The Philips MD1 chassis from the mid to late nineties seems to be one of their more difficult sets to service. And unfortunately for muggins here, I had two arrive in short succession. These were both 1996 MD1.1A chassis and both were DOA (dead on arrival).

The first was a 29PT886A/75R and the client said that it was smoking just before it died (emphysema, I expect – I guess it should have read the warning on the packet!). Sure enough, when I switched the set on, there was no sign of life and even the smoke had gone.

Access to the motherboard or LSP (Large Signal Panel) is rather difficult in this set. Philips has gone to an awful lot of trouble on page 4 of their service manual to show you how to get the set into the service position – even supplying a special support pin. The trouble is, the leads aren’t long enough and the instructions didn’t completely make sense to me (what on earth is the AVM/AMV panel?).

Anyway, having positioned the set as best I could, I gingerly poked a long probe into the switchmode power supply. There was no voltage coming out of the supply so I assumed that it was was faulty and stupidly unplugged the LSP without marking any of the plugs. In fact, it would have been nice if Philips had labelled these.

After recovering from the 315V shock I then got from the undischarged main electro, I used an ohmmeter to check for open and short circuit plugs and to measure the start-up resistors, etc. I couldn’t find anything wrong, so I replaced electrolytic capacitors C2530, C2533, C2527 and C2520. I also socketed and swapped the switchmode IC (IC7520, MC44603p) and then checked all the diodes, fuses and FETs on the A1 power supply circuit. These were all OK, as was the line output transistor.

When the plugs and sockets were finally sorted out, I switched the set on to find that nothing had changed. My hunch now was that perhaps a protection circuit was closing the power supply down, so I decided to switch tactics. I removed the deflection yoke plug (L17) and connected a 100W globe on the 140V link input (pin 2).