While stripping and repairing the mass-o-migs I have, I found this.
The pitting was caused by the nuts being loose on the stud. The nuts were holding the lug supplying current to the mig gun.
A bad joint here has caused arcing and damage to the threads.
Best result of this is a loss of weld quality. Worst case- fire
I have been sanding all contacts smooth and tightening them to effing tight.
These connections are usually in a location that the user can get to and check.
A little care and they will last a lot longer. I can't help but wonder if this was part of the reason these welders were offloaded in the first place.
by the end of today one more will be functioning. Resurrected