23 Jul
2007
23 Jul
'07
9:42 a.m.
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. Obviously I haven't
run it for more than a few seconds like this :).
There are no obviously bad parts (burned parts, swelled/leaking
capacitors, etc...) that I can see.
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.
Disconnect the PSU from the rest of the computer before you try anything else.
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).
Richard