Old welder, low output in DC (AC is OK)

I bought an old Miller 330 A/BP. I don't have the 104 amps required per the manual, so am testing it on 50 amps in my garage (1 phase,

240V). I have it setup for stick welding right now (waiting for TIG torch to arrive).

I am having two problems:

1. Low amperage output at DCSP and DCRP. I haven't measured the amps (unsure how to on a welder) but cannot successfully strike an arc with a 5/64th 6013 rod. The arc is very weak. I've tested in the low range (5-48 amps) and the bottom of the middle range (~60 amps). AC is fine and readily strikes an arc.

1. At idle and in DCSP/DCRP, it trips the breaker at about 70-80 amps output. However, in AC mode, I can run the amperage all the way up the the maximum 460 amps (again in idle).

I am suspicious of the large diodes. As best I can read the schematic, it shows that AC bypasses them. Am I on the right track? Any suggestions are appreciated.

Thanks, John.