Ok I definitely do not have a sound knowledge of electronics, one thing I have noticed is I did take a look inside the welder at the panel and at the AC DC change over switchLikely possibilities are: faulty AC/DC front panel switch; commutator board fault; a fault with the board that is sending control signals to the commutator board. A sound knowledge of electronics will be required to fix this.
After looking at the wiring diagram I can see there are 4 diodes built into the AC DC change over switchLikely possibilities are: faulty AC/DC front panel switch; commutator board fault; a fault with the board that is sending control signals to the commutator board. A sound knowledge of electronics will be required to fix this.