68kMLA Classic Interface

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Macintosh Classic under-voltage

Posted by: ahahthunder on 2018-03-04 11:39:18 I got a Macintosh Classic in decent but non-working state. After washing and recapping the logic board it worked great. A few days later I've noticed what seems like the analog board failing. The computer turns on with wavy horizontal lines for a few minutes until the logic board comes on. Then it takes another minute for the hard disk to start spinning. (I've plugged the hard drive into a known working power supply and it spins up right away.) However last time I used the computer, after warming it up and using it for a few hours it crashed into a restart loop. Now it won't ever get to start the logic board. I suspect this is a power supply issue or at least a known, common problem. How do I fix this?

Posted by: Bolle on 2018-03-04 12:10:53 Analog board caps are notoriously bad on the Classic. You will have to recap the analog board as well. Mine was the same... started with wobbly screen, then took some time to actually boot after applying power and then failed completely once warmed up. Recap and adjusting the voltage regulation so 5V and 12V outputs get into good ranges again and it worked will start working again - mine is rock solid now.

Posted by: bibilit on 2018-03-04 23:53:40 Yes, as Bolle says, first thing is to replace capacitors. I will recommend replacing two diodes as well (DP3 and DP4) seen all becoming leaky recently.

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