Thunderbolt TU?

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I purchased my Miller 225 Thunderbolt in 1983 new.  I had an emergency repair out in the field to do and all I could round up for a generator was a 5 KW.  I used it for a while but near the end of the repair I was having trouble holding an arc.  When I tried using it later on commercial power it will strike but will not hold an arc.  I took the cover off and everything inside looks almost new, no corrosion evident and nothing smells burnt.  It is so simple looking it is hard to identify the problem.   Any ideas?
Reply:I'm a bit surprised you managed to get much out of that machine on a 5kw genset. Rough numbers gives you maybe 20 amps at 230v max from that genset assuming the 5KW rating is continuous rather than surge. Usually you are limited to the lower end on a 30 amp breaker.My 1st thought is that you might have dropped the input voltage below acceptable levels, if the genset was bogging down under the heavy load. Low input voltage might have damaged the welder. I'm wondering if you blew a diode. Is this an AC machine or an AC/DC machine?.No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:The Thunderbolt is an AC machine.
Reply:I I checked my parts manual for my Thunderbolt and I can not find any reference to diodes.  I looked it over and I can not find any.  Any body else have an idea what could be wrong with it.