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Serviceman's Log

It's time I bought a new TV set!

By The TV Serviceman

Items Covered This Month

  • Philips 32PW977/75 TV set (GFL2.2A chassis)
  • JVC video cassette recorder
  • JVC AV-28S4E TV set (MXIV chassis)
  • Telefunken SDX290
  • Hitachi HMV-8300 Stereo Amplifier
  • NAD 317 Stereo Amplifier

My own personal TV at present is a 1995 Philips 32PW977/75 (GFL2.2A chassis) which is a fully-featured widescreen set. The reason I own it is that it has an annoying intermittent contrast level problem which over the years successive technicians – including myself – have failed to repair. However, the general consensus is that it’s the picture tube that’s causing the problem and this is far too expensive to replace.

Most of the time though, the picture is perfectly acceptable to watch and it only plays up occasionally.

Recently, I had an identical set come in for repair. This was somewhat surprising, as this fully-imported model is as expensive as it is rare. The fault with this particular set was no picture.

When removing the back from this set, you must remember to unscrew and remove the sub-woofer cover first. That’s because the connecting lead is rather short and you can otherwise end up ripping it out of the PC board.

This set, commensurate with its features, has a lot of electronics on board. This meant that the easiest way for me to locate the problem area was to initially try swapping modules with my own set.

I did this and it didn’t take long to trace the problem to the CRT socket board (R). With my board in this set, the picture was excellent and the intermittent contrast fault did not move over from my set.

An examination of the faulty board soon revealed that R3273 (10Ω) which supplies the 200V rail was open circuit, as was R3272 which supplies the 12V rail. The circuit shows the latter to be 6.8Ω but the set had an 18Ω unit fitted (by the factory) in this location.

Click for larger image

The reason these two fusible resistors had failed was because all three TDA6111AQ/N3 output ICs (7230, 7240 & 7250) were short circuit to ground. I ordered and replaced all these parts and then fitted the board to my own set to test it.

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