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Macintosh Classic II Analog board randomly restart


New member
Hi everybody,
I am curently trying to revive a Macintosh Classic II. I have trouble with the analog board : it randomly restart. Most of the time, on a cold boot, the computer first enter in a restart loop. It restart every 5 or 10 seconds and after some time it finally boots and works perfectly.
The logic board is recapped and works perfectly with an other analog board.
The analog board is also recapped (I have replaced all the electrolytic capacitors). In first time, it presented a low voltage issue. I solved this by replacing both DP3 and DP4 diode and the QP1 optocoupler. I now have a stable 5.08v on the 5v line and 12.2v on the 12v line. I have also no display issue.
The only issue is these restarts.
I have tried to replaced the TDA4605 ic. It has changed nothing. I have mesured at multimeter all the diode on the board. They all seems correct to me.
Do you have any clue about this issue ?
Many thanks for your help.


New member
Yes, maybe a defective component is shourted out by intermittent but how can I find it ?


Active member
The restarts could be happening from the attempts of the Classic to boot and due to low voltage failing to do so. It is repeats and repeats due to voltage being too loo and flactuating around that value.