Member: Sideband

In the early 80’s I was still in the Philips workshop at Waddon and at that time, things were changing rather rapidly in TV design. I’d gone through the G8, G9 and G11 and was getting to grips with the KT2, KT3 and K30 using the new (at the time) switched mode power supply. There was also an ‘oddball’ set that had appeared in the form of K12. This was an upmarket set boasting a hi fi output stage. It was certainly impressive with a large bass-reflex speaker enclosure built in to the cabinet. It was designed and built by Philips in Sweden and had a definite continental flavour to it. Build quality was superb but it was quite complex.

At the time this set appeared in the workshop, switched mode supplies were still relatively new. We were learning their strange ways though and only few presented real problems. This particular K12 had been sitting on another engineers bench for the better part of two weeks and still refused to produce any sound or picture other than a grunt when first switched on. Now the engineer in question was no spring chicken. He’d had years of experience from the late 50’s onwards and I knew him well. He could fix colour TV’s from G6’s onwards in the blink of an eye (or possibly the flick of a soldering iron) and most other things as well before some of us had even got the backs off of ours…..well he was very quick anyway. He didn’t like these ‘new-fangled’ power supplies though.

During the course of trying to get the K12 going, he’d poked, prodded, soldered and changed many suspect components, none of which made the slightest difference to the problem. He was convinced the problem was in the line output stage but nothing he tried seemed to work. His last attempt at reviving the set had been to change the line transformer and for a few seconds after initial switch-on, thought he had at last solved the problem. He had a meter hooked to the HT rail, a scope attached to the line driver transistor and at the moment of switch-on the meter indicated the correct 140V HT and there was a nice line drive waveform on the scope accompanied by the rustle of EHT. This was augmented by a loud ‘YES’ from the engineer and then followed by a loud groan as the HT once again collapsed and the set relapsed into silence. The initial elation of the engineer was replaced by the throwing down of the test prods and some expletives that are not repeatable here! The set had been pushed to the back of the bench and there it remained. I think he was hoping to come into work one day and find that it had self-destructed, thus removing the problem of its repair!

Anyway I’d had a good week and on the Thursday of this particular week found that I was ahead of the ‘budget time’ (I’d basically done Fridays work a day early) so I had some time to play with. This was good as it meant I could sort out any awkward sets without affecting the weeks budget. The engineer (Bob) was fixing a G11 when I asked him if I could look at the K12. If he could have thrown the set over to me he would have done and took no time to put it on a trolley and wheel it over along with a small bag of components that he’d replaced. This was engineers language for ‘It’s your repair now…..’!

Well it wasn’t yet lunchtime and when I looked at what he had already tried and replaced, I began to think that perhaps I should have kept quiet…..! I asked him what the original fault had been which turned out to be a shorted line transistor, when this had been replaced, it had arced violently from the bottom of the transistor which promptly went short again. This was caused by the insulator having failed. He’d replaced the transistor and insulator and this is where the problems had started. No more arcing but nothing else worked now. Lunchtime intervened…..

Revived and recharged afterwards, I set to work on the K12. First I disconnected the HT feed to the line stage and established that the power supply was OK. The 140V was present and it also ran a 60W lamp. Now as was common with most sets, the line transformer provided all the auxiliary voltages for the rest of the set so something was loading the line stage. I spent half an hour disconnecting various feeds from the line transformer and Murphys law dictates that it will always be the last one that you disconnect that will be the faulty one. This was no exception and it was the 12 volt rail which fed the I.F’s, chroma, tuner and switching chips in the tuning drawer which seemed to be causing the excess load. With this feed disconnected, I got the set running with a bright blank raster. All I had to do was find out what was loading the 12 volt rail. I spent the next hour or so working my way along the 12 volt rail, disconnecting this and that and worked right back to the tuner…still no joy. That only left the chips in the tuning drawer. I pulled the plug out, tried the set and…..that was it! I now had just a snowy raster but the 12 volt supply was now present. Why didn’t I disconnect the tuning drawer first? Anyway having removed the top cover of the drawer, there were three chips that plugged in. I removed each one in turn and only with all three removed, the 12 volt rail stayed up. Quick order placed with spares and soon I had three new chips. These were fitted and at last the 12 volt rail stayed up. The set produced a snowy raster but now responded to the tuning commands and a few moments later had tuned in the local channels and produced a really excellent picture. All that trouble caused by an arcing line transistor that had not only destroyed itself but three chips in the tuning drawer as well. It had taken a good three hours to sort it but was a good education and another chapter in the book of experience.

Bob and I became great mates after that and often exchanged ideas when dealing with unusual faults.

This repair had a hand in a future change for me….!

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