Repair Log… (15/5/11) - MPU3 – MOD4 - LAMP FAULT

 

Following on from the previous repair on this board, I realised that not all the lamps were lighting as they should. Initially, I had thought that the bulbs were blown and this proved to be true in some cases, there were even some bulbs missing, but some bulbs were good and yet they did not light.

 

I went after the transistors with a vengeance and ended up changing them all. T12-T26, T29-T39 and T13-T23, none of which cured the bank of 8 missing lamps. Then I changed IC20 and IC11 none of which make any difference. This is one of the downsides of ‘shot-gunning’, you don’t always get a hit. £10’s worth of transistors used and I still have the original fault. All the old parts have been saved though, so it’s not the end of the world. (some transistors did meter bad)

 

Before moving on, I consult with Magik and he suggests a more measured approach. Time to start probing IC’s.

 

I set the board up with the cover open on a stool behind the machine and what do you know the lamp fault is gone! Disconnect board and solder on a battery and the fault is back but strangely different. All the lamps are right, but some are very bright.

 

Bad connection blue plug. Re-flow solder to pins on blue plug. The fault has gone! Once again I fail to check the obvious first.