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
Using the newly constructed booze.net 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.