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SERVICEMAN'S LOG The search for the missing link I hardly need emphasise that the service game is mostly about diagnosis. Once a fault has been diagnosed, the repair is usually routine. But finding the fault is the real job & this one took months to find & seconds to fix. The set concerned was a Samsung 34cm colour TV set, model CB 349Z. The "Z" suffix indicates a remote control version, while an "F" suffix indicates a standard version. Both use the P50F chassis. The owner uses this set in two locations: at his Sydney home and at a holiday cottage down the coast. As a result, the set has been programmed for city VHF channels 2, 7, 9 & 10, SBS UHF channel 28, and south-coast UHF channels 30, 33, 42, 45 & 48. But that is more or less by the way, except that the large number of channels emphasised the fault's nuisance value. The fault itself was a tendency to spontaneously change channels. In greater detail, it didn't simply jump from one channel to another; it would go into a search mode which took it 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 Rd.) Mount Eliza 3930 Tel:03 787 2077 Fax:(03) 787 3460 ACN006893438 42 SILICON CHIP right through the 10 channels, then· repeat this procedure, before finally going into standby mode. Turning the set off and on again would usually cure the fault temporarily and it might then work normally for a few hours, several days, or even weeks. And, as I established later, it didn't seem to matter whether the set was hot or cold. Nor was there any other pattern to the fault. In short, it was completely unpredictable. Search function The search function just described is a quite legitimate feature in this set. Basically, it is a self-seeking, automatic tuning function, its primary purpose being to facilitate the initial setting up of the channels. There are two sets of UP/DOWN buttons in the system: the search buttons and the channel selection buttons. It is important to clearly differentiate between the two. The setting up procedure is initiated by activating one of the two search buttons (UP or DOWN). This causes the set to search until it receives a signal of adequate strength to lock onto, while very weak signals are ignored. In the unlikely event that no signals are detected, the system will search right through both the VHF and UHF bands, then go back to the beginning, repeat the search, and go into standby mode. In practice, of course, it would normally lock onto a signal somewhere in either the VHF or UHF band. When it does, it stores this channel in memory, waits for the button to be pressed again, then continues the search, stores the next channel it locks onto, and so on. When all the available channels have been stored, they may be selected in two ways: (1) sequentially by using the UP or DOWN channel select buttons on the front of the set; or (2) directly by pressing the appropriate channel number on the remote control unit. And how doEls the system know when it has found a channel? The relevant part of the circuit is shown here, including the PWB-CONTROL board and a section of the PWB REMOCON (remote control) board. Most of the work is performed by RIC01 (M50431), the main IC in this section. Pin 12 of this IC is fed with sync pulses and these provide the locking command. So inuch for the general background. When the customer described the fault, I immediately took the precaution of warning him that, since it was capable ofrurining OK for weeks at a time, it could well take me many weeks to find the cause. Fortunately, he had anticipated this and added that he had another set which would serve in the meantime. And just as well as it turned out. RR65 • RC11 ', UND : 1 SY '' -l RR36 1W,24»< RC06 1 t60V 1, '' ' '' RR40 I., 11< RR4 I 4. ?K sov,1,.. RR4 RR37 10K ~ K5C815-Y ROOS l-: 25C1815-\ RC09 2K RQ04 K~C81S·Y 2SC1815-Y RC01 Tip, 334 I0334 f tOOV I • -: IOOV RC08 22 130CH I R014 ROO? UPC574J JN4148 IBfil2l I START l 1:, _ _ _ " ROIS €e ~;;~,22 RC12 30 RR63 10K C !1i 0 ' a, ' ,... 82 I Reos• 103.SOV ~ RR67 30K RR33 30K RR32 I0K RR34 10K RR35 30K Fig.1: the relevant circuit section of the Samsung CB 349Z TV receiver. The microprocessor controller (RIC01) is on the PWB-REMOCON board & locks onto sync signals on its pin 12 input. The PWB-CONTROL board is to the left of this board and carries the display unit and most, but not all, of the control buttons. The fault was hidden under other components. Knowing how the system sensed the presence of a signal, my first step was to check pin 12 ofRlCOl for sync pulses. Because of the intermittent nature of the fault, all I could do was switch the set on and wait for the fault to appear. It took its time but, after a couple of days, it eventually showed up. I checked pin 12 with the CRO but there were the sync pulses, exactly as before. So that ruled out that theory. I wasn't quite sure where to look next , so I decided to see whether Samsung's service department had any ideas. After all, there is not much point in spending many hours trying to track down a fault when someone else has already been there and the · details are on record for the asking. So I rang Samsung outlined the symptoms, and asked whether anyone else had encountered the problem. The technician had not heard of any such fault but immediately made his own diagnosis. "Oh yes, the problem is the microp'rocessor chip, RICOl. Replace that and it should solve your problem". Frankly, I was highly sceptical of this suggestion. It had all the earmarks of a snap diagnosis, based on the obvious, but without too much thought about the fine details of the symptoms. On the other hand, the set was still under warranty, and if they were prepared to cover the cost of supplying and fitting a new IC -with 42 pins - who was I to quibble? So a new IC was ordered and duly fitted. And that solved the problem, or so it appeared. The set ran for over three weeks and never missed a beat so, when the customer called one day to check my progress, I suggested that he take it away. After all, it might just as well be tested in his lounge room as on my workbench. But I warned him that, in spite of Samsung's suggestion, and my bench testing, I was still not convinced. If it failed, he was to bring it back immediately. Several more weeks went by with no word from the owner and I was beginning to think that Samsung had been right after all. The next thing I knew, the set was back on the shop co·unter. "It went real beaut until yesterday. Then it was up to it's old tricks; really bad". Control board So we were back to square one. Well, almost; at least I could now rule out the IC. In the meantime, I had noted one more aspect of the fault; it always searched up, never down. On this basis there seemed to be only one line of investigation left: the control board (PWB-CONTROL) and, in particular, the channel UP and search UP pushbuttons. If one of these somehow jammed on, either mechanically or electrically, it could create this effect. The' PWB-CONTROL board measures about 200 x 600mm and sits at the front of the set beneath the picture tube. It is offset to the right, with the speaker in the remaining lefthand space. As well as the pushbutton controls just mentioned, it also carries the volume, contrast, colour, fine tuning and on/off controls, plus the remote control receiver and the channel number display. MARCH 1993 43 Fortunately, this board is fairly easy to get at by slipping the main chassis out and undoing several screws. The main chassis can then be replaced, giving a working set with the control board accessible behind the cabinet. So, with everything set up like this, I waited for the fault to re-appear. The set then ran for weeks without any sign of trouble but eventually it did misbehave. When it did, I tried twisting and bending the control board and this provided the first confirmation that the fault really was on this board. I could more or less create or cure the fault in this way. But I say "more or less" because I could not establish any exact pattern that would affect the set's behaviour. On some occasions, the fault seemed to be at one end of the board, while at other times it seemed to be at the other end. And sometimes twisting was the most effective, while at other times bending seemed to work. Whatever it was, I just couldn't pin it down. But with the board clearly established as the culprit, a replacement board seemed the logical answer. The only problem with this was that, when I called in on Samsung, there were no boards in stock. Instead, they were "on back order, delivery date indefinite". How often have I heard that phrase! At this point, I had two choices: either wait, for who knows how long, for a new board or fix the old board myself. If it was to be the latter, then I needed help more than ever. I tackled 44 SILICON CHIP the service department again, determined to check out everyone if necessary in order to get a clue. Fortunately, the first bloke I buttonholed was quite helpful. He listened carefully and didn't make any snap diagnosis. On the contrary, I could tell by the expression on his face that the wheels were turning. Yes, he had heard of such a fault. Well, that was the good news. The bad news was that it was so long ago and had reached him by such a devious path, that he couldn't recall much in the way of detail. In fact, he couldn't recall anything initially but, after much brain racking, he came up with one thought. It concerned the wire links on the PWB-CONTROL board and, in particular, the possibility of two of them touching. And that was all the help he could offer. I thanked him and went on my way. At least it was something but, on the other hand, it didn't seem to make TETIA TV TIP Hitachi VT-640E VCR Symptom: Machine does not want to accept cassette. If the cassette is pressed into the cassette carrier and held there for several seconds, the machine will accept it briefly then immediately eject it. A repeat performance may finally persuade the machine to accept the tape. Cure: Replace the tape end sensors in the cassette housing . Faulty sensors give the control microprocessor the wrong information and makes it think a cassette is already installed. A clue is that the machine can be put into play mode without a cassette in place. TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute ofAustralia. Contact Jim Lawler, 16 Adina St, Geilston Bay 7015. much sense. There are several links on this board but I couldn't imagine how any of them could touch. This was confirmed when I checked the actual board - there was no way any of the links could touch each other, or anything else for that matter. Yet, for all his vagueness, the technician had been adamant that the fault involved the links. Display unit More or less in desperation, I went back to my twisting and bending routine. The set's behaviour was much as before except that I gained the im, pression that the area around the channel number display unit (top left of Fig.1) was marginally more sensitive. This didn't make sense. The display should have no electrical connection whatever with the control functions. Nevertheless, I had to be sure. The display unit is a dual-inline, 18-pin package, fed independently from the PWB-REMOCON board via plug/socket CNR03. This all seemed perfectly straightforward until I took a closer look at the copper side of the board around the display unit. It was easy enough to account for the copper tracks running to the display unit but there were other tracks as well, with several connections coming through from the component side that obviously did not belong to the display. Well, there was clearly no alternative; the ·display unit had to come out. It wasn't all that difficult and when I finally pulled it away, the cause of all the bother was plain to see. Hidden under the display unit were four extra w,ire links. And two of them had been bent so that, at best, they were only a gnat's eyebrow apart and, at worst, were actually touching. In this situation, almost any variable (temperature, vibration or attitude) can change the status of these links, which is a flash way of describing an intermittent. The irony was that having found the fault, it took less than 10 seconds with a screwdriver blade to cure it. But that's the way it goes; finding the fault is the real job. What caused it? But how did the faulty condition come about in the first place? It seems unlikely that the links were bent in that formation when they were fitted, so what happened? From what I could work out, it would appear that the links were sitting too high on the board when they were initially fitted, either because they had not been pushed down far enough or because they were bowed slightly upwards. Either way, they would be pushed sideways when the display module was fitted. It wouldn't have mattered if they had both moved in the same direction, but Murphy saw to it that they moved in opposite directions towards each other. And he also took care to see that they did not touch each other while the set was tested; that had to wait until it was in the field. Well, that's enough from my bench for the month. Let's now take a hop, step and a jump across Bass Straight, for a story from our colleague J. L. in Tasmania. The fishnet VCR This story comes from a colleague who lives and works in the country, 100km from the city. It concerns a Teac video cassette recorder, model MV307, with a very obscure problem. The customer complained that watching a video recording or the ABC on VHF channel 2 was like looking at the program through a nylon fish net. The commercial VHF station and SBS on UHF showed no signs of the trouble. In fact, the "fish net" turned out to be shimmering white lines that flickered across the screen in a diagonal pattern. They were never still and moved at a constant speed. In all other respects, the picture was normal and was quite watchable if one could only ignore the white lines. The customer put up with the lines while watching video recordings but took to feeding the TV receiver directly from the antenna, bypassing the video recorder, to watch channel 2. But this introduced another problem. They were in a remote area and he had come to rely on the extra gain provided by the video recorder's frontend amplifier. Without that gain, the picture from channel 2 was not so good and so he eventually decided that the recorder just had to be fixed. My colleague soon found that the fault was a fundamental one. It not only appeared on the off-air TV program and pre-recorded video tape signal, but could also be recorded onto -WHA1'"'S MORE: 11"" WAS STlt-1IHe: SAME. ~'HEAVY 'RA\N .. .,. tape and replayed on any other machine! This particular fault was not an easy one to diagnose. At first glance, it looked as though it could be a power supply problem since similar problems in TV sets had been traced to this source. However, the nature of this problem - though not the extentcould be changed by altering the tuning of channel 2 on the recorder. This suggested a possible fault in tuner. Since my friend did not have a service manual for this model, he contacted a colleague who does have a manual and asked if he would care to take over the repair. This serviceman works in the city but, after the most exhaustive testing, he could find absolutely nothing wrong with the machine. It tuned and recorded channel 2 perfectly. He did reset the tuner AGC in case of problems there but otherwise he could do nothing to help. And so my friend took the machine home and tried it again. The fault was still present, which suggested that it was related in some way to the weaker signal at his location. For a time, he thought that the trouble might be due to interference from noisy power supply lines. However, the trouble was identical at both the customer's home and at my colleague's workshop. What's more, it was still the same after heavy rain. MARCH 1993 45 '1k«,, 1:J~ ,l!:euu;e ,li:eteaae li : i:11::~:1,t:m:1:mi::i i :i l :il i:l :¼JE;l:1: 1;1:1 :1 :11il il i ffl.il !l i! : 1:1: ~11:1;:: : :i :il i ~ ll::~:m:1:1:1:!l il il i: : 1: 1 11 1:1:l:imil il l!;.1:~::i ~ ZI - I ~ OFFERING i-"'""'"!'_ 11 _ _ _i EXTRA SIZES a MORE ACCESSORIES LOWER PRICES 11 NATURAL or BLACK ANODISED FRAMES II FOREST GREY or BLACK TEXTURE COVER FINISHES READER INFO NO. 6 THE UNIQUE ZIP-RACK DESIGN SIMPLE,FAST & EFFECTIVE THATS ZIP-RACK Twelve lengths of ZIP-RACK hollow section aluminium extrusion are interconnected using black ABS plastic corner connectors. This requires only a rubber mallet or similar tool. The assembly is self aligning and can be further strengthened by 'pop' riviting the overlapping flanges at each corner. TYPE 46 RACK DATA: RACK WIDTH 540mm. RACK DEPTH 460mm. 1U (RACK UNIT) =44.45mm OR 1 3/4". FOR EQUIPMENT/PANEL CAPACITY DEDUCT 90mm FROM TOTAL RACK HEIGHT ZIP-RACK FRAME sro'.DEN[IliAPK lll 1) :; ~si~j 268 357 490 F04 F06 F09 II C12 C15 C18 624 757 890 C24 C30 C40 1157 1424 1868 6/01BK 6/02 6/03 6/03BK 6/04 6/04BK CT2 CT3 CT4 ZIP-RACK is made by "The ZIP-RACK Company" Factory 2, 7 Michellan Court, Bayswater Victoria 3153. Australia. Under licence to AUTOTRON AUSTRALIA P/L. The ZIP-RACK Designers Beware of illegitimate copies without the ZIP-RACK name. For product sales, wholesale pricing & product literature Phone: {03) 729 7255 or Fax: {03) 729 6017 SERVICEMAN'S LOG - CTD So the idea of mains-borne interference had to be discarded. In any case, the interference pattern was nothing like that normally caused by faulty mains insulators and hardware. Next up, he phoned Teac in Sydney and asked their technical department if they had any answers to the problem. He wasn't particularly pleased when their only suggestion was that he should check the antenna. I suppose that might be a typical first response to a question posed by an uninformed owner but it's hardly likely to impress an experienced serviceman. Anyway, the conversation continued for a time and eventually all concerned came to the conclusion that the trouble must be in the tuner. So a new tuner was duly delivered and fitted but to no avail. The problem continued. Another call to Teac produced the suggestion that perhaps it was a power supply problem. Nobody could offer a convincing explanation as to what was happening but an offer by Teac to investigate the supply was too good to miss. My colleague removed the supply from the chassis and packed it off to Sydney. But again it was all to no avail because Teac could find no fault of any kind. After this, the machine sat on the bench for several weeks. In the meantime, the owners had borrowed another machine. This machine worked perfectly at their location and finally scotched any suggestion of local interference being the cause of the trouble. After that, each time he came to town, my colleague would ask some of our mutual friends if they had ever seen the trouble. He had a recording of the effect which he would play to anyone who showed the slightest interest in the problem. It almost reached the point where he took on the nickname of "Fishnet". But nobody had ever seen or even heard of the problem. · And so time passed and the machine sat gathering dust, until one day my colleague was called to another job which really did involve mains-borne interference. In this case, the owner had obtained a copy of the Department of Communications booklet on TV reception problems. He was into self-help and only called on my friend when he reached the stage of needing professional help. It was while browsing through the DOC booklet that my friend came across an idea that he felt might be worth trying on the Teac recorder. This suggestion involved winding a TV receiver power cord around a ferrite rod to minimise mains-borne interference into and out of the set. Back at his workshop, he lost no time in salvaging an antenna rod from an old portable radio. It was about 13mm in diameter and 100mm long. He wound the power lead onto the rod, fitting as many turns as possible. At switch on, he was amazed·to see that all traces of the fishnet interference had disappeared. The picture on all channels was as clean as one could ever wish. Just to make sure, he began to remove turns from the rod. The first three of four turns made no difference but then a faint trace of the fishnet pattern became visible. And, as he removed further turns, the pattern became stronger and stronger, until it had returned in all its annoying glory. He repeated the experiment at the owner's home and found that it worked just as well there. So the machine was eventually set up with a dozen or so turns of its power lead wound around a ferrite rod, tucked into the back of the video cabinet. We have had long and detailed discussions on the problem and the most likely answer is that it is a power supply fault. Radiation from the recorder's switchmode supply is one possibility, perhaps caused by the accidental omission of a suppression component during assembly. Ferrite beads are often fitted to the emitter leads of chopper transistors for just such suppression purposes , so this suggestion is not entirely fanciful. The problem is that the fault doesn't exist in strong signal areas. This means that it would never have been detected during factory tests, nor would it have ever been detected if the machine had gone to a city-dwelling owner. At this point my colleague doesn't feel like recalling the machine and working over the power supply. If it ever comes in again, he will look into that possibility but for the time being the makeshift mains filter solves the problem. Footnote And now for a footnote. At a recent meeting of the Tasmanian Division of TETIA, the fishnet problem was raised yet again. Discussion circulated ad nauseum but one member joined in with more than a little interest. It turned out that he had an identical machine with the same problem in another low signal area. Our colleague could not wait to get home to try the ferrite rod exercise. We've not yet heard whether it was succei,sful. So far, on the basis of just one unit, the idea of a factory error appeals strongly. However, it could be a design problem - the Teac MV 307 is a fairly new model, so there may not be too many of them in country areas where weak signals . allow the problem to reveal itself. It will be interesting to hear if the fishnet turns up in other places. SC MARCH 1993 47