1985 Millermatic Auto Arc 130

[Choocher89]

I have a Millermatic Auto Arc 130 from 1985 that’s been a good machine but recently has developed a problem. The wire feed speed stays all the way maxed out no matter where the control knob is set to. Iv been using it like this for awhile I just have to set the voltage all the way up to match the wire feed speed. I really like this welder and I’d like to fix it. Iv checked the potentiometer on the back of the dial and it changes ohms as I turn it up or down. I don’t know too much about working on welders and was wondering if anyone could help.

[MAC702]

Include serial number and spec number for best help. Does the potentiometer go to a control board or directly to the motor?

[Choocher89]

It goes to a “motor control board” thinking a transistor might be bad based off the research Iv done but I don’t know much about working on boards. It welds pretty good with the voltage all the way up. Iv just been using it like that and it’s been working fine for me but I’d like to get it fixed.

[jjohn76]

How comfortable are you with electronics trouble shooting? The schematics for the motor control board are in the manual, and most of the components should be easy to find. I'll have to take a look at the schematic later in the week, but should be able to recommend some places on the board to check.
Jon

[jjohn76]

I don't think anything is wrong with your motor transistor(Q51 page 36 of your op manual), or you would not have any trigger control - the motor would be full on all the time until resistor R72 burned out, and then you would have no motor turning. For some reason, you're not getting the back emf (voltage across the motor when it's turning is based on rpm and current through it). I think it's a matter of figuring out what is connected to either U50 or A50 that's shorted to ground. With the board removed, what do you get when you do a resistance check with your multimeter between pin 9 (positive/red lead) and pin 5 (black/com lead) on U50 (LM3524A PWM integrated circuit, you can search for it to find the pin locations)?