We have an older Lincoln Na-5 that when you press the button to start welding the wire runs in reverse. So basically you press the start button and the wire sucks back up into the tip.  This has happend more than a few times and it is a quick and easy fix I know bc the repair man changes no parts out and is in and out in minutes,though When he comes he doesn’t like anyone looking at what he’s doing so it’s kind of pain. We have blown the boards off bc we thought maybe some grinding dust had bridged a gap Between two prints on the board but to no avail. I’m not an electrician but I can use a multimeter and run tests by directions but I really have no idea where to start. My thinking of I knew nothing would be that somehow polarity switched making the motor run backwards but that can be the case bc when you inch in it feeds correct. It only feeds backwards when trying to weld. Any help at all would be very appreciated.