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Serviceman's Log

The one-day TV technician

By The TV Serviceman

I had a 2002 Sony PFM-42B2E flat panel plasma monitor come in with lines on the screen. This unit uses a Fujitsu PDP (plasma display panel). Fortunately, we had another identical set which had just been fixed and was on soak test, so we were able to swap modules and isolate the problem to the B Y-SUS board.

As described in a previous article, when working on these sets, we use the cardboard packaging that protects new plasma screens. The raised foam blocks glued onto this cardboard support the screen, while fragments of mirror tiles placed between the blocks enable us to observe what is happening on the screen during the course of the repair.

A small problem is what to do with the enormous amount of screws and hardware that accumulate as you strip down the machine to access the boards. Most times, you have to have an array of plastic boxes to put all this stuff into. Unfortunately, with the pressure of work and the demand for quicker and quicker service, the screws are invariably left resting on the back face of the plasma panel.

Items Covered This Month

  • Sony PFM-42B2E plasma monitor
  • Sony KV-E29SN11 TV set (BG1L chassis)
  • AWA W6900S/SF MkII TV set
  • Sanyo WF2-00 TV set (EB7-A32 chassis)
  • LG CA-20F898 TV set (MC-994A chassis)
  • Panasonic Dimension 4 Genius Convection Microwave oven NN-C855B
  • Panasonic PT-LC50E LCD projector
  • In this case, I was under pressure and being distracted by a colleague who has a strong accent and can sometimes be difficult to comprehend. Anyway, I unscrewed the metal screening cover and instead of putting it to one side, I left it lying on top of the power supply (Switching Regulator APS-179). And as you can probably guess, when I put the new board in, I switched the set on to check whether all was OK and completely forgot about this screen.

    Click for larger image

    Of course, that was a big mistake. The metal screen shorted out one of the live heatsinks to an earthed heatsink, which completely killed the set.

    My colleague thought that this was hilarious. "Mate", he said with this heavy accent, "you know why you are called the wonder technician? Because you are one day technician, no? Ha, ha!"

    Oh well, humour is all in the eye of the beholder. One day, I’ll give him his "one day technician" stuff right back in his face!

    OK, so I’d started with a working plasma panel, got it to give a good picture and then, because of my carelessness, had made it to stop working altogether! However, it really wasn’t that bad – power was getting into the board and some voltages were present. What’s more, there were no obvious burnt components, nor had there been any major noises or explosions when it failed.

    Encouraged by this, I removed the board and checked all likely fuses and fusible resistors. Nothing was immediately obvious until I measured R101, a 10Ω resistor, which was open circuit. Replacing it fixed the problem and I was back in business.

    But I was lucky – you really can’t afford to be careless or complacent when working with this kind of gear. And I’ll remember my colleague’s "one day tech" jibe.

    Storm damage

    TV sets hit in electrical storms are always dodgy – especially when it comes to warranty claims later. Lightning, being the erratic animal it is, can damage all sorts of odd circuits in a set, causing all sorts of bizarre trouble. And it’s not just the parts that immediately fail that cause problems. Many other parts can be put under so much strain that they barely survive but then fail later.

    Recently, I had a Sony KV-E29SN11 (BG1L chassis) which was dead after an electrical storm. The switchmode power supply was still working, though its output was low.

    I removed the screen covers over the microcontroller and found that the 5-pin 5V IC regulator (IC601, L78LR050-MA) had burnt up, creating a hole through the board. I also found that a 22Ω 2W resistor had been fitted where link JW158 is shown on the circuit. This resistor had been getting very hot from the current it feeds to IC601.

    Further up the food chain, I found that Q601 (2SA1315-Y) and R606 (18Ω) were similarly getting hot supplying current for IC601. Interestingly, this is only a back-up circuit for the main 11V rail that also feeds IC601 via a diode. This back-up circuit is also part of the power-on circuit and the 135V control circuit (which caused the 135V rail to drop).

    I replaced all the parts and though I had some success, the results still weren’t satisfactory, with too much current still being drawn – probably by faulty internal diodes and zener diodes in the microprocessor IC set on the 5V rail. In the end, this set was written off and the client got a new one with the insurance money.

    An easy intermittent

    A Chinese-built 2002 AWA W6900S/SF MkII came in with an intermittent no picture fault.

    Of course it took a very long time for the fault to re-occur and as luck would have it, it only appeared just after we had told the client to come and collect it. Fortunately, it was an easy fix; the CRT filaments were intermittent and the fault was traced to a hairline fracture on R920, the 1Ω series resistor from the flyback transformer to the CRT heaters.

    I wish that all intermittent faults were as easy as this.

    Doing a Lazarus

    A newish Sanyo WF2-00 (EB7-A32 chassis) came in DOA (dead on arrival) and still under warranty. Our job was to raise it from the dead – just like Lazarus.

    Click for larger image

    Anyway, the set’s switchmode pow-er supply had spat the dummy, blowing chopper transistor Q613 (2SC4429), its driver (Q612, 2SC3807) and the surge resistor (R695, 1.8kΩ 7W). These parts were all replaced and the set indeed rose from the dead.

    The only awkward part of this miracle was that its OSD was in a foreign tongue. I didn’t have the instruction book but I struggled on with the menu before I worked out that it was stuck in Croatian! Once I knew what I was doing, I was able to set up the tuning for the local stations.

    Personally, I feel that all manufacturers should get together for a convention on the OSD language options and colour code, or place them in the same position in all sets. It would make it all so much easier if, for example, English was always in red and Croatian in orange (say) and if the language option was always the fifth item on the menu list. Still, who listens to the technician?

    The ticking LG

    A dead 1999 LG CA-20F898 (MC-994A chassis) 48cm TV was brought in to be repaired. Nowadays, these sets really aren’t worth looking at, considering the price of new ones, but the owner insisted.

    Anyway, this set was ticking (pulsating), which meant that the switchmode power supply was in protection mode, probably due to a short circuit.

    In a beachside suburb, the usual culprits are the flyback transformer and the line output transistor which would make the cost prohibitive in such a budget model. However, the owner was persistent – he felt sure it could be fixed within his budget and told me to do the best I could.

    Well, of course, the line output transistor (Q402, 2SD2499) had gone short circuit so I replaced it, half-expecting it to instantly destruct again at switch-on. Instead, I got a vertical white line down the centre.

    This was unusual and is almost always due to a dry joint in the deflection stage – which in a 48cm set is really simple to fix. However, there were no dry joints and the yoke and the horizontal linearity coil were both OK.

    That just left yoke coupler C412, a 0.33μF 400V high-current capacitor. Replacing it fixed the problem and the repair came in within budget – a win-win situation for both of us.

    Squashing the cockroaches

    Edith Montague is in her late seventies and still drives a car – well, barely. One hot summer day she arrived at our place unannounced and her first words to me were that she had got horribly lost and it was all our fault.

    We are obviously in the wrong place and all our signs are invisible!

    When she finally finished blowing us up for getting lost, I learnt that there was a microwave oven in the back of her car, which Edith repeatedly told me would not give a display. I carried the microwave into the workshop for her and placed it on my bench.

    This particular unit was a 1995 Panasonic Dimension 4 Genius Convection Oven, model NN-C855B. I plugged it in and the display came on straightaway. What could she have been thinking? Well, it might be intermittent so I thought I would give it the once over and took the covers off.

    As soon as I did this, you could hear the rustle as hundreds of cockroaches ran for cover. I smartly replaced the cover and took it outside where I sprayed a large amount of insecticide inside and beat a hasty retreat. The effect of this poison was pretty rapid and I subsequently blasted the corpses out with an air compressor before carrying the oven back into the workshop.

    Click for larger image

    The cockroaches alone would be easily enough to cause corrosion, in turn giving an intermittent display. However, the problem had to be investigated further, so I removed the front control panel and the PC board. When this was out, I removed a few more jammed corpses and egg sacs before examining the PC board itself. This carried the general mess from a cockroach infestation and there was also some of the infamous brown glue spread over some of the components. And as if that wasn’t enough, there were dry joints on pins P and E of the mains transformer (T1).

    I also noticed that one of the oven’s lamps had failed and so gave her a quote to fix all the above. She wasn’t prepared to just take my word for it though and wanted a detailed explanation of all the faults. However, after having a good whinge about the very reasonable estimate I had given her, she finally agreed to the work.

    The job was straightforward and when I had finished, I gave it a thorough test to make sure it was working properly. The display was now excellent, especially after a good clean on the outside of the window. I then advised Edith that the oven was ready and she came straight over.

    When she eventually arrived, I got another tongue lashing over her getting lost a second time. Now, I can understand someone getting lost once but twice is somewhat careless. After all, we have been at this address for over six years and no-one else had ever got lost.

    I wasn’t in a position to demonstrate the oven but I assured her that the display had been fixed and that I had tested it. With that, she paid and left, presumably to get lost on the way home. Anyway, I thought that would be the end of the matter but it wasn’t to be.

    A few hours later, Edith was on the phone, not only complaining vociferously that she still had no display but demanding that I come round immediately and fix it in her home. I was incredulous. How could this be? There had to be another factor. "Are you quite certain there is no display?", I asked. "Absolutely", she insisted.

    Perhaps she hadn’t plugged the oven in – after all, that would give no display. I asked her if the light came on when she opened the oven door and was told that it did, so scratch that theory.

    Click for larger image

    This had me baffled. As a compromise, I grudgingly said I would call around but explained that I had no facilities to fix anything in her home. If there was a genuine problem, it would have to go back to the workshop.

    When I got to her place, I finally understood what she meant by "no display". The display wasn’t blank at all but was instead showing just the two little dots which form the colon between the hours and minutes digits. And that to her meant no display.

    I switched the unit off and then on again at the power point and the usual "WELCOME" display message immediately came on. However, it would then go into "9 LAMB" and 10 CHICKEN" alternately. Apart from the STOP/RESET button, nothing else worked and I was just left with the time colon. I couldn’t even set the clock and it was fairly obvious that the membrane switch was jammed on the 9 Lamb/10 Chicken selection.

    Back at the workshop, I removed the membrane switch altogether and it was indeed faulty. Switch Q17 was intermittently short circuit and had probably only previously worked in my workshop because it had been a hot day, which kept the air expanded between the two halves of the membrane switch.

    Clearly, the unit would have to be replaced but by now, Edith had lost confidence in both me and her microwave oven. She phoned to inform me she had bought a brand new microwave and asked what part of her money was I going to refund because she "hadn’t got anything for it"!

    There was really no point arguing but I still wanted to recoup my labour costs for the work I had already done. Anyway, I told her that if she left her microwave behind, I would tear up her cheque.

    For some reason, this didn’t register with her and she just kept repeating her question over and over again – like a needle stuck in a groove. Eventually, I was forced to put a stop to her tirade. I bluntly told her to stop talking and listen very carefully. What had I said to her?

    Well, she couldn’t remember. So, very slowly, I repeated myself to her until the message sank in.

    She was relieved to get her money back and I got to keep the old oven as compensation for my time and effort. In the end, I replaced the part and then sold it, so I did manage to make some money after all.

    LCD projector

    We had a Panasonic LCD projector PT-LC50E come in requiring a new lamp unit (Part No: ET-LAC50 – a mere $600!)

    However, after replacing the special globe, the projector intermittently wouldn’t work, giving no functions except an error code CØ in the display. This means an incorrectly installed air-filter.

    After fiddling around with the unit for a while, we discovered that the cause was due to the door and its sensor switch. However, measuring the plug with an ohmmeter showed there was perfect continuity when the switch was closed.

    This was quite baffling until it dawn-ed on us that when measuring the plug, the meter probes were pressing in the pins and making contact. The fault was actually in the plug itself, with the connectors being dry jointed. We just kept missing this with the way we were measuring it. Resoldering them fixed the problem.

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