Items Covered This Month Masuda MGV28AV TV set Blaupunkt IS70-33VCT TV set Mitsubishi CT-29ATS(A)TY TV set

My first story involves a typical customer type; easy to get along with and prepared to pay what was necessary to solve any awkward problems. The set was a Masuda MGV28AV, bought from the now defunct Brashs chain of shops.

The Masuda was a Chinese-made set and I explained to Mr Bull that there could be problems obtaining parts or service information. However, he was eager for me to try; as I said, he was easy to get on with and prepared to pay.

The problem was a very annoying intermittent brightness variation – it sometimes became brighter when the set got hot. Well, the first thing was to confirm that the set was exhibiting this fault and so I put it to one side where I could monitor the picture. Nothing happened for the first two days but on the third day, towards closing time, the fault began to show, the picture gradually becoming brighter and brighter. I couldn’t do anything about it just then, although I did remove the back and set it up on the workbench so that it would be ready for me to tackle the next day.

The next morning, while waiting for it to misbehave again, I tried accelerating matters by covering it with a blanket. And I rummaged among my circuits to see if I had a diagram that might match. As luck would have it, I found a circuit for a 1993 Teac CT-M715B, which is very similar.

Where does one start? I needed to make measurements consistent with the symptoms so I started by measuring the screen voltage to the picture tube (pin 7) to see if it varied when the fault occurred. When it did occur some hours later, the voltage was rock steady, so I wasn’t looking at an EHT fault.

Eventually, I determined that it was some kind of video fault, because the tube cathode voltages were drifting lower when the problem occurred. I even went so far as to trace this drift back to IC304, a TDA3504 which is well known for causing problems (usually resulting in loss of picture). This was encouraging but the job was still proving to be extremely frustrating because it took so long for the fault to show.

Calculated gamble

As a result, I took a calculated gamble and plumped for replacing IC304. Unfortunately, it wasn’t meant to be, so I spent yet another day leaving a meter connected to pin 17 of IC304, the brightness control input. This was normally at about 2V but under fault conditions it was all over the place.

Where to from here? I followed the beam limiting signal path from pin 17 via R355 (56kΩ) to R433 (150kΩ), then to the 143V rail and R403, C425 & D406, but these all measured OK. I also checked D302 which links the contrast DC tracking, before going on to check R322 and transistor Q302. Once again, I drew a blank.

That left the brightness control circuit itself which comes out of pin 3 of microprocessor IC601 and also involves pin 25, the mute line, which controls transistor Q302.

I wasn’t getting anywhere – I knew I was in the right area but I couldn’t isolate the exact cause. Finally, I decided to check the sub-brightness control circuit involving VR301 but initially couldn’t find its location. The reason was that it was, rather ridiculously, situated under a large resistor which in this instance had been bent down so that it was literally touching the plastic former of the control.

Naturally, the heat from the resistor had distorted the control former, resulting in its function being intermittent when hot. A new control fixed the problem completely and I also relocated the resistor so that it wouldn’t happen again. The only tricky part now was telling Mr Bull how much all this cost – plus the GST! But he didn’t baulk.

Crook Blaupunkt

My next customer, Bill Strong, is another reasonable bloke; not the sort to whinge for no reason. He brought in his Blaupunkt IS70-33VCT complaining about the sound – or lack of it. He was very apologetic in admitting it was extremely intermittent.

He wasn’t wrong. I put it on the soak bench with the sound on low while I hunted up a circuit. This set used an FM310.32 chassis with part No. 7663 700. The nearest I had was for IS70-31VT with part No. 7660 800 but it would have to do.

The fault didn’t occur for the first couple of days but did on the third day and continued to give trouble. Unfortunately, I couldn’t do anything about it because I had my hands full with other more pressing jobs.

On the fourth day, I transferred the set to the main bench and took the back off. But much to my frustration, it worked perfectly all day. By the fifth day, I was losing patience with the set – I needed something I could work with but it refused to play up. In the end, I switched it off and pulled the chassis out, looking for bad connections and faulty joints but could find none.

I switched it back on and tapped the chassis with a screwdriver handle, then tried heating and freezing it. Nothing I did made any difference at all. Fed up, I decided to put it back on the soak bench and switched the set off to do this. But – you’ve guessed it – as soon as it was back on the soak bench, it started playing up again.

By now, I was beginning to notice that, at times, the fault would occur almost from the moment I switched it on until I switched it off. At other times, when I switched it on, it would come good and stay good until I switched it off. Bells were beginning to ring. Perhaps it was the way I switched it on and off. I was using the master switch on the soak bench but perhaps it was the set’s switch itself?

I tried switching it on and off many times and it did seem that there was something about the switch that was causing the problem. But what was it and more importantly, why did it only affect the sound?

I removed the switch (S601) and checked the contacts. The main AC power contacts were fine but the standby or "Temp contact" (as Blaupunkt calls it) was intermittently sticking on. This momentary switch controls T801 (BC548C) and the U WISC line P1.0 to pin 15 of the microprocessor I811 and, if left permanently on, mutes the sound!

A new switch fixed the problem completely.

An overbearing customer

And now for a change of scene. Mrs Ruddock did not strike me as a reasonable type and the screaming and demanding children she brought with her only made matters worse.

Anyway, she arrived with her Mitsubishi TV set in the back of the station wagon. She said that it wouldn’t come on and asked if I could fix it. I said that I was sure I could – not realising that she meant immediately. But having lifted the 68cm 46kg TV set into the workshop, all by myself, I wasn’t in the mood to return it to the car when she made that point obvious. And she reinforced this idea by closely following me into the workshop and standing there expectantly.

Somehow or other, I had allowed myself to be painted into a corner – almost literally. For lots of reasons (including safety), I regard it as an unwritten rule that clients do not to come into the workshop. Yet here I was plugging the set in and switching it on. And before I knew it, I had the back off and was making measurements.

This Mrs Ruddock had powers beyond my understanding. Indeed, I felt as though I was part of some Greek mythology, where I was being controlled by one of the Gorgons – probably Medusa. I even tried to avoid Mrs Ruddock’s gaze; after all, I didn’t want to be turned to stone!

Anyway, this was a 1993 Mitsubishi CT-29ATS(A)TY with an ATMT691 chassis. This set employs a lot of advanced features and is a real bells and whistles job. It even had a motorised swivel stand so one could rotate it by remote control.

Apparently, the fault had been intermittent but had progressively become worse until now it was completely dead. I had never come across this model before and didn’t have a circuit. However, I did have a few notes from a trade meeting I attended, at which this type of dual switchmode power supply had been discussed although not in great detail.

I could measure voltages all over the place and all the fuses were OK, so I knew this wasn’t going to be easy. But Mrs Ruddock was still pressurising me to fix it now.

I located the standby switch transistor (Q9132) and was trying to measure the voltage on its base when suddenly the set came on. At the time, I didn’t have a clue why and could only conclude that my shaking hands had accidentally shorted something. What’s more, the set now came on perfectly each time it was switched on, with all functions working (including the power/on/off/standby). And nothing I could do would recreate the fault.

Mrs Ruddock was initially pleased that it was working but wanted more – basically, a lifetime guarantee was the kind of thing she had in mind.

However, her slight sign of pleasure released the psychological hold she had over me; the image of Medusa instantly vanished and she was now just plain Mrs Ruddock. The spell had been broken and I took full advantage of it.

Emboldened, I told her that whatever I had done could only be considered temporary and that, sooner or later, the set would give trouble again. I also told her that the only way to fix the problem was to leave the set on test while I acquired the correct service manual (which costs $60), so that the real fault could be tracked down.

She immediately tried to reassert control but failed. I had won; well, sort of. In any event, I was happy to replace the back of the set and put it back into her car. But I wouldn’t budge from my position and I didn’t charge her.

That was the last I heard of the set until some nine months later, when the set suddenly reappeared with the same fault. The lady’s attitude had improved a little. She said that the fault had recurred within a few days (just as I had predicted) and so she had taken it to a Mitsubishi agent. She also complained that they had kept it for eight weeks before they fixed it.

I am also a Mitsubishi agent but I resisted the temptation to ask her why she had taken it elsewhere. Nor did I ask her why she hadn’t taken the set back there now. But I insisted on my basic ground rules – I must have a circuit, time and space!

This agreed to, I proceeded to connect the set and see what was cooking. Well, to begin with, the set came on straight away with severe hum and picture distortion and with the picture jumping. Mrs Ruddock thought I was splitting hairs when I mentioned these faults, pointing out that it came good after five minutes.

I then asked her what the other Mitsubishi agent had done. Their bill was produced, which showed they had changed microprocessor IC701 (M50436-566SP) but, in fact, they had done more. I discovered this when checking the -30V rail on the power supply – capacitors C9F1 and C9E9 had both been changed.

My insistence on getting the correct service manual had paid off but it was irritating to find different voltages marked along the same supply rail. This makes it difficult to be sure of the correct value. In the end, I found that the 12V rail was significantly low, with the CRO showing significant ripple on it. The culprit turned out to be C9E5, a 470μF 25VW electrolytic that was leaking badly.

Because I had to take the board out to change this capacitor, I took the opportunity to examine the rest of them. C9E1, another 470μF 25V electrolytic was also leaking badly and I could see that most of the other electrolytics were in poor condition. Before changing all 25 or so, I thought that I would confirm that the two I had just replaced were the significant ones. I was gratified to see that they were, the set coming on immediately with picture and sound.

I then set about replacing all the remaining capacitors and cleaning up the damage caused by the leaking electrolyte from the C9E1 and C9E5. These had even corroded the heatsink a few centimetres away.

Finally, when it was ready, I replaced the board and checked the six main voltage rails (130V, 33V, 15V, 12V, 9V and -30V), all of which were OK. With soak testing I had the set turned around within one week but received only moderate praise from Mrs Ruddock. I guess there is no pleasing some people but maybe she learnt something. Who knows, she might even call on me the next time something fails!