I picked up a 1978 Millermatic M35 at a garage sale this weekend for $40. Dude told me It didn't work. Brought it home and 3 diodes are bad, plus the contactor was shot. Ordered 4 new diodes at $17 a pop and contactor for $15. I noticed the owners manual said something about gas only welding, can I reverse polarity and use flux core?