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Repair Log… (12/4/11) MPU4 – MOD2 - Lamp fault


No point in powering on the board, the area around PL11 is burnt and all the components R33 to R43 and D51 to D60 are completely destroyed,


Start with the obvious, replace all the damaged transistors and diodes. I use pulls from a scrap board for this because the resistors in my parts stock don’t look up to the job. The board is severely burned and has warped from the heat. I was wondering whether this repair would be viable. Clean up all the charcoal and the tracks are OK and the pads have survived.


Power on and the machine reports MUX selects alarm. I replace IC23 to illminate it and IC13 and IC14. Still no joy. I suppose it could be a shorted transistor? Well that’s what the machine is telling me.  Still MUX alarm. Bugger!!! By now I’ve been on it all day and my heads gone. Magik is online so we do some tests with the probe. Anyway, not to be out done, replace IC3 as well. Still alarms.


Had to give in and go to bed. Lying in bed thinking about something Magik had said. If green is the power and blue is the ground, then blue needs to be connected to complete the circuit otherwise it's open. (he said) So if there was a way of grounding each pin on the blue connector individually then we could establish which transistor is shorted. (I will construct a small PCB to do this in the future and use PCB switches or jumpers)


Using the newly constructed bad transistor-sniffing out loom. Machine says MUX selects alarm, so it should be on the blue side. I cut up the blue loom and connect each wire via terminal block (see pic).


So start with all connected, MUX alarm, remove pin 17 - alarm, 16 - alarm, 15 - ADC INIT. It boots! It's a bad transistor on pin15 of the blue connector.

Got it!!!