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SERVICEMAN'S LOG Did lightning strike at all? The old saying that “lightning doesn’t strike twice in the same place” has a new twist these days. Did it strike at all? Many insurance claims for lightning strikes on TV sets are best described as dubious; and they can put the serviceman in the hot seat. Mr Lancia (not his real name) had already sought service elsewhere for his Toshiba 2529SH, 21 System, 63cm stereo TV receiver. It had been purchased overseas but I only found this out later. When he brought it in, he said that it had died as the result of a lightning strike and could I prepare an estimate for insurance? Estimates are tricky things and are best avoided in other than exceptional circumstances. The only way one can “estimate” is to actually fix the set. But by doing that, the estimate concept disappears; it now becomes a factual situation. Material costs have been incurred and hours of labour have been spent. The estimate becomes the final account. And if the customer doesn’t like it, what then? The best that I could hope for in this current situation would be that a lightning strike might produce enough visual evidence to support a rough guess as to the minimum cost. But there is no way of knowing whether the obvious damage is covering a host of additional “invisible” problems. In any case, if an insurance claim is involved, it’s necessary to first determine the validity of the lightning strike theory before going any further. This set was probably made for the Middle East market and certainly not for Australia. I tried to order a service manual from the Toshiba agent, only to be told that they had never even heard of this model, far less have any parts for it. They were pretty emphatic that there would be no support available in Australia for this set. I was on my own. I removed the covers and made a visual check. Significant­ly, I was unable to see any signs of lightning damage. Encouraged by this, I switched on; there was no sound or picture but it was far from dead. First, I could switch the power indicator LED from “Standby” to “ON” from the remote Fig.1: this circuit section from a Toshiba 3418DA helped solve the problem with a Toshiba 2529SH, which isn’t sold locally. It shows the TA8783N jungle IC and, in particular, the Vcc pins (6, 40, 61 and 63). 20  Silicon Chip control, as well as from the front panel controls. Also, there was 124V on the collector of the horizontal output transistor (Q404) and the horizontal drive transistor (Q4022). But there was no voltage anywhere on the 64-pin jungle IC (IC501, TA­ 8783N). I wasn’t familiar with this device but it ap­peared that pin 39 was the horizontal drive output and that the crystal on pin 37 was part of an oscillator. I couldn’t work out much more without a circuit and I especially needed to find out which was the supply rail pin (Vcc). Next, I went to the power supply and measured the rails on the output from the chopper, Most had voltages but it was far too complex to work out what was what. I did notice that a relay on the primary was switched off by a link across the base emitter of its driver transistor. When I removed this link the relay began to chatter loudly. Once again, without a circuit diagram, it was too hard to find out how it was meant to work, so I refitted the link. This was about as far as I could go and the set was put aside. I told the client the situation and he liaised with his insurance company, presumably with a view to writing the set off. Lateral thinking Months later, I was fixing another Toshiba for a different problem and I needed technical assistance from the local Toshiba agent. They weren’t able to help directly, suggesting instead that I try one of their larger agents in Melbourne. I telephoned them and they were able to help with my immediate problem. Buoyed by this, I thought I’d push my luck and asked if they had heard of a 2529SH model. They hadn’t but, as an after­thought, I asked them if they had any Toshiba circuitry using the TA8783N IC. The technician offered to a look and after a minute or two he came back and said he had a circuit for a 3418DA which used this IC and that I could buy a copy if I wished. I agreed to go ahead and when the circuit arrived I found that two Vcc rails are fed to this IC: 9V on pin 40 and 12V on pins 6, 63 & 61. I took a punt on pin 40 and traced this back to the power supply before the trail went cold. The power supply was completely different from the 3418DA circuit and the only chance I had now was to trace out the circuit. This was a fairly ambi­tious approach; it was very complicated with about 20 transistors and it was going to take some time. I had already established that the 12V rail was derived from the horizontal output transformer via a 3-pin IC regulator. This regulator is designated as a UPC2412HF but I am unfamiliar with it. However, I was still following the idea that the 9V supply to pin 40 was critical. Two issues now had to be resolved before I spent more time on this theory: (1) was the set worth fixing?; and (2) was I going to get paid for it? To answer the first point, I hooked up an external 9V power supply and switched it and the TV set on together. This produced both sound and picture and everything looked fine. This confirmed that the fault was confined to the 9V source – apart from the previously mentioned relay. I revisited this part of the circuit briefly and found that when the set was on, the relay was silent – it was only in the standby mode that it chattered. I also worked out that its func­tion was to short out two large, low value resistors which limit­ed the 240VAC supply to the main bridge rectifier. Finding out how all this worked and fixing the problem was going to be time consuming. I phoned Mr Lancia, and he said he was quite prepared to pay for all my efforts if I could do it for less than $300. He already had a new set from the insurance company and he could use this set as a second set if I could fix it. So in the next month or so, whenever I had some spare time, I would draw the circuit of the power supply module. The result was four voltage sources: 124V, 8V, 16V & 17V, all of which were present and correct. The 17V rail fed an NPN transistor (Q870) which generated 9V at its emitter, the 9V supply then going to pin 40 of IC501. However, there was no 9V on the emitter of Q870. The transistor driving Q870 was another NPN transistor, designated Q871. Its base was controlled by a network of transis­tors and other assorted devices. When I shorted Q871’s base and emitter leads, its the collector voltage rose and turned on Q870, restoring both picture and sound. I was on the right track at last. Safety circuit But it was still complicated; the network of transistors controlling the base of Q871 was also part of the remote control on/off power circuit, including the safety cutoff. I already knew that the remote system was working properly, which left the safety circuit to investigate. From what I could work out, it involved an SCR (D859), which was controlled by the horizontal output stage via a series of small unidentified glass diodes from the 8V rail to the 16V rail. I tried disconnecting various parts of this circuit, one at a time, to see whether the set would fire up. Items Covered This Month •  Toshiba 2529SH TV Set •  JVC 775AU TV Set •  JVC CX-60ME TV Set •  Hitachi C33-P900 TV set •  Mac LC630 computer And it wasn’t until I disconnected these diodes, which I realised were unidentifiable zeners (D890, D891, D892 & D893) that the circuit finally fired up. Because I couldn’t identify the diodes, I decided to recon­nect them and measure the voltage across each one. Believe it or not, when I did this, the set continued working and nothing I could do would recreate the fault. I froze them and heated them but they wouldn’t fail. I assume that, in the process of unsol­dering them, the fault had somehow been fixed. Regretfully, the circuit involving the relay was in the too hard basket. I tried to draw the circuit but it used over 10 transistors, plus optocouplers, in all sorts of bizarre configu­rations. I can only guess it was some sort of current sensor safety circuit but I couldn’t work it out, so I left it as I found it. I left the set on soak test for couple of weeks before calling Mr Lancia. I quizzed him on this and other parts of the fault which I really couldn’t attribute to lightning. Finally, it emerged that he had the same faults before and that someone else had done the same sort of thing as I had, which kept it going until the recent storm. So, was the storm really involved? I don’t know. Nor did he explain why he hadn’t returned the set to the previous repairers, nor did he say who they were. For my money he was really a rather October 1999  21 dodgy sort of bloke – but at least I was paid for all the work I did. Two elderly JVCs Two elderly JVC TV sets came into the workshop this week – one a 51cm model and the other a 15cm battery/ mains portable. Both were over 10 years old and belonged to different customers, and both sets were dead. The 51cm unit was a 7755AU, while the baby was a CX-60ME with an attachable mains power supply. Some­what surprisingly, their circuit diagrams were similar, with both employing an AN5900 pulse width modulator IC for the low voltage supplies. I started with the baby, which I found would still work with a 12.5V bench power supply. I opened up the AC adaptor (AA-60ME) and what was left of resistors R09 and R10 told the story. Fortunately, having the circuit for both sets was a bonus. The two resistors were in the emitter circuit of the power output transistor Q03 (2SD­1453), which turned out to be short circuit. I replaced these and applied power. There were no fire­works; in fact there was nothing – the unit was dead. Transistor Q03 takes its collector voltage from the bridge rectifier, D01, via the 22  Silicon Chip primary of transformer T03. And the collector was sitting at 340VDC; the unloaded voltage from the bridge and the mains. Nothing was oscillating and it is a little obscure as to exactly how the circuit worked. However, I figured out that the problem was in the starter circuit Q01, Q02 & Q05. The ohmmeter indicated that Q01 had a base-emitter short but the others were OK. This circuit supplies the start-up voltage to start IC01 oscillating until the secondary 12V takes over. I fixed this and the whole power supply started working but was producing a fierce whistling sound. This implied that it wasn’t oscillating properly, probably due to a leaky electrolytic capacitor. There were about 10 of these and I started to hang addi­tional capacitors across the most likely ones until I reached C22. Replacing this, a 47µF 16V unit, finally stabilised the circuit and it behaved properly, giving a 12.5V output. I had already soldered any potential dry joints and I left it on soak test. Moving on to the larger set, there were no low tension voltages at all from the power supply and a relay was not acti­vating. I checked the voltage at the emitter of Q03, which is fed directly from the bridge rectifier and there was no 11V as indi­cated on the circuit. Nor was there any voltage on Q03’s collec­tor, test point TP98, out of bridge rectifier D11, or even from the power transformer T02. It wasn’t until I removed T02 that I found that its primary winding was open circuit. This didn’t look good but I persevered. I fitted an external bench power supply across the bridge rectifier and ran it up to 62.5V, as indicated on the circuit. This produced low voltage rails of -28V, +12V and + 5V, the latter derived via IC001 (LA7930). But the 110V main HT rail was low and causing trouble in another part of the power supply. This turned out to be due to horizontal output transistor Q551 (2SD1453), which was short circuit. I thought that replacing this would be the end of the story but worse was to come. While I now had sound, there was no pic­ture and there were blue sparks inside the picture tube. All the indications were that the picture tube was down to air and this was subsequently confirmed. It was a good job I hadn’t ordered a new T02 transformer, because the sick picture tube was the death knell for this set. There was no way one could justify the cost of a new tube (assuming one could be found) and the labour costs involved. It was a write-off. That’ll teach me to muck about with sets over 10 years old! The big Hitachi Mrs Belrose asked me to service her 78cm Hitachi C33-P900 (G8P chassis) TV set at home because the prospect of bringing it back to the workshop was horrendous. She complained of obscure intermittent faults such as intermittent loss of picture, intermittent flickering and a monochrome picture for three minutes or so when cold. These faults hardly ever occurred when the set was warm, although occasionally it would flicker for few seconds when switching from the AV (audio/visual) sockets to the tuner. I removed the back of the set and looked around. I was looking for the AGC control and the associated circuits when I happened to see one of several small black electrolytic capaci­ tors on the video board. It had literally spat the dummy – or, more correctly, the electrolyte – all over the PC board. Then I saw another, and another, and realised all 30 or so electrolytics were leaking badly. Most, if not all, would to have to be replaced. It was a big job, and one that couldn’t be done in the customer’s home. Though the set looked a lot younger, I learned from the service manual that it was 10 years old. But apart from these problems, the picture on the 78cm picture tube was still good which meant that the set was worth fixing. After all, a replace­ment would cost $1500 or more. I wasn’t looking forward to taking the set back to the shop. However, I managed to round up three helpers and it wasn’t too bad for the four of us, considering the set’s dimensions and weight (80 x 72 x 57cm and 64kg). I started by replacing all the electro­ lytics, mostly 10µF, on the video signal board, hoping that would fix most of the problems. It didn’t. I then examined the small electrolytics on all the other boards. Two of these, C918 (1µF 50V) and C950 (470µF 60V), were particularly bad but I also changed a few other low-value units which might be causing hum on the power supply rails. Altogether, I replaced 40 electrolytic capacitors, cleaned up all the corrosive electrolyte from them and felt confident that I had cracked it – but I hadn’t! The thought of replacing every last one of the remaining capacitors was too much to con­template. It was time to get technical. The main clue I had was that it was temperature sensitive so, using the hair­dryer and freezer, I followed the video rails from the tuner as they branched out all over the set. Being a multi-system set, it was fairly complex to follow. During this procedure, I followed a few false trails, espe­cially at pin 8 of IC501, which I noticed earlier had hum on it when connected to the oscilloscope. I had also used a signal source plugged into the AV input sockets and noticed that the stronger the signal, the less chance there was of the symptoms occurring. For example, the set was much better in the AV mode and SBS (the strongest signal in this area) was better than all the other channels. Anyway, after messing around for a long time around IC501, I found I wasn’t really progressing and continued until I reached IC701 (TDA­ 2579A), which is the jungle IC. There I noticed that freezing it caused the horizontal frequency to change (I could hear it) and then the picture vanished. I gradually isolated the area down to the components adjacent to pin 6, especially C619 (2.2µF 50V). Replacing this capacitor removed all the remaining symptoms. The easy bit was over – all I had to do now was manhandle the set back to Mrs Belrose! Secondhand Mac Our local school was given some secondhand Macintosh com­ puters but one, an LC630, had died, so it was brought in to me to see what I could do. I hadn’t worked on these before and the first problem was figuring out how to get inside the unit. It was like a Chinese puzzle. This meshed with that and that slid into this, to unlock something else, and so on. However, after a lot of time, I even­tually managed to locate and remove the power supply. Eventually, I managed to unscrew the final metal screening can and remove it. But, in so doing, my hands touched a part of the PC board and even though the set had been switched off for hours, I received an awful shock. I instinctively let go if it and the wretched device tumbled to the floor. After I had recovered with a very stiff cup of coffee, I carefully picked up the module and examined it closely. Fortu­ nately, it hadn’t been damaged by the fall and the incident had given me a major clue. It was obvious that I had received the shock from the fully charged filter capacitor following the mains bridge rectifier which meant that no current was being drawn from it. This in turn suggested that the power supply wasn’t oscillating. So the first step was to find the start-up circuit. That wasn’t too difficult. I soon found two 220kΩ resistors in series (R16 & R17), one of which had gone high. Replacing them (after shorting out the remaining charge on the main electroly­ tic) completely fixed the power supply. The only remaining prob­lem I had was trying to remember how everything went back togeth­er again. I charged for two hours labour and the bill came to $114. I was worried the school might think that this was too expensive, especially as one can buy complete PC power supplies for less than $60. However, the school was delighted as they had been quoted SC over $200 for a replacement! October 1999  23