To me it indicates a high resistance developing as some poor connection
heats up in the circuitry feeding the filter capacitors. If you have an IR
temperature gun, look for hot spots when the welder malfunctions.
(If you don't have a noncontact temperature probe, unplug the welder
and *immediately* feel around for hot spots when it acts up.)


Good idea Gary. I'll keep that in mind.

Likely places to look would be the bolted connections to the SCRs,
primary or secondary connections to the transformer, the circuit breaker,
etc.


SCRs were taken off for testing. Transformer is a likely suspect.
Circuit breaker can probably be ruled out as he was using another
welding machine on the same circuit. Although the power switch of the
welding machine in question might have something to do with it.

I know you said you checked the filter capacitors, but they could still
be at fault. One or more of them could be developing a high ESR as
they heat up under full load. But I'm betting on a loose or corroded
bolted connection somewhere in the circuit before the capacitors.


The capacitor issue. I had the same feeling. I mean after 13 years of
high current operation and they still maintain their original
capacitance?
We might just change the caps as well as the SCRs on spec. Just
because...

But sure enough we will check all the connections...

Thanks Gary!

Alex