had a second set, I put it aside for a couple of days. When I finally set it up on the bench and fished out the manual, I fully expected to find that the ver- § ··· ···. < ,.._ THE: CUS1"'0M~'R AANG .,-o s~ THA, Hl'S 1V WF\S ?l..A'-flNG UP. 34 SILICON CHIP se:.T tical output transistors, Q903 and Q904, had packed it in (a fairly common fault). They usually take out a few other components as well, particularly diodes D461 and D462. But no. A careful check of these and other relevant components in this part of the circuit drew a complete blank; as far as I could see it should be working. It was time to stoke up the CRO and start chasing waveforms. At least, that was what I would have done normally. Now I realised that the set was exhibiting a couple of symptoms that I had overlooked earlier. Other sets were operating on the bench and I noticed that, when I turned this set on, it created random white dots all over the screens of the other sets. Sizzle, sizzle At the same time I became aware of a faint sizzling noise, exactly like an EHT leakage, though not quite as loud as these usually sound. Nevertheless, I assumed that it was an EHT fault and that this was responsible for the interference to the other sets. Such faults can be quite destructive, so it was important that I fix it before worrying about the vertical fault. But it wasn't that easy. One can usually pinpoint such faults visually if the ambient lighting is not too bright, but not in this case. I turned off all the lights and it didn't help, so I drew the blinds on the windows and tried again, paying particular attention to the ultor cap, the tripler and the horizontal output transformer. Still no luck. More or less on impulse I reached for the EHT voltage probe and measured the EHT. And that set me back somewhat. In these sets the EHT normally runs at around 24kV to 25kV but in this set is was varying between 30kV and 32kV. I snapped the power off quick smart. Excessive EHT can cause a lot of damage, even puncturing a picture tube in extreme cases. I needed time to think. The most likely cause of excessive EHT would be a higher than normal (120V) main HT rail due, in turn, to some kind of fault in the regulator circuit of the main power supply. But I didn't fancy the idea of trying to track down such a fault while the EHT system was doing its imitation of a sausage sizzle. On the other hand I needed to apply power. The simplest approach seemed to be to fire the set up via a Variac, bringing the voltage up gently to assess behaviour, while keeping the system below the sizzle level. So that was what I did, connecting a voltmeter across the main HT rail to keep tabs on it. I brought the rail up to about 100V, at which point all was quiet, but I had been paying more attention to the meter and the vital EHT points than the screen, which was facing away from me. When I did check it, I received another setback - in a cockeyed kind of way - for there in place of the thin bright line was a full size picture. From a serviceman's point of view I don't know which is more puzzling; finding a line when there should be a picture, or finding a picture when there should be a line! I checked the EHT at this point and it was close to normal so I edged the Variac up gently, trying to keep an eye on everything at once. And suddenly, as the HT rail was approaching the 120V mark and the TRIPLER EHT --------------7 rI I TO BEAM I L~ R607 1k R617 220k R612 470k R611 330k R613 R614 R616 220k 270k 270k I v~~ I I I I I 700k I I L I I R605 270k o2 CRT UNIT 85PH ------------------ I j Fig.1: this diagram shows the divider network which supplies the G2 voltage in the Sanyo CTP 8631. Note C603 (1µF) which is common cause of trouble. EHT was getting dangerously high, we were back into sizzle country again and the picture collapsed to a thin bright line. I switched off smartly. Weird symptoms Well, here was a weird set of symptoms if ever there was one. But there were a couple of others I had noticed while the picture was on the screen. It was overly bright for what I took to be a normal brightness setting and more importantly, was exhibiting vertical retrace lines. These were vital clues but I didn't appreciate this immediately. I was still trying to figure out the source of the sizzling and why it was causing the vertical system' to collapse. I also wanted to know at what point the vertical deflection was failing. The vertical and horizontal drives, along with a number of auxiliary functions, are generated in a 24-pin chip, IC401, type LA1463. The vertical drive comes out on pin 5 and runs straight to the base of vertical output transistor Q904. I attached the CRO probe to this pin and with the set running on reduced input voltage, confirmed that the waveform here was essentially as shown on the circuit. Next, I wound up the Variac until the vertical deflection collapsed, held it there just long enough to check the CRO, then backed off. The answer was that the waveform out of the chip vanished when the fault appeared. This at least cleared the vertical output stage but did little to suggest why the excessive EHT was shutting down the vertical oscillator. On the other hand the real fault was the one causing this excessive voltage. Find that and I shouldn't have to worry about what was really a side effect. But what could possibly cause the EHT to go high? While horizontal output transformers develop faults from time to time, I've never known one to behave like this. Reduced voltage, yes; excessive voltage, never. Much the same applies to triplers. But it so happened that I had a spare tripler on hand and it took only a couple of minutes to patch it in. After all, funnier things have happened. But it was a waste of time; the results were exactly as before. G2 voltage It was then that I realised that the symptoms, when I forced the vertical system to work, were more significant than I had previously appreciated. A common source of trouble in these sets - and some other brands as well - is a voltage divider network from which the picture tube's G2 voltage is derived. In this set the voltage divider is connected between the lower end of the EHT winding and chassis (the chassis connection also involves the beam limiter network). There are seven resistors in the main divider network: R607, R617, R612, R611, R613, R614 and R616, in that order (see Fig.1). In addition, there is a 700k!l pot (VR601) connected across R612 and R611, the moving arm of which supplies the G2 voltage via R605 (270k!l). Another vital component here is C603 which is a 1µ,F electrolytic capacitor. Partial or total failure of this capacitor will either darken or black out the screen. I'm not sure of its exact function but I imagine that it's there to filter out any remaining ripple which might otherwise upset the beam limiter circuit and bring it into action prematurely. It is a fairly common fault. Screen brightness can also be reduced by total or partial failure of resistors at the high voltage end of the divider (eg, R616). This lowers the G2 voltage and is a fault I have encountered several times in the past. But I was looking for increased brightness plus excessive EHT. Could some other failure here account for this? I pulled the neck board off and one glance was enough; all the main divider resistors were showing signs of distress and one of them, R617 at the chassis end of the network, measured open circuit (note I said "measured"). I promptly replaced the whole chain, plus C603 for good measure. Then I switched on and up came a near perfect picture, being just a little on the dark side due to the Variac being turned down. Restoring full mains voltage cured this and there were no further complications - ie, normal vertical scan, no sizzling and no interference to other sets. That was it. Unfortunately, I can't answer all the questions which this fault raises. The hardest one is why the fault shut down the vertical oscillator. Quite frankly, I haven't a clue. Next, where was the sizzle? I never did find it but my tip is that it was on the neck board, on the component side, facing the front of the cabinet. This cabinet is fairly deep and there is no way that one can view the front of the board while it is in situ. And why did the open circuit resistor cause excessive EHT? How, in fact, did the circuit work at all? I suspect that the resistor DECEMBER1989 35 SERVICEMAN'S LOG -CTD which measured open circuit on the ohmmeter behaved quite differently in the circuit. More specifically, it was probably behaving more like a very high value resistor, breaking down across the fault and generating the sizzle. And in this condition it jacked up the EHT, either directly due to the increased resistance, or indirectly because it reduced the current drain. In any case, any voltage increase in this part of the circuit would be multiplied by the tripler. Which is about the limit of my speculation. If anyone has any other ideas I'm sure readers would be interested. But, as I hinted earlier, never take a "routine" fault for granted. To change the subject, here's a story from my colleague J.L., in Northern Antarctica, across Bass Strait. As so often happens, something which started out looking like a lost cause finally had a happy ending. Here's how he tells it. J.L.'s story Sometimes customers give the most misleading description of the symptoms their sets are displaying. One such came in recently with the rep9rt, "No colour". The set was an early model HMV, a 12613, otherwise known as a "Braddon". At first I took the customer at his word. As is typical in such situations, when I set it up on the bench it produced a perfect colour picture. This introduction to the problem set the stage for my battle over the following two months. The set normally lived in a country town, some 100km away amid high hills. It was possible that the colour loss was due to a weak signal, so I set about reducing the signal with a series of coax attenuators. All of which proved nothing because the colour remained steady until the picture was about to disappear in the snow. I could do nothing more than return the set to its owner with instructions about a good antenna connection and proper fine tuning. I heard nothing more for a week. When the set came back it was with the explanation that the colour had been good for several days, then it disappeared and returned at irregular intervals until the owner decided to bring it with him on his next visit to town. On the bench it upset its owner by firing up with perfect colour. But only for five minutes. Even before he left the colour had gone, only to return intermittently for the rest of the day. But more importantly, I had seen the fault for the first time and realised that the customer's decription was only partly correct. There was some colour with faces being shown on screen as red. Just the faces - the rest of the picture was indeed monochrome! When there were no faces on screen, any red or orange item was coloured but everything else was monochrome. It was rather like a newspaper illustration in which highlights have been picked out in garish colour. In between times the colour returned and produced quite a good picture. At these times I tried all the usual tests, like thump and bump or heat and freeze, in an attempt to disrupt the colour. The only thing I learned from this was that adjusting the fine tuning could produce the strange "red faces" effect. Unlocked chroma 7) '7NE SUCH C..AMe: IN W\1'\,\ "TI-\~ 'R~'PoR-r '' NO COI-OU'R '' 36 SILICON CHIP But there was another strange effect; tuning from sound bars into the picture produced nothing unusual but tuning the other way, from a monochrome picture towards colour, produced a picture with unlocked chroma. I couldn't imagine why the chroma should be locked when fine tuning one way, and unlocked when tuning the other way. But it was a possible clue and I spent much time swapping crystals and integrated circuits in an attempt to secure stable locking. The intermittent nature of the fault gave me many false alarms but in the end it was always back to either red faces or unlocked colours. The only hint of a clue was that the go/no-go state might have been slightly sensitive to the angle at which the chroma board was sitting on its socket. This made me hope that there might have been a dry joint under the socket, so I upended the chassis and resoldered all the pins under the chroma board socket and a few others that looked a bit doubtful. Then I re-tensioned the contacts on the board itself, just for good measure. After all this I thought I might have really cracked it because the picture came up perfect, with no sign of unlocked chroma no matter which way I fine tuned the set. After all my hassles, this had to be a cure so the set went back home next time the owner came to town. I should have known better because it was back again within a week. It was the same problem and thoroughly intermittent to boot! The symptoms did not point to any clearly defined fault. The unlocked colours might have indicated a reference oscillator fault (and note that this set has two oscillators, running on different frequencies!) while the fine tuning requirement might have indicated a bandpass amplifier fault. Several other less likely troubles also suggested themselves. As an aside, the use of two reference oscillators is a rather novel arrangement in place of the usual flipflop, line alternation, PAL switching system. One oscillator runs at the conventional frequency of 4,433,618.75Hz and the other is offset by half the line frequency and runs at 4,441 ,431.25Hz. There are two decoder circuits, one for the B-Y signal and one for the R-Y signal. It became clear that I was going to need a set of extension leads so that I could test and adjust the chroma board out of the chassis. I had no idea where I might get such a set, so I cast around for bits and pieces to make into a suitable lead. Funny sockets You mightn't believe this but I tried dozens of plugs and sockets from dozens of different sets and all of them were either too big or too small. It seems that this particular HMV was fitted with odd sized sockets that don't match any other set sold in this country. It seemed that I was going to have to hardwire each plug pin to each socket terminal. ~-~ ~ 0 I ' I I' ~ -t' / , ~ , / I :t -rR ,~o AL-L- -o-t«;: \JSU~l-"'11::S-rS,L\K ~' SUMP P\ND 11-\UM'P ~ OR l-\~-r AND 'f'RE:E'Z.~ •.., There was only one adjustment on the chroma board that I was prepared to fiddle with while the board was in situ. This was the killer threshold trimpot and a small movement of this control brought back full colour, without any of the earlier tuning problems. What's more, the set ran for a fortnight in my workshop without a glitch. The only problem, if it could be so described, was that we now had confetti on the blank channels. Putting the killer back where it had been stopped the confetti but restored all the problems! So the set went home with the killer set fully high and my hopes for a long and trouble free life. I wasn't really happy about the job because I hadn't found and fixed the fault - I had only masked it. The customer had no grouch so long as the set worked normally onchannel. He had no interest in what it was like off-channel but it still worried me. As a result I was not too surprised when the set came back a couple of weeks later. It was back to its old capers and this time I had to fix it once and for all. Eureka - as they say At about this time I had an inspiration. I remembered that my son had an identical model. So I gave him a loan set and borrowed his HMV to give me a known good board for comparisons. My first test was to put the good chroma board in the doubtful set. This confirmed that the fault was on the chroma board because the client's set was now perfect. More importantly, when the client's chroma board was fitted to my son's set, it became as troublesome as the client's set. I had already replaced the three chips on the faulty board so I assumed that these were OK. Next I checked the rest of the semiconductors and compared them with their equivalents on the good board. Only one transistor showed any difference and I replaced this but without any improvement. Next, I checked all the fixed inductors on the board by testing DECEMBER1989 37 SERVICEMAN'S LOG -CTD I I ~w•,rri~--1~4~-~2 cso2 i I them for DC continuity. They were all OK so I began testing other coils and transformers, about eight in all. None of the transformers gave any indication of being faulty during my initial continuity check. It was only when compared with the equivalent unit on the good board that any sign of a difference in one of the coils showed up. It was the first coil on the board (although by Murphy's Law it was still the last one to be checked), L501. This is the chroma input coil and consists of a tunable shunt coil and a series capacitor. It is fed with raw chroma from pin 1 via a 120pF input capacitor and in turn feeds clean chroma to pin 2 of IC501. My first test was for continuity from pins 1 to 3 and this checked out at less than rn on both boards. The second test was from pin 4 to chassis, thus checking the integrity of the capacitor. Here the good board showed 2.2kn while the bad one showed much less, varying between 4200 and 8200, depending on which digital meter I used. I removed the whole coil/capacitor assembly and rechecked the resistance from the (now empty) pin 4 connection on the chroma board to chassis. It was 2.2k0, just as in the good board. As a quick test, I shunted connection points 1 and 4 on the board with a lOOpF capacitor. At switch-on the picture came up trumps - good though noisy colour and perfectly stable. Now that I knew what the trouble was, I was faced with the problem of curing it. This chroma input coil would have been an unlikely spare part, even when the set was new. Now, 12 years on, I wouldn't give tuppence for my chances of getting a replacement coil from any of the usual sources. It seemed that the best thing to do would be to find a similar coil in another set. The most likely source would be a Rank Arena because TV TEST EQUIPMENT (AUSTRALIAN MADE) I csoi L501 I 1 J I I I I__ 47pF 5 ~~ I I 3___ .J Fig.2: this is the circuit for the chroma input coil in the HMV 12613. While simple electrically, it is difficult to work on mechanically. they use the same chroma amplifier chip. However, examination of several Rank boards showed there was no similarity between the equivalent coils. So that line of investigation came to a dead end. So, if replacement was not going to be easy, would repair be any easier? Closer examination of the L501 package strengthened my conviction that the capacitor was the culprit. It seemed to be leaky, although there remained a slight possibility that the leakage was in the coil base material itself. There AUDIO TRANSFORMED SHORTED TURNS TESTER Built in meter to check EHT transformers including split diode type, yokes and drive transformers. $78. 00 + $3.00 p&p HI-VOLTAGE PROBE Built in meter reads positive or negative 0-50kV. For checking TVs, microwave ovens. $84.00 + $5.00 p&p. 1 TELE·TEST TUBE TESTER & REJUVENA TOR Removes cathode grid shorts & rejuvenates picture tube. Suitable for all colour tubes. 12 months warranty. $420.00 + p&p. NEW PRODUCT DEGAUSSING WAND: A must for all workshops. Efficient design, strong magnetic field, low heat, double insulated with momentary on/off. $75.00 + $1 0 p&p. ALSO TUNER REPAIRS FROM $17.00. Cheque, Money Order, Bankcard or MasterCard Y.V.TuNERs) 38 SILICON CHIP 216 Canterbury Road, Revesby, NSW 2212. Phone (02) 77 4 1154 FOR PROFESSIONAL, OEM OR AMATEUR Broadcast qual ity audio transformers ex-stock Mo re than 70 standa rd types availab le Fast protot yping se rvice for non-standard types Co mprehensive data available on requ est Locall y manufacture d Competitively priced HARBUCH ELECTRONICS PTY LTD 90 George St .. HORNSBY NSW 2077 Ph one (021476-5854 was only one way to find out. The trouble was, any kind of work on the package didn't look to be particularly easy. It is very small, the can being about 10mm square and 12mm high, and removing the assembly from inside the can without damage is difficult to say the least. So the alternative was to work inside the can to isolate the capacitor. Using a powerful magnifier and working with the fine tip of a sharp craft knife, I cut one of the capacitor leads. This removed all trace of the leakage between pins 1 and 4 and proved conclusively that the capacitor was the culprit. i carefully cut the other lead and tried to remove the capacitor from the coil body. It came out in two pieces so I don't know if that was the problem or the result of my ministrations. Whatever it might have been, there was no further sign of leakage across the base pins. I re-fitted the coil, and replaced the missing capacitor with an external 47pF ceramic tacked onto the back of the board. The problem licked At next switch-on I had the problem licked. Perfect colour, no chroma hash, and no sign of instability due to the fine tuning. I still had the confetti off-channel but that was easily curable with the killer threshold pot. I'm sure that the set will never come back with that particular fault. But in retrospect, I can't help wondering how I would have found it if I had not had the good board from my son's set for comparison. The extension leads might have helped by makin_g scope .inspection possible. But I wonder . It was the ohmic disparity that really solved the problem. As a matter of interest, I tried to duplicate the fault on the good board by dabbing a 5600 resistor across the capacitor in 1501. It killed the colour all right. But totally! Nothing I could do would give a repeat of the red faces or unlocked chroma. Comment Well that's J.L.'s story and it prompts a couple of comments. It so I 0\ s c; <at>J~g,. ...... S-,;;- s~v~AAL 011-\ER l-~'SS L..\K~l-'1 -r'ROU'Bl-E'S A.L.'SO s \JGGes-r-E,0 ~e:M~eL-ve:s.o. happens that I have one of these chassis in the workshop right now, exhibiting a particularly stubborn fault. Tracking it down may well make a story in its own right but that's for the future. The point is that this one is a General chassis, model GC18A, which is the true origin of the HMV 12613, which was also marketed under the Healing label. More importantly, good stocks of spares are still available from the General Corporation Japan (Aust.) Pty Ltd, 10 South St, Rydalmere, NSW 2116. I recently obtained an IF transformer from them and they also supplied my extension leads for the chroma board. They came as a kit of four leads (three 6-pin and one 3-pin). This was some time ago now but they may still have some. So, next time you tackle one of these J.L., this information may make things a little easier. ~ TETIA TV TIPS AWA C6319 (Q chassis) Symptom: Overbright screen, with no control of brightness. Retrace lines are obvious and persist at all settings of the CRT bias control. Replacing the usual electrolytic capacitors has no effect. Cure: R251 (1. 5MQ ½W) open circuit between VR25 1 CRT bias trimpot and the Vee supply to the video output transistors. This fault is confusing because the bias trimpot seems to adjust the screen voltage as might be expected. Al pine Cassette Deck, Model AL51. Symptom: Mechanism completely inoperative although panel lights show that power is reaching the unit. Cure: Either or both of two faults: ( 1 ) the counter belt is broken or misplaced or (2) the reed switch on the counter is stuck. A magnet on the counter shaft rotates in normal operation and causes the reed switch to produce a series of pulses which unlatch the auto stop circuit. Without these pulses the machine thinks it has come to the end of the tape. TETIA TV Tip is supplied by the Tasmanian branch of The Electronic Technicians ' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, Tasmania 7015. DECEMB ER 1989 3-9