23 Jul
2007
23 Jul
'07
2:59 p.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).
I'd check all electrolytics. I've had this fault in another PSU caused by
a small capacitor on the primary side of the supply which smoothed the
supplie to the chopper control IC. With that open, the supply couldn't
keep running. An open (or high ESR, which amounts to much the same thing)
capacitor on the output side will put high-ish votlage spikes on the
output (say around 10V amplitude) which will most likely trip the
crowbard circuit and cause just what you are seeing.
Of course it could be a short in the laod, I'd unplug the PSU from the
rest of the machine and connect dummy loads (car bulbs are good for this)
just to be sure the fault is in the PSU.
-tony