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SERVICEMAN'S LOG From a snack to a 3-course meal! The above gastronomical analogy may appear a trifle strange but it's quite appropriate in the circumstances. What presented itself as a simple snack, turned into a full meal and a severe case of technical indigestion. The device involved was a Samsung 50cm colour TV set, model CB5025J, and the fault was plain for all to see. The antenna socket was broken out of the tuner, which is quite a common one with this particular Korean-made unit. And this same tuner is used in a whole range of other sets: Goldstar, Orion, AWA, NEC, Sharp and Tandy, to name just those I can recall offhand. So, as I implied above, it looked like a snack; particularly as the set was under warranty and it was a simple job to replace the tuner. In these circumstances, I didn't pay a great deal of attention to the owner's comments when he showed me what had happened. Had I done so, I would at least have been better prepared for subsequent events. Our loss is your gain Fracarro's ESVU2. The really low loss VHF/ UHF mixer with areasonable price tag. Peter C. Lacey Services Pty. Ltd. P.O. Box 678 (74Fulton Ad.) Mount Eliza 3930 Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438 36 SILICON CHIP In fact, he had stuffed the socket back in position and secured it with sticky tape and a prayer. And although the broken connection was not restored, there was apparently enough capacitive coupling to provide a picture of some kind, even if it was snowy. And, he added, there wasn't any colour. Well, I didn't take much notice of that last remark; what else could one expect with a signal that weak? So I simply went ahead and fitted a replacement tuner. And, of course, this resulted in a nice bright, snow-free picture; one which anyone would be happy to watch - except for one problem. There was still no colour! Suddenly the owner's, comment took on a lot more meaning. The lack of colour was obviously a separate fault and, had I taken the trouble to question him, I would have learnt that this had been obvious before the tuner socket collapsed. A fair dinkum fault Anyway, that was history. I now had to track down a fair dinkum colour fault. Fishing out the circuit, I concentrated on IC501 (TDA3561A) which performs the decoding functions. I established that the video signal goes in on pin 3 and that, according to the CRO, there was a normal signal at this point. The gating pulse, to admit the 10Hz reference burst, goes in on pin 8 and this was also normal. The next logical check was around the reference oscillator crystal, X501, between pins 25 and 26. And here it seemed that I had struck oil; this time the CRO report was negative - there was no reference oscillator signal. So, was it a faulty crystal? This seemed the most likely cause and, in any case, was simple enough to check. I fished a 4.43MHz crystal out of stock and fitted it. Result - still no colour, even though I now had a reference oscillator signal. First whinge However, this signal was very low in amplitude. And that brings me to the first of several whinges I have about the circuit and the data supplied by Samsung. The circuit and manual are seriously devoid of data of this kind, which meant that I was ·on my own. I took a punt on a signal of about 2V p-p and, in the event, this estimate was pretty well spot on. But right now, I had only a fraction of this value. I woke up when I took a closer look, through the jeweller's loupe, at the minute markings on the crystal I had taken out. The crystal was not marked 4.43MHzbut8.86MHz. Whythisvalue is used I don't know but it appears to be the latest trend. " I didn't have such a crystal, which meant that I had to order one from Samsung, and this put the job back a couple of days. But it arrived fairly promptly and I lost no time in fitting it. And this confirmed my previous estimate of the reference oscillator amplitude; it was almost exactly 2V p-p. But that's all it did- I still had no colour. After some thought, I reasoned that the new crystal, while almost certainly within tolerance, was sufficiently different from the original to prevent it from locking onto the burst signal with the setting used forCV0l, a 22pFtrimmer in series with the crystal. In any case it is good practice, when a new crystal is fitted, to adjust this trimmer so that the reference oscillator is running as close as possible to the burst frequency. This then ensures "test points", I would have been an hour ahead. HATIIIX-ADJ CSl4 Anyway, I went ahead and bridged TSOI A524 SIO these "test points". This should have RS:ZO produced colour in one form or an12K ~~ I other. If the reference oscillator's free~ HATIIIX-AllJ running frequency is sufficiently accurate, it can be an otherwise normal C521 221 0$01 pattern, but with the colour slowly 1N4141 drifting from one hue to another as R521 m tlO the phase of the oscillator drifts, relative to the burst. More usually the error is a good deal greater, producing horizontal colour bars, superimposed on the vertical bars, which also drift in colour as the phase changes. And the greater the degree of error, the more horizontal colour bars. But while that may be all very fine (201 :ol.4111' in theory, it was oflittle practical value in this case. There was still no sign of colour in any form after I bridged pins 23 and 24. So where to from here? As I have already noted, I had checked the sigFig.1: the colour decoding ciFcuitry in the Samsung CB5025J. IC501 nal into the chip at pin 3 and the burst is the colour decoding chip & the crystal is at top left, between pins gating signal at pin 8, and found both 25 & 26. The delay line (DL501) is located directly above the chip. Note the poor qualify of the diagram which makes troubleshooting to be correct. But what about the sigjust that much more difficult. nal out of the chip? This appears at pin 28 and goes to the delay line, DL501. Or that's what should have happened. But there was nothing at pin that the oscillator locks in reliably, and no waveforms. Nor is it always 28. And, in view of all the other tests even on a weak signal. strictly correct and , to make it even I had made, I was forced to conclude The standard procedure for this harder, the printing quality is often so that I had a faulty chip. Fortunately, I adjustment is to first disable the lock- poor that it is literally impossible to had a chip in stock, so I simply fitted ing function between the burst signal read the component values. The cir- it. and the reference oscillator. This lets cuit used to illustrate this article is a And that fixed it? No way - there the reference oscillator run free and typical example. was still no colour and I was looking Try to read the IC component at a brick wall with no ideas left. All I the set's behaviour is then monitored using a standard vertical colour bar number and type number directly could think of was to talk to the maesabove the righthand end of the chip tro at Samsung and hope that he had pattern from a colour bar generator. diagram. The type number inside the some ideas. The reference oscillator is then adjusted to produce the closest approxi- diagram - top left corner - is much The result was a lengthy discusmation to a stable and correct colour clearer but is wrong. The correct des- sion during which we covered the ignation, as noted earlier, is TDA- history, the symptoms and all the display. 3561A, not TDA3562A as shown here. things I had tried. In fact, we thrashed Second whinge This mistake caused me to order the the whole problem about pretty thorWell that's the broad theory. But wrong component on a previous oc- oughly. But it was all to no avail; they how does one unlock this particular casion, resulting in an inconvenient were just as much against the brick delay. (There is a TDA3562A. It is wall as I was. So I was on my own. system? And this brings me to the second of the whinges I have about used in circuits designed to decode I went over the circuit again, both the data supplied. The manual states both SECAM and PAL signals). on paper and on the board, checking But back to the unlocking proce- voltages, resistors and capacitors, that the system may be unlocked by bridging test points 23 and 24 but dure, I eventually worked it out by many for the umteenth time in case I gives no indication as to where, on reverting to basics and doing a lot of had missed something. I drew a com, the board, these test points are lo- circuit tracing. But I wasted over an · plete blank. cated. Nor are they indicated on the hour in the process and that's not funny. And where did I finish up? At Transcendental meditation circuit or anywhere on the board. The system falls down in other ways pins 23 and 24 on the chip. So, if the When I run into a brick wall like too. The circuit carries no voltages manual had said "pins" instead of this, I find that it is often beneficial to ll!1..J 1m ; -- - JUNE 1992 37 SERVICEMAN'S LOG - CTD put the offending device to one side and, as far as it is concerned, go into transcendental meditation mode. And by that I don't mean squatting on the ground and contemplating one's navel. (Physically impossible at my age anyway)! No , I mean tackling a completely different job and banishing the problem completely from one's mind - at least as far as this is possible. And it is surprising how often it works. More to the point it worked this time. Suddenly, when I least expected it, I realised that there was one component I hadn't checked - the chrominance delay line. And I hadn't checked it because it was obvious that it couldn't possibly have any bearing on the problem. For one thing I couldn't recall ever having had any problems with chroma delay lines; luminance delay lines, yes, but not chroma types. In any case, what kind of a fault would create these symptoms? An open circuit could create problems further down the video chain but how could it kill the signal at the IC output pin? Nevertheless, the thought kept nagging away at me and I was grasping at straws. Fortunately, I had a fair stock of these devices on hand, mostly salvaged from scrapped sets, and it was only a few moments work to prove that my theory was wrong. Full colour Only it wasn't wrong. The set came to life in full colour and, of course, there was now a nice clean signal of around 0.5V at pin 28. So, in some mysterious way, the original delay line was loading pin 28 to the point where it was producing virtually zero output. And I use the word mysterious deliberately. No one, including those at Samsung, has yet been able to offer any explanation. The temptation is to assume that the input transducer had developed a short circuit. But, as I understand it, these transducers are piezoelectric devices, presenting a capacitive load to the input signal. How could they develop a short circuit? So the mystery remains. But at least the fault had been found and the job completed. Well, almost completed. I had previously fiddled with the crystal trimmer (CV0l) and so I unlocked the reference oscillator/burst system again and set the crystal up properly. In fact, it was a fair way out, though still locking on a strong signal. And that's pretty well the end of the story. But I can only classify this job as a real "stinker" and I think readers will appreciate why I described it as a snack that turned into three course meal; three faults - two of them nasty ones - where I had expected only one simple one. However, I must add one more thought. A lot of the trauma I experienced was due to the shortcomings of the manual and circuits, as I explained in the story itself. But, in fairness to Samsung, I'm happy to report that they have taken these criticisms - and mine were not the only ones - very much to heart. The latest manuals are a far cry from the ones I have criticised. They are now well printed and contain all the voltages, waveforms and other data that we have come to expect from other manufacturers. Full marks mate! The Panasonic portable PR.OM t,.. 'SN~C.\< -ro ~ 38 SILICON CHIP ~~5 COU'R.~e:. N\E.A\.-~oe My next story is about a Panasonic 35cm colour set, model TC-1401AR. It, too, was still under warranty, being virtually brand new, and having been in the field for only three months. The owner's complaint was, basically, that there was insufficient brightness but he went on to expand on this (and I did listen carefully this time) saying that, in his opinion, the brightness control was not working at all. And he added that the set was fitted with a remote control and that other functions which it controlled, such as contrast, colour and volume, all seemed to work normally. Naturally, I couldn't hazard a guess as to the nature of the problem and simply suggested that he bring it in. When he did, I turned it on while he was there and, yes, it was exactly as he said; no brightness control but all other control functions normal. More to the point, the effect was the same whether the command was via the remote control or the front panel control. I also noticed that the bright- R1142 181< IC1102 CDLOUR31 · BRIGHT30 01110 P-+-+tH-+-¥N_..._-i R1144 12k IC601 VIDEO/CHROMA J188 JUNGLE LINK P--o--o~-018 BRIGHT J45 LIN L__ ---------~-:>--f. 5V 1 Fig.2: this simplified diagram shows the main components in the brightness control circuit of the Panasonic TC-1401AR. Unfortunately, Link J45 is not shown on the circuit and link J188 was hidden on the board. ness was virtually normal, although perhaps just a trifle on the dark side. Whep I finally dug out the manual and opened the set, I went first to IC1102 (MN15142TEAI),amicroprocessor which handles the functional commands from both the front panel and the remote control. In greater detail, the colour saturation control voltage comes out on pin 31 and the brightness control voltage on'pin 30. On checking these voltages, pin 31 showed a varying voltage, from about 1.5V to 6V, depending on the setting of the colour control. But the brightness control voltage sat at a steady 5V, regardless of the brightness control setting. Well, at least these measurements fitted in with the observed symptoms. But why was it happening? After some preliminary checking around the IC, without finding anything obvious, I felt it was most likely an internal IC fault. After all, it was only a single function that had failed. 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 So I decided it was worth replacing the IC. Granted, I had to order one and there are 42 pins involved, but this job has become pretty much routine these days - one like this taking about 10 minutes. And, more often than not, the IC is the culprit. Unfortunately, it wasn't the culprit this time. Oh well, it was worth a try and a spare IC wouldn't go amiss. I'll use it eventually. My next approach was to track down the source of that fixed 5V on pin 30. Tracing the circuit (see Fig.2) from pin 30, I came first to diode Dl 110, which is shunted with an 18kQ resistor, R1142. And from the pin 30 side of the diode is a 12kQ resistor, Rl 144, to the 5V rail. So the 5V on pin 30 was legitimate enough, except that it was from a relatively high impedance source. Following diode Dl 110 was a 27kQ resistor, Rl 143, after which the circuit ran through a maze of twists and turns to pin 18, marked "Bright", of IC601, which is described as a "video/ chroma jungle" IC. And, fairly obviously, it was the voltage on pin 18 which controlled the brightness. But ANTRIM TOROIDAL TRANSFORMERS HIGH-VOLTAGE PROBE Built-in meter reads positive or nega- .. . tive voltages from 0-50kV. For checking EHT and other HT voltages . . $98.00 + $5.00 p&p 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 ~~ ~ _TUNER REPAIF!_~ --.:?" ~ ' From $19 exchange plus post & pack Cheque, Money Order, Visa, Bankcard or Mastercard 216 Canterbury Rd, Revesby, NSW 2212, Australia. Phone (02) 774 1154 Fax (02) 774 1154 • • • • Now made in Australia Standard range _15VA to 2kVA Large ex-stock quantities available Competitive pricing available for OEMs & resellers • Simply the best transformers all round HARBUCH ELECTRONICS PTY LTD 90 George St, Hornsby, NSW 2077. Phone (02) 476 5854 Fax (02) 476 3231 JUN E 1992 39 TETIA TV TIP AWA video recorder AV11 Symptom: very poor vertical hold on own recordings or EE picture but OK on pre-recorded tapes. Any light to dark transition in picture content shows several white lines paralleling the outline. Cure: C116, a 0-47µF 50V electrolytic capacitor, defective. This capacitor forms a bypass on part of the tuner/IF AGC circuit and the fault the voltage on pin 18 was, not surprisingly, also locked solid at 5V. causes the IF stage to go into oscillation which destroys the vertical sync pulse. The capacitor tests OK, with no leakage and correct capacity at low frequency, but fails at the much higher intermediate frequency. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. to secure another component to the board. The other way to open a circuit The missing link without cutting a track is to find a At this point, it was clear that I had series component which can be lifted. to separate these two pins before I In this case, there was a small induchad any hope of determining the tor, L1113, shown near pin 30 of source of this voltage. Now, in theory, ICl 102 on the circuit and also close to it should have been easy to break this this point on the board. It took only a circuit. This board carries a number few seconds to lift this. oflinks, designated with a "J" number, This removed the 5V from pin 30 of which make it easy to open a circuit ICl 102 but still left it on pin 18 of for testing. In this case the link was IC601. So the search area had been narrowed to a handful of components: designated J188. diode Dl 110 and resistors R1142, This link would have made things Rl 143 and Rl 144. As already menvery simple, had I been able to find it. But search as I might, I simply could tioned, Rl 144 goes directly to the 5V not locate J188. I did find it eventu- rail and, in effect, feeds pin 30. I asally but only after I had solved the sume that, when functioning norproblem by other means. It was on the mally, pin 30 can pull this value down, edge of the board and hidden under a as required, via R1144. So, with Ll 113 disconnected, I shut layer of Silastic which had been used the set down and proceeded to check these few components individually. And to help isolate them, I lifted R1142. I then checked diode D1110 , which proved to be perfectly OK. The same applied to Rl 142 and Rl 143. This leftR1144. And thi s was the breakthrough . Instead of measuring 12kQ on the meter, it registered a dead short. And I do mean a dead short; it wasn't a few hundred ohms, or even a few ohms - it was zero. ~ i f ~. . . . Now resistors don't -nu~ OWNE:.~'s C.OM?LP..\NT wA.s, go short circuit and so l!,ft...StCAL-L-Y, iHi\T i\-\1;::."RE: WA..-S I began a detailed ex\I\\SUFFIClENT :B'R.\GH,NE'SS amination of the board 40 SILICON CHIP and copper tracks involved. And this emphasised something I hadn't taken much notice of before. In series with the 5V rail feeding Rl 144 was another of the links that I had mentioned earlier, J45 . Strangely enough, this is not shown on the circuit but is shown on the component side of the layout diagram. The copper track in which this is fitted runs parallel with the track in which Rl 144 is fitted. More to the point, R1144 and J45 sit next to each other, with their solder pads side by side. In fact , one pair of pads is joined to provide the necessary connection between the 5V rail and R1144. The two pads at the other end of these components are separate - or at least they should have been. But they weren't. The solder pad at the pin 30 end ofR1144 was -yes you've guessed it - bridged across to the J45 pad alongside it. (The tracks are only 3mm apart). It wasn't simply a sliver of solder but a genuine solid bridge which had flowed between the two pads during the wave soldering process. Little wonder there was no brightness control; pin 30 of ICl 102 was connected directly to the 5V rail. Of course it was easily fixed; a quick touch with the iron and the set was better than new. And I make that point quite deliberately; this fault must have been present when the set was made and had passed through all tests without being deh:icted. It would appear that the sub-brightness control had been set to produce a typical brightness level but no-one had checked to see whether the brightness control was working. As it was, I now had to reset the sub-brightness control (R312 on pin 18 of IC601) to establish an adequate brightness range. So how did it happen? It probably has to do with the way manufacturers check their sets as they come off the assembly lines. As I understand it, most factories now employ a sampling-type final check, with most sets given only a cursory examination to confirm that they are actually working. Only a small a percentage (or sample) are actually given a thorough check out. It's a far cry from the old days in a radio factory but that's the way it now is. Anyway, I had another satisfied customer. SC