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Bolacore

Macintosh SE wont power on if logic board is plugged in

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Hello, I've picked up a Macintosh SE that was said to have a broken CRT, and did not power on at all. Indeed if I powered it on with everything connected, the 5V rail seemed dead and only 0.5V was on the +12V rail. Since I had them on hand, I replaced all the electrolytic caps on the analog board and inside the Sony CR44 supply. I put the Sony power supply on a load after and it seems to be able to put out the rated current on all rails. Powered it up with just the analog board, plus CRT + yoke boards connected and the fan spun up and I saw 5.6V on the 5V line and 11V on the 12V line. 

 

However, if I connect the logic board, the fan no longer spins and I get the same really low voltage levels as before. Nothing seems to get hot though - if I look at the logic board after 10 mins with a thermal camera everything is cold, and everything on the analog board is too. There's a short somewhere (I'm assuming), but I cannot seem to find it. If I measure the resistance at the analog board of the voltage rails everything seems fine. The logic board seems fine too, although it's 5V line measures 110 ohms at the J12 connector. 

 

I'm not quite sure where to start, as it's not obvious to me which piece is at fault. Has anyone else seen this behaviour before? 

 

Thanks in advance

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