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SERVICEMAN'S LOG The TV set that smoked Warranty work is not always plain sailing, due mainly to the over-reaction of some customers when a near-new set fails. And in this case the set didn’t help matters much by first smok­ing and then working normally. Mr and Mrs Clarke were not happy. Their nearly new NEC FS-5185 TV (MM-2 chassis PWC4034A) had thoroughly disgraced itself and had had the audacity to not only stop working but actually smoke in their living room. Anyway, the set was under warranty and though they would really have preferred a new set, they had to have it fixed straight away. On these occasions, one has to be as polite and understand­ing as possible, whilst remaining aloof and firm as to what can and cannot be done under warranty. One way to reduce the acrimony is to attend to the problem as soon as 56  Silicon Chip possible. The set was actually under an extended war­ranty with a major department store, for which we are the service agents. So it was with trepidation that I removed the back and had a good look around before connecting it up, switching it on and standing back. Well, what an anticlimax – the set came on and performed perfectly. What’s more, when I examined the one-piece motherboard, I couldn’t really determine any components that had burnt out at all. A number of resistors had got a bit warm and some of the white/clear glue the manufacturers now use to hold wires and components in place while they are soldered had melted, but really there was nothing to write home about. I got onto the blower to report the situation to the Clark­es. They were not amused. Apparently “clouds of black smoke had come out of the set” and, what’s more, they were definitely not going to have that set back their living room unless it was fixed properly. I went back and gloomily examined the hapless set once again. First, I checked the HT rail and it was spot on 115V which is correct. Next I attacked the circuit board; I shook it, froze it and heated it and it kept right on going with a really good picture. I then put it aside on test and left it for three days. It didn’t miss a beat. I phoned the Clarkes again – “are you sure that the smoke came out of this set? Was it perhaps the VCR or the stereo system which are nearby? What about the power point? Does anyone in the family smoke?” Fig.1: the power supply circuitry in the NEC FS-5185 TV set. IC602 (bottom, right) monitors the HT rail and drives optocoupler IC601 to derive a feedback control signal for chopper transistor Q601. Mrs Clarke assured me that none of these had been the case. I was perplexed but even after a week the set was still working perfectly and so I returned it to the less-thanunder­standing Clarkes. The set bounces It was nearly a fortnight later that the set bounced right back into the workshop with the same complaint. This time, fortu­nately, the fault was genuine – the set really was dead. It didn’t take long to establish that the 130V ZD621 R-2M safety zener diode was short circuit across the main HT rail. And for this to fail, it meant that the HT rail had risen above 130V. I replaced the diode and the set once more came on perfectly with the correct voltage. However, when I replaced the cabinet back and put it aside to test, the set was dead again. Ah, ha, I thought. It must be a faulty back! But no; actu­ally the zener had gone short circuit again. Rather than risk yet another, I foolishly decided to run the set without it and initially all was OK. However, predict­ably, as soon as my back was turned, the B+ rose dramatically and wisps of smoke (but not a lot) came from numerous areas. This time, the line output transistor (Q502, 2SD2499) had gone, along with a couple of electrolytic capacitors (C313, C622). D631 had also failed by going short circuit. After restoring everything, the volt- age was constantly high and the set could only be run for a few seconds before the damage would re-occur. Because of this, I used a Variac and a lamp to keep the voltage down while I sussed the problem out. My first suspects were IC602 (SE115N) and IC601 (a PC817 optocoupler) which are at the heart of the voltage control feed­ back network. Basically, IC602 monitors the HT rail derived from transformer T601 and drives optocoupler IC601 which in turn varies the drive to chopper transistor Q601 (2SD1710). I replaced both ICs and the associated 22kΩ resistor (R623) and resoldered everything in sight, in case a dry joint was lurking somewhere but to no avail. In fact, the control feedback circuit of IC602 and IC601 was responding to voltage changes and I could see the mark-space ratio change on the collector of the 2SD1710 chopper transistor (Q601) as the Variac was altered. However, it refused to lock and there was no control of the secondary voltage rails. Well, to cut a long story short, I was removing and check­ing every component on the primary (“hot”) side of the chopper transformer when I noticed that the board had been slightly modified by the factory. Some of the PC board tracks had been cut and a number of components had been mounted on the print side of the board but the circuit was still the same as published by NEC. A closer examination revealed that some of the parts on the component side around this area were rather awkwardly mounted and so I decided to remove these parts one by one for checking. When I desoldered one leg of R699 (120Ω, 1W), I noticed that the other leg simply fell away from the board. In other words, one leg of this component had not gone right through the hole but was actually just touching the solder pod on the other side. The resistor checked out OK so I rein­stalled it, this time making sure that both its leads were cor­rectly soldered. And that solved the problem. This time, when the Variac was wound up, the mark space ratio on the collector of Q601 became fixed when the HT reached 115V and the power supply was once again stable. I soak tested the set for another week whilst fending off the ever-persistent Clarkes. I had, in fact, given them a loan set when the problem re-emerged but it is the nature of some customers to be impatient. The set was eventually returned but the Clarkes remained totally unimpressed with my efforts to help them. You win some and you lose some. The Beovision gear Bang and Olufsen, or B&O, are among the “Rolls Royce” brands when it comes to producing home entertainment equipment and this is reflected by their prices. Their products are beauti­ful to look at, even when switched off, and their performance is definitely up-market – all of which you would expect for the prices demanded for this luxury equipment. B&O has been quite innovative with some of their technolo­gy over the years and often have neat little features built into their equipment; eg, wave your hand in front of the CD player and the drawer will open! Their Beolink system, which allows you to have different selected music in every room of your house, was one of the first designs of this nature. On more familiar turf, their TV sets introduced automatic grey scale adjustment years ago. And of course they have an amaz­ing ability to cram electronics into very thin, unobtrusive cabi­nets with hardly any wires protruding. But I digress – I am beginning to February 1998  57 Serviceman’s Log – continued sound rather like one of their salespersons. Mrs Smythe-Jones was from the old country and lived in an exclusive suburb along with her pedigree Siamese cats and of course, her Beovision Type 3854 2502 stereo TV, her Beocord VHS91 type 4493 VCR and her Beolink 1000 hifi system. These approx­imately 10-year old items had been reinstalled by B&O when she arrived some years ago and now, because of their age, they were beginning to show signs of trouble. How much is it? Despite her obvious affluence (or perhaps because of it), she immediately enquired about the price of servicing the equip­ment and wasn’t too impressed with my answers. However, she was in too deep with her investment in B&O and so, reluctantly, she instructed me to go ahead and sort out the problems. Living in an exclusive suburb does have its drawbacks, one of them being poor TV reception due to the tall trees that char­acterise the area. Installing a separate high-performance UHF antenna certainly improved things a lot but her main complaint was poor reception through the Beocord hifi video recorder. In particular, she complained about the picture which had a vertical line down the screen about 5cm from the lefthand side. And the video tuner gave a snowy picture with lots of patterning. I decided that the only way to handle all this was at the workshop, as both the TV set and the VCR are connected via a SCART lead. In addition, the remote control for the TV set con­trolled the VCR and the stereo hifi sound. The first thing I did was track down B&O’s service and spare parts section in Melbourne and order in the service manuals and instruction books which cost $120. To give them their due, they were nicely bound and thick. Unfortunately, what ever they gained in presentation they lacked in detailed substance. For example, there was no circuit description and no PC board compon­ent layout diagram for the TV set although there was for the VCR. As it turned out, the latter was made by Hitachi for B&O and is similar to a 1986 VT-860E(AU). The B&O service manager was very helpful and suggested various courses of action, including fitting the various serv­ice/modification kits. I started with the Beocord VCR which I connected to a dif­ferent TV and tuned in on approximately Ch37. The patterning and snow was easily fixed by replacing the electrolytic capacitors (as was suggested) in the inverter power supply on the VS tuning board. The main culprit here appears to be C715 (100µF, 25V) which is on the +A17V rail to transformer L701 (this rail comes from the power regulator board). Interestingly, the Beolink 1000 remote control would not operate the VCR in a standalone configuration –only when con­nected to the TV. However, when I used a generic programmable remote control (Quadrant Plus) with the B&O numbers programmed into it, the VCR worked. I was about to install the service kit (3375102 at $129 plus freight) when a routine check on playback showed the hifi sound to be intermittently distorted and critical with tracking – all indicative of worn heads (normal mono sound and picture was OK). This was the death knell for this VCR, as the heads (8600097) cost a cool $600 trade price. It was too hard to work out the equivalent Hitachi part number and the risk that it wouldn’t work properly was too high. B&O advised us that the unit was too old to repair anyway and that we shouldn’t proceed. The TV set So it was on to the TV set, with the problem described as “curly beads” (the aforementioned speckled line down the screen about 5cm from the lefthand side). This is usually due to para­ sitic oscillation in the line output stage and can be fixed in most cases by fitting a ferrite bead or two to the emitter of the line output transistor. B&O had not heard of this problem in their sets before but suggested we install their service kit 3390454, which prevents failure of the line output transistor (4TR11). An S2000A was fitted here but a BU508 is shown on the circuit diagram. The kit of 10 components cost $30 plus freight and included a BU508A replacement transistor. A ferrite bead (FE1) was already in­stalled in the set. It was all fairly straightforward to fit, the only excep­tion being PNP transistor 4TR15. This was originally a BC328-25 but the replacement was a BC369 which has a different pin ar­ rangement (bce instead of ebc). Wrong leads Fig.2: the line output stage in the Beovision 3854 2502 stereo TV set. B&O now recommend a BC639 for TR15, instead of a BC328-25 as originally specified. 58  Silicon Chip You guessed it – in the melee of installing this and sol­dering any suspicious-looking joints, I inadvertently connected its collector and emitter leads around the wrong way. When give the board a good going over by re-soldering any suspicious joints. I also spent some time cleaning the board and cleaned and lubricated around the ultor cap. A few days later, I received not one but two BC328-28 PH27s instead of the specified BC369 PH72. Anyway, I fitted one of them and the picture was back to normal. As an experiment, I then refitted the original transistor only to find that it too now worked perfectly. More questions than answers I switched the set on afterwards, the picture was perfect and the “curly beads” were gone. It was only when I was checking my work and cleaning up afterwards that I realised my mistake and quickly corrected it, only to find that the fault was back. I double-checked everything to confirm that it was all as per the instructions and even fitted another BC369 I had in stock (without result) before phon­ing B&O for clarification. To cut a long story short, I was told that I shouldn’t use a generic substitute as all B&O components are carefully selected for optimum performance – in this case for its “slew switching rate”. I was also told that they had done this modification lots of times and that I was extremely lucky I hadn’t blown the line output transistor. Suitably humbled, I delicately asked if I could have a genuine B&O selected BC369 (PH72) – to give it its full title – to replace the original BC328-25 PH27. The manager kindly agreed to send me out one free of charge. In the meantime, I decided to Well, despite fixing the problem, this left more questions than answers. First, which component or components fixed the fault? I replaced the originals and undid any modifications to try to find out but it made no difference. I now think that the problem could have been caused either by a dry joint, by sparking where the ultor cap connects to the tube, or by sparking at the CRT socket. In the process of cleaning, soldering and lubricating everything, I had inadvertently fixed the problem. I remain unconvinced of the necessity for highly graded components, as the original line output transistor had not failed in 10 years and it showed no signs of stress, even with the collector and emitter leads of 4TR15 reversed. Besides, I was always told that a well-designed circuit should operate with any generic component. Doesn’t the saying go something like this? – “an en­gineer is someone who can design something for five bob that any damn fool can make for a quid”. I refitted the service kit again – after all, it had been paid for – and put it aside to soak test. We abandoned the VCR and sold Mrs Smythe-Jones a new Loewe-Opta hifi unit which did everything and more than the original did except use the Beolink 1000 SC remote control system. February 1998  59