MM35 guns and diodes!!

[Chdhpkns]

Hello! I picked up a 1978 MM 35 (sn# HJ 13331) at a yard sale for $40 not working. Guy didnt know why? Brought it home pluged her in fan came on wire fed could hear contactor engage no arc. Took diodes out 3 were open, also contact points worn down to copper. Ordered 4 diodes 300v 150 amp 2 pos 2 neg $17 each and contactor 24v coil 40 amp 3 pole $15. Just want to make sure I dont blow the new diodes. Also read the original gun is a P.O.S not sure what gun I have (see pic). Single dad tight budget can I get by with the gun I have and how do I go about finding the cause of the diode failure? Any help input guidance greatly appreciated

[cruizer]

It's a really old machine, likely was used over its capability, or merely failed due to age. Hard to say