Advice/Suggestions for repairing Apple III power supply - test-drb@ccmp.vtda.org

23 Jul 2007

On 7/23/07, Richard Smith <richard.smith at mewgull.com> wrote:
...
  Hi all --

Picked up an Apple III this weekend in non-working condition, and I
suspect the power supply is at fault.

Symptoms are:  On power up, a rapid clicking noise is emitted from
the power supply (maybe 4Hz or so), and the power LED on the
system's motherboard pulses at the same rate. 
 It's an early  switch-mode power supply and the ticking is the device
 trying to start up, the control circuit says something is wrong, so
 it shuts down. And cycles all over again. 
I concur.

...
  Disconnect the PSU from the rest of the computer
before you try anything else. 
Indeed, though you might get different behavior with no load
whatsoever.  This is where those 6V headlamps come in handy.

...
  I would trouble-shoot by looking first at Q1 since
that is the
 switching transistor, and then maybe all the electrolytics on the +5V
 and -5V rails since they are used to provide the reference signal for
 the feedback circuit. It's all very simple (compared to modern
 switchers). 
Agreed.  One other thing to consider - I've seen this behavior in
other machines when there was no problem whatsoever with the PSU - the
problem was with the motherboard - a dead short, typically.  If you
test the PSU with no load or a dummy load, you might get the PSU to
start up.  If that happens, check the resistance of each voltage input
of the motherboard to ground - resistances near 0 or 1 ohm would
suggest a problem on that rail.

I last saw this behavior when I attempted to use a modern ATX supply
on a Blue&White Mac G3 - the PSU ticked and wouldn't start.  Turns out
that Apple recycled the pin for -5V as an additional ground.  Clipping
that pin solved the problem (the Mac doesn't need -5V) - it all
started normally and runs fine.  Stock ATX supply and stock G3 mobo
results in ticking.

Without further investigation, you can't easily tell, but it could be
either the PSU not starting or it could be the protective circuit
kicking in.  You can do a lot with a VOM and some resistive loads.

-ethan