They brought me a Miller GoldSeal 160i that had two blown diodes.
The board is this:

The blown diodes are on the other side, somewhere between the two big heatsinks

As you can see, part of the board is burned out.
The diodes were already removed when I got the board on my hands so I don't know what type they were. I installed a couple of 1N4007 that I had handy, turned the welder on and BOOOM! the diodes blew again in milliseconds.
The traces on the board that lead to the diodes are thin, so I don't think these diodes handle too much current. So either there's higher voltage there that the 1000V the 1N4007 are rated for, or there is a short somewhere else on the welder. Also, since part of the board was missing, I am not sure about the direction of the diodes. I installed both in the same direction as the schematic on the board. Could the other diode be in the opposite direction?