Apple II Plus power supply strangeness

Mike van Bokhoven mike at
Sun May 17 00:51:14 CDT 2009

I've had a little more time to poke at the Apple II Europlus. It's a 
240V model - reason I mention that is that the power supply is doing 
strange things. If I power it up with no cards, it will do nothing for a 
minute or so. Looking closely, the power light glows very dimly; the +5V 
PS line is sitting at about 1V or so. After a little time, the supply 
will suddenly come up completely, and the machine will work perfectly 
and pass all tests for maybe 30 seconds, after which the memory slowly 
becomes more and more corrupt.

If, while the PS is not running correctly, I hit the Reset button, that 
will kick it into life. Odd, but I guess Reset lowers the load on the 
PS, allowing it to start properly.

My intention is to replace all the caps on the output side of the PS. In 
the meantime, does anyone have any insights into why a power supply 
might behave in this way? Seems odd to me, though my understanding of 
switch-mode power supply theory doesn't stretch terribly far.


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