Welder Repairs, Yeah I Do That Too...

[Latinrascalrg1]

i can only speculate, but here it goes.....
the nuts used to attach the 460v/230v bus bars to the PCB board are galvanized.

what i think happened, is that the nuts rusted over time.
the rust added to the circuit resistance on 2 separate poles on the PCB board. ( the other hardware on the board is all brass, except for these screws)
the resistance increased slowly over time and as a result the increased resistance generated more heat than the Board, and fan could dissipate.
but this is only speculation....

I think you hit the nail squarely on the head with your theory of how things went bad.....It makes perfect sense, thanks for sharing.