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SERVICEMAN'S LOG The things I do for money In my never ending quest to earn money, I never cease to be amazed at what I am asked to repair. Recently, I ended up repairing a video games machine on the footpath but first I faced up to a couple of TV sets. I must admit that I feel more comfortable working on TV sets and VCRs but you cannot be too fussy these days if you expect to earn a living. Computers, computer monitors, microwave ovens, video cameras and even the odd video games machine are now all part of the job as far as I am concerned, although many organisations prefer to specialise in just a few areas. But I still like working on TV sets (mostly) and so the first two jobs I tackled when I arrived at the shop this morning were a couple of Philips TV sets that had come in late the previ­ ous day. The first was a 34cm Philips GR1-AX chassis (model number 14GR1224/75R) which apart from making a buzzing noise was otherwise quite dead. This popular model is now well known with few surprises left. But to those not familiar with it, it can be a little daunting the first time it is encoun­tered. If the main fuse is intact like this one was, there are really four main conditions possible for the main B+ rail which normally sits at 95V. First, if it measures only 1.5V across C2660, the crowbar circuit is probably faulty and you have to check SCR 6641, zener diodes 6638-6640 and diode 6642. Second, if it measures only 5V, the procedure is to check the standby circuitry from the remote control, the microprocessor and from transistor 7631. Third, if it measures 10V, FET 7610 is probably open cir­cuit and D6610, D6613, R3616 and R3680 also have to be checked. Finally, if it correctly measures 95V but the set is still dead, inductor L5524 is a likely culprit (it can go open circuit) but it could also be the flyback transformer (5530) that’s at fault. In this case, it was the latter and a new one quickly cured the problem. Fig.1: the 11V rail in the Philips 21MK2460 is derived from pin 16 of the transformer winding via a 1Ω safety resistor and diode 6180. 38  Silicon Chip I also reworked a few suspect solder joints on the deflection yoke plug and socket connections and also on some of the inductors, transformers, transistors and even IC7020-B. By the way, the 9V and 12V rails are critical on this model and so I always replace C2523 (6.8µF) with a 10µF 63VW 105°C type (and also occasionally C2542). Finally, a few words ought to be mentioned about the “Hotel Mode”. Once again, for those unfamiliar with this model or who never read the instruction book, it is fitted with a childproof lock which prevents the tuning from working and limits the maxi­mum volume level. To unlock this, all you have to do is select program site No.38 and press STORE and CONTROL+ simultaneously. Another Philips The second Philips set was a late-model 21MK2460 using an Mk.2 chassis. The customer’s complaint was that there was no picture or sound and, as far as he was concerned, it was dead. However, turning up the screen control soon revealed a blank raster but that’s about all there was. This was my first time with this model although I did have a circuit for it on my files. My first thought on the problem was that perhaps it was stuck in the AV (audio-video) mode but con­ necting an external video source made no difference. The problem seemed to be situated in the small signal circuits and as it was affecting both sound and picture, I checked the low tension 12V rail supplying them. This measured OK and so I next turned my attention the logic circuits and the 5V supply rail to them. This time, there was a clue – the 5V rail was down to just 3V which meant that I now had something to go on. By tracing this rail back towards its source, I quickly discovered that it is derived from an 11V rail via transistors 7182 and 7184. And this 11V rail revealed as a very faint hairline fracture in the copper track immediately adjacent to the pin. This was defi­nitely an 11 out of 10 on the “dj” (dry joint) scale. The rest was an anticlimax – soldering this microscopic crack fixed the problem once and for all. But what caused it in the first place? My guess is that the transformer pin was slight­ ly crooked and the fracture occurred when it was forced into its mounting hole during assembly. A job for the Terminator was also low, measuring just 7V from diode D6180. I then measured all the other rails from transformer 5161 and found them all to be correct bar this one. The circuit here is quite simple (see Fig.1). The top of the transformer winding terminates on pin 16 and goes to diode 6180 via a 1Ω safety resistor. The rectified output is then filtered using a 680µF electrolytic capacitor (C2180) to produce the +11V rail. The 1Ω resistor checked out OK and it made no difference when I substituted C2180 – the rail was still too low. However, when I disconnected the 5V rail, the 11V rail came good. In fact, disconnecting anything from this rail caused it to rise but I could not determine whether it was an excessive load problem or an insufficient supply problem. I disconnected pin 42 of the microprocessor (IC7200) and all the other components one by one but they all restored the 11V rail. They couldn’t all be faulty, could they? Surely not. My next thought was that perhaps diode D6180 was intermit­tent, so I tacked another one in parallel with it to the copper side of the PC board. This made no difference, so I decided to short out R3180 by connecting the new diode’s anode directly to pin 16 of transformer T5161 via a short length of hookup wire. When I did this, the 11V rail immediately came good again and the set burst into life. What’s more, the 5V rail was also now correct so what was happening? I removed and replaced R3180 but it still wouldn’t work properly without the wire link between R3180 and pin 16 of the transformer. By now, it was obvious that the connection between pin 16 and R3180 was not kosher, despite the fact that it looked per­fect. However, when I checked this path with an ohmmeter it meas­ured no resistance. Of course, that only meant that the fault was not showing up under no-load conditions. There was a lot of white paint surrounding pin 16 and so I decided to scrape this away with a utility knife. And there at last was the fault, I was about half-way through the second Philips set when the local video store rang and asked me to fix their Mortal Combat. “Mortal who?” I responded. “You know, the one-arm bandit”. This sounded dangerous. “You mean you want me to fix a one arm mortal combat bandit? Are you sure you got the right number?” “No, No, I mean the video game machine”. Ah ha! A cyborg repair – a job for the Terminator. Eventually the story unravelled as follows. The video store, which is a pretty small shop, was supplementing its meagre income with a video games slot machine. This was located near the entrance and the kids had been fairly pounding it recently be­cause first the picture gave lines and then it went dead. Because it was a fine day, I decided to abandon my stuffy workshop as soon as I had finished the Philips set and have a go at fixing this. Normally, I would be quite reticent at taking on such unfamiliar equipment but I was feeling unusually optimistic. It couldn’t be anything too complicated, could it? I didn’t realise that I would be fixing the thing on the pavement but, as it turned out, that was the only way I could find sufficient space to gain access to the machine. The game consisted of a sturdy wooden case on wheels with a 20-inch CRT on top and the controls on the front. The case was reinforced with steel and multiple locks and chained to the front of the shop. Good neighbourhood, this, I thought. Anyway, I eventually removed the front and rear panels and found that the parts were generally quite accessible. What’s more, the unit appeared to be very well-made. On the base of the cabinet was an expensive looking September 1997  39 Serviceman’s Log – continued of things that needed doing. First, both the protective glass and the tube surface were covered with dust and dirt. However, the glass is easily removed by unclipping two suitcase-type latches inside the cabinet and hinging back the control panel. Once this has been done, the rest is easy and the picture looked a lot better after I had finished cleaning the glass surfaces with detergent and a damp cloth. On the downside, my cleaning efforts revealed a further problem in the form of visible retrace lines. These were elimi­nated by reducing the screen control setting and setting up the sub-brightness. All that remained to do was to relock it all up and extract my fee from the proprietors. This had turned out to be a rela­tively straightforward job and for once my optimism had paid off. The crook microwave power supply and this was neatly terminated, with all the voltages clearly marked. The actual games circuitry consisted of a large PC board that looked just like a computer mother­ board and, finally, there was the TV moni­ tor board. These boards were connected by a large wiring loom to the various controls and to the money collector. Power supply checks The whole machine looked quite dead so I decided to start by checking all the different rails in the power supply. I was surprised to find them all correct and so took the next step of looking at the monitor board. The CRT heaters were on but because of the street noise, I couldn’t determine whether the line output stage was working. However, the B+ voltage on the line output transistor was correct, as were the screen voltage on the CRT 40  Silicon Chip socket and the voltages on the RGB output transistors. It all looked pretty good so far, so I turned my attention to the computer board. This was clamped to the side of the cabi­net and was awkward to get at. Anyway, while I was loosening the clamps, the large edge connector fell off with an ease that was all too apparent. I reconnected it and pushed it home as far as it would go. It could travel quite some distance and I noticed that the wiring loom connected to it was heavy and quite tight. It didn’t take a genius to figure out that the connector could easily come loose if someone was giving the case a good thumping. And that’s just what young kids are inclined to do when the game isn’t going too well. I repowered the machine and it immediately came to life with sound and picture. But although the picture was reasonable, there were a couple My next job in this day’s potpourri was a Sanyo Micro-Convection microwave oven, model EM-5710. This particular unit is now approximately 20 years old and features a stainless steel oven, a heating element in the roof and a fan. The problem that it had first became evident seven years ago but, on that occa­sion, the owner had decided not to opt for a full repair and instead went for a temporary one. I first saw the oven back in 1990, when it was brought in dead. The 8-amp power fuse was open circuit and the 25W SES globe had failed. I also noticed at the time that the convection fan in the roof was loose due to worn bearings. However, the customer was adamant that he didn’t want to spend money on getting this “minor problem” fixed up. This time, seven years later, it was sparking badly in the roof from two different locations. First, there is a ceiling partition between the element, fan and the main oven and this material had, over the years, been caked with variety of differ­ent foods that had gradually become carbonised. Once it reaches this state, the microwaves regard it as unwanted metal and the area absorbs a lot of energy. As a result, it gets hot and generates sparks which creates even more carbon. The second source of the sparks was the fan itself. This was lurching around in its now totally worn out bearing case and was striking the stainless steel roof, all the time giving a shower of sparks. It is quite possible that minute molten metal particles were spraying the partition below and were responsible for starting the fire there in the first place. Fairly obviously, the first job was to clean up the mess and I spent some time removing the dirt, rust and carbon deposits around where the fan was hitting the roof. The carbon on the partition was removed by cutting it out, just as a surgeon would cut away a cancer. Despite the oven’s age, the fan bearing was still available from Sanyo, as indeed was the belt. After reinstalling it, the acid test came when the power was connected and all the sequences tested. A mug of water was used to ensure that the magnetron had a load to absorb its energy, while a small fluoro tube (with its metal ends removed) from a discarded portable torch was placed in the water to check for the presence of microwaves. When the oven came on, the fluoro tube glowed brighter than it ever did in the torch. More importantly, the sparks had stopped and the rest of the oven was neutral, even when the water boiled. Purple patch My next story concerns a Compaq Presario 14SV monitor, vintage 1995, with the complaint that “first it had purple lines, now it has no picture”. This super VGA monitor is made by Samsung for Compaq and, as is my policy, I first connected it to a com­ puter to confirm the symptoms. This was just as well as the symptoms I observed were completely different to those described by the customer. I did get a picture of sorts but there was no horizontal control and I also noticed that the green LED power indicator wasn’t working. I prepared for surgery and dived in. Removing the back is quite easy provided that you know about the two concealed catches at the top. Once inside, you realise that the bottom PC board is inaccessible unless you remove all the metalwork and then the motherboard. My gut feeling was that there was a dry joint somewhere and the clue would have to be the LED. So once I got the chassis out, I checked the LED and the connections all the way back to the power supply where, to my relief, I found that there was indeed a dry joint on 3-terminal regulator IC202. On the dry-joint scale of 1 to 10, this was about a 6. I resoldered the regulator and then carefully inspected the rest of the chassis for similar faults. This revealed no further prob­lems, so I reassembled the motherboard into its metalwork and reconnected all the plugs and sockets. When I switched it on, the monitor’s picture locked perfectly and the power indicator LED shone green and so it was put aside to soak test before being finally pronounced cured. The old Sanyo My final story this month comes from K. Sims of Black For­rest, SA). I’ll let him tell the story in his own words. I’ve been in the lighting and sound business for around eight years now. TV’s certainly aren’t my cup of tea but, on this occasion, I decided to tackle a project that would be a bit more of a challenge. The set in question was a Sanyo CTP 5604 that was found in the “equipment graveyard” room at a night club that I have been working for. Apparently, it had been there for about two years but no-one knew where it originally came from. As the manager was junking all the rubbish from the offices, I decided to take a look at the set to see if it could be rescued. Plugging it in for a quick inspection revealed a squashed image no more than about 8cm high across the centre of the screen and there was no sound. This seemed to be two rather unrelated faults but as the picture is the most important of the two I decided to start there. Because the set had its back towards me, I decided to remove the rear cover and have a quick look inside for any obvi­ ous signs of trouble. As I was about to switch the set off, I noticed that the picture had “grown” to the extent that it now almost filled the screen. This indicated that the fault was heat sensitive and that a search of the deflection board for dry capacitors would be a good place to start. After shifting the set to my home and making a list of the capacitors that needed replacement I also noticed that 0.5W resistor R452 had been overheating, to the extent that its outer insulation (and colour coding) had burnt off the resistor’s inner core. PCB POWER TRANSFORMERS 1VA to 25VA Manufactured in Australia Harbuch Electronics Pty Ltd 9/40 Leighton Pl. HORNSBY 2077 Ph (02) 9476-5854 Fx (02) 9476-3231 After a few calls, I was informed that Sanyo had a techni­ cal information line that you can call for information on assort­ed topics regarding repair. They kindly faxed out a schematic for the deflection board and informed me that R452 was actually a 33Ω resistor. It still tested OK but I replaced it for good measure. After replacing half the listed electrolytics, I realised that the worst one (C444, 4.7µF) was actually a non-polarised type. Eventually, I ended up replacing this capacitor and the remaining high-value electrolytics with low-leakage types ob­tained from a TV repair centre. On powering the set up again, I was rewarded with . . . absolutely nothing! I could still hear the whistle from the scanning coils and the tube heater was a lovely shade of orange but there was no picture at all. On closer examination, I then noticed that a 3W resistor adjacent to R452 had come adrift, probably due to a dry joint. A little resoldering fixed that problem and that did it – the set came good. It seems that replacing all those suspect capacitors cured the fault and the set now delivers a top class picture and sound. I also spent around an hour looking for the vertical cen­tring control only to finally find that it wasn’t a pot but a jumper lead that goes to one of three connecting posts on the deflection board. The UHF tuner was also removed and the tuning mechanism relubricated, as it had frozen SC solid from lack of use. September 1997  41