RoyJ wrote:
If the charger doesn't work but the main unit does work, the two usual
culplrits are a bad contact on the selection switch or a blown diode.

In most electronic equipment, switches are not much of a problem but
welders tend to have high currents, dirt, open design, and not high
useage of the switch. Look for cruddy contacts, buff them lightly with
some soft abrasive (NOT steel wool!) and see what goes.

Simple ohm meter check from both directions shouls spot a bad diode.

Odds are good you could find the culprit in half an hour of open case
diagnostics.

The contacts in this machine are submerged in oil, and there are no
vents at all, so crud can't enter. Do you think this makes it more
likely that it's the diode that has failed? Would you expect just one
diode or four? Any idea what a replacement diode is likely to cost?

If I open the machine, should I also change the oil? The guy at the
welding supply store suggested this, but I wasn't sure if it was really
necessary, or if he was just trying to make money out of a service. He
said something about the oil getting contaminated with moisture.

Thanks for the advice.

Chris