Advice/Suggestions for repairing Apple III power supply

John Honniball coredump at
Mon Jul 23 14:52:19 CDT 2007

Josh Dersch wrote:
> 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.

This is a classic symptom of a switch-mode power supply that's
failing to start up, possibly due to a short on its output.

Such a short could be caused by a tantalum capacitor that's
simply gone short-circuit with age.  The trick will be to
find out which one of the (possibly several) tantalum decoupling
capacitors is the bad one.  An ohm-meter (multimeter on Ohms
range) will read the duff capacitor as a short circuit.

John Honniball
coredump at

More information about the cctalk mailing list