I have a Miller Shopmaster 300, s/n KD408963. My manual is January 1993.

It has worked fine until recently. Now it will sometimes mig weld, sometimes it pops the breaker. The breaker pops right when I pull the trigger, before the wire strikes an arc. I looked at my liner, it had a couple of minor kinks so I replaced it, no change in symptoms. I was welding 1/8" steel, it will weld fine for a bit, pause, pull the trigger again and the breaker pops.

Stick welding (reverse polarity, 170A) works fine.

I am using a 50A breaker which has always worked for me. I changed to the 50A circuit for my plasma cutter (different breaker), same problem.

Advice will be greatly appreciated.