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SERVICEMAN'S LOG The fireball TV set from hell It might seem over-dramatic to describe a rather ordinary looking NEC 51cm TV set like this but this was indeed a wolf in sheep’s clothing. And it lead me on a merry chase to discover the cause of the problem. The set, an NEC N4840 using a Korean Daewoo C-50 chassis, was brought in by a young, brusque woman who was succinct and to the point: “it smoked, burned and then went black”. I barely got her name and address before she left as quickly as she had ar­rived. I was rather busy at the time and a couple of days passed before I was able to examine the set. It didn’t seem wise to switch the set on immediately, as her description of the fault suggested that there may have been a fire. As such, it would be all too easy to exacerbate the problem and, in any case, there would probably be obvious visual evidence of the damage inside. And so it was that I gingerly removed the back, carefully examined the various circuit boards, and tried sniffing for the telltale smell of fire. But there was nothing. The set, which I guessed was about seven years old, was reasonably clean and everything look OK. I especially examined the flyback transformer and power supply circuitry but all was fine. Perhaps the damage was on the inside of the deflection yoke but that would have to wait for the moment. Eventually, I concluded that there was nothing for it but to go for a smoke test – in this case, literally. I plugged the set into the power and switched it on. At this stage, I didn’t know what to expect but the result was something of an anticli­max. The set momentarily spluttered into life and then died – no sign of smoke or flames or anything dramatic yet. OK, so where to start? My first volt- age check was at the collector of the line output transistor. This measured 103V which seemed reasonable and so I switched the set off and fished out the file on NEC/Daewoo sets. Unfortunately, the only circuit diagram I had was an abysmal photocopy with virtually illegible component values and type numbers. However, by using a magnifying glass, I could just discern that the B+ rail was indeed 103V which meant that the power supply was probably OK. So why was the set dead? Well, maybe because a safety cir­cuit had turned off the horizontal oscillator. I initially con­firmed this by measuring the collector voltage of the driver transistor in this stage – it was floating at the B+ potential, which meant that it wasn’t turning on and off. What’s more, a quick check with the CRO showed that there was no waveform on pin 20 of IC I501 (TA8718). that was going too high. To test the latter theory, I decided to try using a Variac to reduce the mains voltage before switching the set on. Of course, the input voltage can only be reduced so far. The set is remote controlled and if the input voltage is set too low, the microprocessor is starved of voltage and will not switch the set on. Nevertheless, I ploughed on and swung the input voltage up to 110V AC. Before switching on though, it is necessary to secure the PC board. Normally, this is held into the front of the cabi­net shell by pressure from the back. As a result, if you try to switch the set on without the back in place, the pressure on the mains switch can be enough to push the whole chassis back inside the cabinet without the power actually coming on. To overcome this problem, I held the rear of the chassis and the very edge of the PC board with one hand, taking extreme care not to touch any of the parts or copper tracks. By this stage, I was really beginning to feel relaxed about the repair and that the Tricks of the trade Because the circuit diagram was so poor, I could not discern how the protection circuit worked or even where it was. Basical­l y, there were two possibilities to consider: (1) either the protection circuit itself was faulty; or (2) the protection circuit was shutting down the horizontal oscillator in response to a voltage January 1997  69 Serviceman’s Log – continued symptoms had been over exaggerated. I was wrong. When I pressed the power switch, the set powered up . . . and up . . . and up, until there was a terrific “crack”. I jumped away, partly in response to this “crack” but mostly due to an electric shock that I received from the two places I had been touching the set. And it continued to crack and spark until I recovered my senses sufficiently to dive for the mains wall power socket and switch it off. It would hardly be an exaggeration to say that we had too much voltage! Now I was going to have to be much more cautious. Obviously the EHT was far too high and it was arcing everywhere – even across the plastic insulation and onto me. The shock wasn’t severe, except perhaps to my wounded pride. EHT checks After a fright like that, it was time for some heavy-duty armour. After checking that the .0056µF high-voltage capacitor across the line output transistor was OK, I reached for the 70  Silicon Chip EHT meter and connected it to the EHT output lead at the ultor cap (ie, where it plugs into the tube). I also connected the multi­meter to the B+ rail so that I could monitor this voltage as well. There was no way I was going to touch the PC board again. This time I wedged the chassis into the front of the cabinet with an old defection yoke rubber positioner and turned the variac down 100V. Wearing a rubber glove, I switched the set on and watched the meters. Interestingly, the multimeter showed that the B+ rail initially rose to +103V for a second or so and then continued to rise even higher to over 200V (fsd on the meter). Similarly, the EHT paused momentarily at about 22kV and then rose to over 30kV, at which point it began to arc everywhere and I had to switch off. I was confused. Why was the B+ rail OK in shutdown mode and why was it rising so high until shutdown occurred? Right now, I didn’t have any answers to these questions but there was one other worry; all this arcing was bound to cause more damage to periph­eral circuits. To overcome this problem, I decided to disable the line output stage until I had sorted out the problem with the B+ rail. Fortunately, this is easy to do; all that’s required is a jumper between base and emitter of the line output transistor. This done, I switched the set on again and to my surprise the B+ rail rose to its correct value of +103V and stayed there dead steady. By now I was really baffled. The only theory I could come up with at this stage was that the power supply was somehow breaking down under load. To this end, I replaced switch­ mode IC I801 (STR50103) and resistor R806 (470kΩ) as I had had problems with that going high in other sets. I also replaced C814 (1µF 160V) as it looked suspicious and connected another meter across the output of the bridge rectifier. Unfortunately, that didn’t cure it. When I removed the shorting jumper from the base of the line output transistor and switched on again, sparks flew everywhere. Reducing the variac below about 90V killed the set completely, while between 110V and 240V the voltage across the bridge rectifier rose to 350V. And, as before, the B+ rail and the EHT rose well above their specifi­cations and the set often closed down. I did manage to reduce the arcing a little by cleaning around the ultor cap with CRC 2-26 and by cleaning around the CRT board but it was still very hairy. But obviously, this was fid­dling at the edges and had nothing to do with the real fault. The EHT stage My next approach was to replace the spike suppression ca­ p acitors around the line output transistor but this only showed that I was still miles off the track. About all I could do was temporarily fit some larger values to reduce the EHT to a more manageable 27kV while I checked the components around the flyback transformer. Eventually, it got to the point where I began suspecting the transformer itself. Perhaps an internal insulation breakdown was causing EHT to arc onto the B+ rail? It certainly seemed that way, although the CRO only showed oversize (but otherwise per­ fect) pulses on the collector of the line output transistor. Nevertheless, I felt sure that I was on the right track at last and ordered a new transformer. When it duly arrived, I wasted no time in fitting it. Unfortunately, it made absolutely no difference. I subsequently fitted a substitute yoke without result and, by this stage, was becoming thoroughly fed up. So much for my initial confidence. Logical thought It was time for some logical thought. The crux of the prob­lem was what caused the B+ rail to go high? It was time to take a closer look at how this rail is derived and where it went. In summary, the B+ rail is generated from a switchmode power supply based on transformer T802 and switching IC I801. And pin 4 of I801 is connected via a diode to pin 2 of the flyback transformer (T402). I did some voltage checks and noticed that the B+ voltage got higher as it got closer to the flyback trans­former – even on the same track! How was this possible? By now, I felt sure that some sort of weird voltage doubling process was taking place and if it wasn’t the diode itself that was at fault it had to be a capaci­tor. So I began hanging extra capacitors onto the B+ rail at different points in the hope of changing something but to no avail. I was about to give up when I noticed that the circuit shows an electrolytic capacitor (22µF 160V) between pin 4 of the flyback transformer and earth. But what really caught my atten­tion was that no internal connection to pin 4 was shown. Obvious­ly, this was wrong – pin 4 had to go somewhere, otherwise why connect a capacitor to it? Fortunately, the circuit of an NEC N4845 circuit (Daewoo C-900 chassis) is similar in many respects and this showed that pin 4 connects to a tapping on the flyback transformer primary. I removed the capacitor and immediately noticed that it was leaking slightly down the positive lead. Could this be it, at last? I was desperate. I soldered in a new capacitor, held my breath and switched on. Hallelujah – it worked! The B+ rail stabilised at +103V and the EHT settled at 22kV, even with 240V input. Unfortunately, all that EHT arcing had created a couple of extra faults, although these proved easy to track down. First, the picture came up as an overbright raster. This was due to the 10µF 160V electrolytic capacitor on the +180V rail to the RGB outputs. It had gone leaky and pulled the rail down to about 70V (the poor beast had nearly exploded from its trauma). Secondly, the set suffered top vertical foldover and there were obvious retrace lines. This problem was traced to the verti­cal output IC (I301, AN5515) which had been damaged. A new IC restored the set to full health. The set was soak tested for a week before it was whisked away by its unknowing owner. I must renew my life insurance. Computer monitors The next day started looking distinctly “computerish”, as three monitors were dropped in by the local computer shop as soon as I opened the door. As usual, they were extremely urgent and their clients wanted free quotes. To cap it off, no faults were specified which is often par for the course but can cause prob­lems if a fault is intermittent. I don’t really consider “free quotes” as being fair as most of the work is in the diagnosis and not the actual fixing. After all, if you go to a doctor, he charges you for the consultation, gives you no guarantee and then you have to go elsewhere to buy your own parts (drugs). In the circumstances, the best I can offer are free guesses. Now that I repair so many monitors, I have set up two old 286 computers with VGA cards running a test program by Koenig, as well as a 386 with Windows 3.11 running a program called Wintach. I also have another 286 with an EGA card for older monitors. The three monitors were all only two years old and were 15-inch digital non-interlaced SVGA types. Two were Moebius CM15VDE models and the other a WEN JD156B. I began by connecting them to my three computers and switched on. One Moebius was initially working OK, while the second one was giving a “pink” picture. The WEN, on the other hand, was completely dead –well, almost. I decided that the “pink-picture” job would be the easiest and tackled that one first. The back was held on with two screws on the bottom and two plastic lugs at the top that are awkward to unclip. Once this was off, I unsoldered the metal screen over the CRT board (PWB1787). It was obvious that the problem was no green so I examined this board for dry joints, glue, corrosion and cracks but all was OK. The fault had to be somewhere in this vicinity because the cable from the computer connected directly to CRT board, with sync pins 13 and 14 going off to the motherboard. Next, I considered the possibility January 1997  71 that the fault was in the cable itself. With this in mind, the DB25M plug was carefully examined for broken or bent pins, with particular emphasis on pin 2 (the green input). I could find nothing wrong. I then checked for continuity between pin 2 and the CRT board plug (P502) at pin 3 and again all was OK. Voltage checks My next step was to make some voltage checks around the CRT board. First, I checked the voltage on the green cathode (pin 6 of the CRT socket), then the red and blue cathode voltages (pins 8 & 11). The latter both measured about 70V, whereas the green cathode voltage was at 60V. This was rather puzzling – I had expected the green cathode voltage to be higher than the other two, because the green gun was being cut off. Because these voltages were not unreasonable (after allow­ing for grey­ scale adjustments), and because there were no signs of any distressed components around the LM2419T RGB power ampli­fier IC, I concluded that the problem was back around the decoder IC (I501, MM1203). It was time to fire up the CRO. Immediately, it was obvious that there was no signal on the green channel. There was no sign of a signal at the input to the decoder IC or even where the plug connects to the CRT board. There just had to be a short somewhere that was pulling the green signal down. To test this theory, I shorted the red input to the green one and the red immediately dropped out. Similarly, when the blue input was shorted to the green input, the blue dropped out. An ohmmeter test between the green cathode and ground subsequently confirmed the existence of a short. All I had to do now was track it down. I began my search by checking all the decoupling components to the green input but they were all OK. However, when I un­plugged the connector to the CRT board, the short on the board vanished. Obviously, the problem was either in the cable or in the DB25M plug. I suspected the plug at first as this Fig.1: the NEC N4845 circuit (Daewoo C-900 chassis) is similar in many respects to the N4840, particularly around the line output stage. Note the capacitor connected to pin 4 of the flyback transformer. 72  Silicon Chip is often abused. Unfortunately, it is directly moulded to the cable and wiggling it while checking between pins 2 & 7 with an ohmmeter made no difference – the two pins remained shorted. Adjacent to the DB25M plug is a cylindrical assembly – probably a ferrite ring core – then there is a metre of cable before it goes through a plastic clamp on the back of the moni­tor. After that, about 15cm further on, there is an earth clamp around the striped cable braid, then another ferrite core before the plug to the CRT board. It all looked OK and nothing I could do would clear the short. Getting a replacement cable probably wouldn’t be easy, so I tried one last gamble – I connected a variable power supply across pins 2 and 7 and wound it up in the hope it might burn off the short. It didn’t work; the current rose to 5A (the supply limit) with no sign of the short melting. But what was interest­ing was that the cable became warm only as far as the entry clamp but no further. That just had to be the location of the short. I removed the cable, ring­barked the outer sheath on either side of the clamp marks and carefully opened the braid. To cut a long story short, I eventually found a small nick in the green signal cable which allowed the inner conductor to short against the outer braid. After that, it was a simple job to correct the fault and refit the cable. And that fixed the problem – the green was fully restored and the display returned to normal. Two to go By this time, Moebius No.2 had decided to show its fault which was a very dark display. On the bench, the tube filament read only 2V instead of 6.3V RMS, so all I had to do was find out why. I traced the source of the voltage to the +6.3V rail off the main chopper transformer and it measured OK all the way from there to a plug designated P501a-1. From there, it went to P001-2 on a small “power saving” board and then from P001-1 to the CRT socket board. And the 4V was being lost on the power saving board. The power saving circuit includes transistor Q003 (2SD667). The 6.3V rail goes to its collector and the output to the picture tube filaments. The An hour later, I had another look at it only to find that it was dead and that the power supply was oscillating again. Ob­viously, my choice of a substitute line output transistor hadn’t been a good one. There was nothing for it but to order the cor­rect transistor. It arrived within a week, was duly fitted and fixed the problem. It had really all been a piece of cake so far. Now for the really difficult part – the “quotes”. The three monitors had taken nearly all day in labour time and estimates of $82.50, $90.00 and $155.00 were given for each job in turn. The first two were accepted readily but the owner of the third monitor baulked at the cost. Later, on discovering the cost of new one, she chang­ ed her mind and decided to proceed with the repair. base is controlled by IC I002-6 (MC­ 14551BCP). As well, there are two other transistors, an SCR and a second IC (I001, HA17555). I checked Q003 and it was OK Because the set had worked initially, it appeared that the fault might be heat sensitive and so I decided to try the freezer approach. And I was rewarded with instant success – when I sprayed C001, a 470µF 16V electrolytic, the picture returned to normal. Replacing the capacitor made the cure permanent and a soak test revealed no further problems. So two down and one to go. The WEN monitor Fortunately, I had dealt with WEN monitors before and already knew about their energy saving functions. In greater detail, this model will shut down when not connected to the computer and will also shut down under software control. However, this one was almost totally dead when connected to a computer, the only sign of life being a high pitched whistle. Fairly obviously, that high pitched whistle was coming from the switch­ mode power supply which was closing down because of a short circuit. On the bench, I managed to locate the line output transistor (Q404, 2SC4924) and found that it was shorted. But it wasn’t going to be that easy. First, access to this transistor is very poor. There are two side PC boards and getting at the transistor mounting screws from the lefthand side involves removing the chopper FET (Q801) and its heatsink, as well as C304 (a 2200µF 35V electrolytic). After that, the transistor can only be reached by moving the CRT board which is glued securely to the CRT itself. In fact, the CRT board required considerable force to prise the socket off the neck of the tube. Fortunately, I managed to do this without breaking anything but I cannot say I was impressed. Nor was I impressed with the general quality of the soldering on any of the boards. The next challenge was to come up with a suitable line output transistor. My catalogs only went up to 2SC4700 and, as with the two Moebius monitors, I had no circuit and no data. The nearest I could get lay my hands on was a BU508DFI which was worth a try. I fitted one and reworked all the dry joints I could see. When I switched it on, there was power and EHT but still no picture. I had postponed tackling the CRT socket board because it was enclosed in a metal screen. When I removed it, I saw that its solder joints were even more horrendous than in the rest of set. Anyway, resoldering the CRT socket connections restored the picture, so I replaced the covers and put it aside to soak test. Another monitor Later that same afternoon, another monitor came in. This time, it was a Videocon 14-inch mono VGA unit (model T-14MS31) and, according to its owner, it was smoking. When I opened it up, I found that two electrolytic capaci­ tors had exploded, leaving small bits of paper everywhere. Fortu­nately, I found the metal/plastic covers and was able to identify their values. One was a 2.2µF 100V bipolar capacitor (C523), used as a yoke coupler, while the other was a 1µF 160V electrolytic (C522). The former is hard to get, so I fitted a 2.2µF 450V elec­tro and a 1µF 250V electro and switched on. The picture was good, so I cleaned up the gunk that was all over everything, reworked a few suspect joints, fitted the cover and left it to soak test. The next day, after it had been soak testing for a few hours, there was a loud bang, followed by a hiss. It had blown up again, destroying the same two capaci­tors. This time, I chose a high-current 2.2µF 400V polypropylene capacitor for the yoke coupler and replaced C522 with the same type as before. I left it to soak test for two days before call­ing the customer and telling him that it was ready. He was grateful for the speedy repair but I did wonder if the service cost was worth it for an old SC monochrome monitor. January 1997  73