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SERVICEMAN'S LOG It took a little longer than usual At the risk of seeming to state “the bleedin’ obvious” – as our English colleagues would say – my main story this month is an unusual one. But then they usually are. OK; so this one is more unusual than usual, if that makes sense. The story really started over two years ago but can only be told now because that’s how long it took to finalise the job. Well, as I said, it is an unusual story. The device involved was a Philips colour TV set, fitted with a KT-3 chassis. The KT-3 chassis was fitted to a whole range of Philips sets and this one was a 48cm model about 14 years old – the age being a matter of some importance as it turned out. And the complaint was simply that the set would fail inter­mittently. Sometimes it would fail at switch-on, sometimes after it had been running for some time. More importantly, from the owner’s point of view, what had begun as a very occasional prob­lem had become progressively worse. It was now likely to be off more than it was on. On the bench it was just as the customer had said; it was only nominally intermittent and faulty most of the time. But when it was functioning, it performed very well. But there was a major symptom which he had not noticed; the power supply was hiccuping away quite merrily. This symptom normally indicates an overload of some kind, such as a failed horizontal output transistor, or something in this part of the circuit. It looked fairly straightforward, as over­ loads are not usually all that hard to track down. The trouble was, I didn’t see Murphy lurking in the corner. I pulled out the horizontal output transistor and checked it. It checked OK but I replaced it anyway. I doubted that this was the culprit. I’ve never known this component to fail inter­ 40  Silicon Chip mittently; when they fail they don’t muck about but there is always a first time. I also checked the insulating washer. It looked OK but I replaced it also. What about the horizontal output transformer? Could be, except that I would expect it would start overheating if allowed to run in fault condition for even a short period. But no; no sign of trouble there. A puzzling aspect of the fault was the effect on the HT rail. This was down to around 35V but varying up to about 50V at times. This seemed to rule out a dead short and, in fact, resist­ ance measurements failed to find any evidence of short circuits anywhere. This seemed to suggest that it was either an AC fault of some kind, or something breaking down at operating voltage. But what? Replacement boards In a sense, I had come to something of a dead end. All the usual approaches to a fault of this kind had failed and I had to think of a new one. Fortunately, over the years, scrapped chassis had provided me with a useful collection of boards for this and similar chassis. So this was my next step; replace each suspect board until I found the culprit. In all, there are six plug-in boards, plus a plug-in IF pack and the ELC2060 tuner which is permanently fitted. I imag­ined swapping a couple of boards would probably be enough to give me a clue, unless I was very unlucky. And the truth is, I was very unlucky. I finished up chang­ing every board and was no closer to solving the problem. The only thing that had changed was that the fault was no longer intermittent but was now permanent. This was a minor plus in terms of convenience but no help otherwise. At this point, I had really run out of ideas. I needed time to think and there were other more urgent jobs waiting, so I put it to one side. I find it is often a help to take a break like this; one can dwell on a problem for too long and frustration clouds one’s judgement. It is surprising how often an idea will suddenly pop up when least expected. It didn’t quite happen like that this time but I’m sure that the break did help. When I pulled the set out of the corner of the bench a couple of weeks later, I could take a broader look at the problem. What had I missed; what hadn’t I checked? Well, I hadn’t checked the scan coils. That idea was a long shot – scan coil failures are extremely rare. I doubt whether I have encountered half a dozen in the last 20 years. More particu­larly, I had never had one in a Philips set. So, against that background, one tends to take them for granted. But I couldn’t take this one for granted; the scan coil assembly had to come out. It’s a simple enough operation – release the neck board and ring convergence magnets, then undo the clamp holding the scan coil assembly and slip it off the neck of the tube. My idea was to hook it up to my shorted turns tester, a shorted turn being the most likely fault. And I was right about that. But I didn’t need the tester to tell me; one glance was enough. There was a large blackened patch on one of the horizon­tal windings, surrounded by spots of green corrosion which had obviously caused it all in the first place. So that was it; the scan coils were a write off. Well, at least I’d diagnosed the problem, even if it had taken more effort than I would have liked. And the solution seemed simple enough – a new set of coils. But it wasn’t that easy. A new set of coils would be in the $100 plus category which, with labour costs, might be difficult to justify for a 14 year old set. And, unfortunately, scan coils for this set was one thing I didn’t have in the junk pile. So I rang the customer with a typical good-news-bad-news report. The set could be fixed but the cost might be hard to justify. All I could suggest was that I might be lucky enough to score a set of coils if another customer’s set was written off. But, of course, we had no way of knowing when, or even if, this would happen. He thought about it briefly, then decided that a new set of coils was not a proposition. On the other hand, he asked if he could leave it with me for the present, in the hope that another set of coils might turn up. He is a good customer, so I readily agreed although I wasn’t very optimistic. And that is about the end of that part of the story, which all happened over two years ago. I relegated the set to spot in the junk store and more or less forgot about it except when another such set came through the workshop. Unfortunately for this customer, they were all routine jobs. Back to the present Which brings us to the present time. And to another charac­ter who became part of this story. I’ll call him Lance for con­venience but that is not his real name. He is a young married bloke who, to put it mildly, has had a pretty rough trot. Lance’s main activity is repairing or rebuilding discarded sets which he donates to a local charity that has stood by him and his family over the years. His main source of scrapped sets is a dealer/serviceman in a nearby suburb but he also drops in on me – and some of my colleagues – from time to time for a word or two of advice and to scrounge a few parts from stuff earmarked for the tip. Soon after I first met Lance, I raised the matter of a set of scan coils for the KT-3, asking him to a look out for such a set. That was over a year ago and he had had no luck whatsoever. Then, a couple of months ago, Lance walked in with what looked like a complete KT-3 set. In fact, it wasn’t complete, consisting only of the cabinet (in very good condition) picture tube and scan coils. There was no chassis. A visit to my store room produced a chassis plus a set of boards and so we had the makings of a complete set. Unfortunate­ly, it didn’t work out that way. When we fired it up, the set went into a hiccup mode exactly like the previous one. I wasn’t sure it was the same fault, of course, and I made a few routine checks initially but I could find nothing else ob­viously wrong. So off came the scan coils and, yes, that was it; August 1995  41 exactly the same pattern of corrosion and self-destruction. Which put us back pretty well to square one, except that we now had two sets needing scan coils. The only difference was that set number two was something of an unknown quantity – we had no way of determining the condition of the picture tube. Neverthe­less, we agreed that it was now worthwhile looking for two sets of coils. That was being somewhat optimistic, I suppose, but he did eventually find one set of coils, which he turned up with a few days ago. They looked to be in good condition, so I lost no time in fitting it to my customer’s set. And it worked. However, before doing any setting up, I pulled the coils off again and fitted them to Lance’s set. Unfor­tunately, it wasn’t such a good result this time. The tube was a write-off, one gun being completely dead. I even tried boosting it but to no avail. So the coil assembly was refitted to my customer’s set and I went through 42  Silicon Chip the setting up procedure, after which the set delivered a first-class picture. Finally, I rang the customer and advised him to come and collect the set. He was happy that the repair bill had been kept to a reasonable level and I was happy to finally be reimbursed for my time and effort. Regrettably, Lance didn’t come out of it quite as well, although he is still looking and hoping. But he needs both a picture tube and a scan coil assembly now, so he will need to be extra lucky. That said, he has scored a chassis and learnt something about scan coil failure and the symptoms it produc­es, so it hasn’t been a completely wasted effort on his part. All of which just goes to show what can be achieved if the customer is prepared to wait. But I must concede that this has to be regarded as a one-off; one that just happened to work out. The flasher My next story is about a lady customer who was troubled by a flasher. No, not one of the raincoat mob – rather, a TV set. To be more precise, it was a Samsung 51cm colour set – a model CB-515F fitted with the P-50F chassis. According to the lady, the problem was random white flashes on the screen. Initially, this happened only occasionally and she put it down to interference from some external source. This theory was reinforced by the fact that she lives in a gully which is a relatively poor signal area. More recently, the problem had become more frequent and, at times, much worse. Sometimes, it was so bad as to make the pic­ture virtually unwatchable. Still thinking that it might be interference, she took the opportunity to operate the set while staying with a friend in a much better signal area. And it did clarify the point; the problem was just as bad in this location. And so the set finally landed on my bench, along with the above explanation. I put the set on the air while the lady was still in the shop but, of course, Murphy was lurking in the corner – it behaved perfectly. All I could do was suggest that she leave it with me, which she did. I set it up in a corner of the bench and it Fig.1: the front end of the Samsung CB-515 colour TV set. The tuner (TU001) is at top left, the IF IC (IC101) at lower right, & the SAW filter & its associated components below the tuner. ran for several days without any sign of trouble. But then came the first hint; a brief white flash, no more that a few centimetres long on one line. Blink and you’d miss it. Nothing more happened for about a week then, one day, it really turned on an act with flashes all over the screen. These became progressively worse until it was quite unwatchable. Well, at least I’d seen the problem and that is always useful. And, in fact, I’d already made a tentative diagnosis – I was sure it was a front-end problem, most likely the tuner (TU001). Fortunately, this was a relatively simple theory to prove – or disprove. I had a spare tuner on hand and it was easy enough to substitute it. And the set ran perfectly after that – for about a day and a half. Then it was back in flashing mode. OK, but I still felt sure that it had to be somewhere in the front end. So the next thing to try was the transistor in the first IF stage (Q161, 2SC388), mainly because it was a simple operation. But again, no joy. Desperation measures Things were looking somewhat desperate now but there was one good point; I had a good stock of spares from previously junked sets, which meant that I could replace almost any compon­ent, at least in the front end. This is a useful approach in such circumstances but it can be time consuming. So, in turn, I replaced the SAW filter (Z101), it’s asso­ciated matching transformer (T101), the video detector coil (T171), the AFT balance coil (T172) and, in desperation, the IF IC (IC101, LA7520). As readers will appreciate, all this took a lot longer than it does to write about it. In fact, the entire process took several days, taking into account the time taken to monitor each change. And in the end it was all to no avail; I was back to square one. So was it in the front end? Or could it be a fault in the horizontal scanning system; a breakdown or flashover which was generating interference? I rang the Samsung service department and spoke to a contact there who has always been very helpful. He wasn’t able to offer any ideas based on actual cases but he did agree that the idea of interference from the scan system was worth investigating. And he went on to offer some ideas as to how the front end could be operated with the scanning systems shut down. As a result, I finished up with the rear end of the set shut down and the front end operating on 12V and 33V from a bench power supply. I then hooked up the CRO to monitor the IF envelope – it has enough bandwidth to do this – and fired up the front end independently. Glitches Sure enough, in the fullness of time, I could clearly see a succession of glitches on this envelope. I had no doubt now that I had been right the first time; the fault was somewhere in the front end. But where? I had checked or changed all the likely components in this section. Or had I? No, there was one section I hadn’t checked; the channel selector pushbutton assembly, shown as PWB-SELECTOR. There are in fact two versions of this unit, an 8-key assembly and a 12-key assembly, the latter being the one shown. This assembly carries the push­ buttons which are used to select the preset channels, plus the preset controls themselves. The pushbuttons are shown in the centre of Fig.2, August 1995  43 SERVICEMAN’S LOG – CTD designated as SW01, SW02, etc. They have two sets of contacts, those on the right performing the channel selection function and those on the left selecting the appropriate band. These latter contacts also activate the associated indicator LED (DL01, DL02, etc) from the 12V rail. These LEDs are connected in series and all but the wanted one are shorted out. There are two preset controls for each channel. On the extreme left are the 3-position band selector switches, marked VL (VHF low), VH (VHF high) and U (UHF). These are preset for the band appropriate to the channel chosen for that position. They are connected to the 12V rail when a channel button is activated and apply base voltage to one of three transistors – SQ101, SQ102 and SQ103. The selected transistor then turns on and connects the main 12V rail to the appropriate section of the tuner. The chosen channel is selected by the corresponding vari­able resistor on the right (VR01, VR02, etc). These are fed from the 33V rail and feed an appropriate voltage to the varicap diodes in the tuner (terminal VT). As an example, switch SW06 is shown in the active position (with the LED illuminated), VR06 is connected to the 33V rail, and the 12V rail is 44  Silicon Chip connected to the VH position of the band selector switch. All of which should give the reader some idea of the com­plexity of these assemblies. And to be truthful, they have more than their fair share of troubles although, until now, these have all involved channel selection problems. These are easy enough to diagnose and there is only one practical solution; replace the entire assembly. However, I have never experienced, or heard of, these units causing the kind of trouble evident in this set. But, with all other likely culprits exonerated, this one had to be a suspect even though it was something of a long shot. Fortunately, I had a spare unit on hand and this was duly fitted. And that was the answer. The set was run for several days with no sign of the fault and I eventually returned it to the customer. But I warned the lady to contact me immediately if it should reappear. That was many weeks ago and all is quiet so far. More flashers That wasn’t the last of the flashers. Within a few weeks, I had no less than three more and all from the same cause. The only difference was that it involved different bands; one was on low VHF, one on high VHF, and one Fig.2: this diagram shows the channel preset & channel selector circuitry in the Samsung CB-515. Note the active setting for switch SW06. on UHF. And they were not all CB515s; one was a CB-349 and one an Akai CT-K115, both of which use the P-50F chassis. Granted, these were the bands favoured by each customer and my checks confirmed that the fault occurred only on the particu­lar band. And there was a fourth set with an even weirder fault in this section. The set worked normally on UHF but suffered from very low gain on low VHF. For some reason (probably due to leakage), when it turned on the low VHF transistor (SQ101), it also turned on the high VHF transistor (SQ102) at the same time. The effect was to completely wreck the low band gain. So be warned; any similar funnies and you’ll know where to look. SC