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Repair Log… (13/4/11) - MPU4 – MOD4 - DEAD



Power on, led is lit, no activity, D49 is broken (physically snapped). Check for broken tracks and go over battery damaged area. Can’t find any broken tracks?


CPU is stuck, address lines are stuck high. Board has no clock. Replace crystal. We have clock but again, CPU gets stuck. IC18 address lines stuck high. Board is stuck!  CPU is halting.


Go for strip down, IC32, IC33 and the resistor pack removed, IC9 RAM, IC2 timer, clean all tracks and buzz out. Work my way round the board. Everything seems in order. Change IC1, replace IC15 and IC16 just in case. Crazy!!!!! So I do IC20 and 21 also. Now it’s making me fanatical, at this point I will replace every component on the board until I find it... I will get it!


Rebuild. Everything is pulsing! No boot. It should be running, why won’t it boot????? You start to have insane thoughts, why is it not booting??? What have I missed. More tea, stay calm.


At this point, there is no option, but to start again. I’ve missed something. I know all the replacement parts are brand new and my soldering is usually good. Start again!


Back to battery area, check again. There must be a broken track? Round again, all buzz out OK. Remove and replace C47, the battery normally sits in-line with this cap. The battery is missing.


Still no boot. Then I had a thought, Magik said the track looked bad. But it buzzes from component side to track side, so it can’t be? I think, what the hell use a small link wire to make sure it’s definitely connected. I solder in a small link wire and it boots. The probe points must have touched through the large hole, see pic to the right.)