View Full Version : Millermatic 130XP - rheostat and PC1 problems (maybe worse!)

05-02-2012, 11:51 PM
This is a copy of a message I just sent to Miller Tech Support. I am posting it here just in case anyone else has experienced anything even remotely similar.
I am a NOOB MIG welder, but learn new things quickly. I only hope what I learn from this experience, I can share with others. This post may seem long-winded, but I have tried to supply as much detail as possible about my welder problem. If I need to supply additional info, feel free to leave your questions and/or comments.

I recently bought a used Millermatic 130XP for home use. The former owner gave me a decent deal due to problems with the wire feed.
Being a handy guy, I figured I could fix it, (thinking it might be a flaky fuse).
When I got it home, I fired it up and noticed the wire feed didn't run at all. I pulled the cover and checked the fuse, it was fine. I disconnected power, disconnected the gun, cable and stinger connector from the unit and removed the wire roll from the spool. I jumped the trigger connections to simulate a pulled trigger on the gun. I then set the power selector to "fan only" and plugged the unit in to the wall. When I turned the power switch on, the fan ran, but the wire feed motor didn't operate, even though the trigger connector was jumped.
I switched off the power then changed the power selector to 4 and switched the power back on. I turned the wire feed to 100 and...NOTHING!
I began tapping in the little black relay (PC1) on the circuit board...HEY, the big contactor clicked closed and the wire feed motor started turning!
When I turned the wire feed rheostat downwards, CLICK, the contactor opened and the wire feed motor stopped running. I turned the unit off and unplugged it from the wall. I suspected a rheostat problem. I removed the wires from the rheostat, drawing a little diagram as to which of the 3 lugs the 2 wires were connected to, (and marked the wire connectors with a sharpie). I then removed the rheostat from the unit for visual inspection and resistance testing.
After checking the 3 lugs with the multimeter, I determined there was a fault in the wire winds because connecting the 2 outer lugs gave me NO resistance or continuity. I found that by connecting to the middle lug (wiper) and the unused lug on the rheostat, I had an almost complete range of resistance from 0 to 16 ohms. When I connected to the wiper and the other "used" lug, I only got 16 ohms at the end of travel, but when I turned the wiper, the resistance immediately dropped to 0. Upon close visual inspection, I noticed there was a break in the wire winding right next to the end of the wiper travel. This explained why the motor only ran at 100% wire speed!
Out of curiosity, I replaced the rheostat back into the unit and connected the wires to the middle wiper lug and the previously unused lug on the rheostat, figuring the rheostat would work, but in reverse.
I plugged the power cord in and turned the unit back on, NOTHING...HMMM!
I then tapped on the little black PC1 relay on the circuit board...CLICK...HEY...the contactor closed and the wire feed motor began running! I turned the rheostat knob back and forth to test my theory, and I was right! I had almost a full range of motor speed in all power ranges!
Seems like I solved one problem, the rheostat is bad, but there's a second problem with the PC1 relay on the circuit board. It needs to be smacked around a bit with a screwdriver handle in order to send voltage to the contactor and wire feed motor circuit.
My question to Tech Support, I suppose, is...DID I diagnose the problems correctly and completely, or, is there yet another problem, other than a bad relay on the circuit board that is causing me grief here?
I figure since the wire feed motor runs well, through all speed ranges and power levels, (albeit with the speed selector being turned the opposite direction on the housing). All seems to work with a rap on the PC1 relay. The only thing is, if I turn off the power switch and turn it back on, NOTHING WORKS!
That is...until I rap on the PC1 relay again.
THIS WILL NOT DO! How do I fix my newly purchased welder? I know it's repairable, somehow!
And you know what? I haven't even checked the arc yet! Or, for that matter, the gas solenoid!
I have a feeling it will work just fine, I just need some good advice!

05-03-2012, 03:03 PM
Did you test for correct voltage to the board relay and motor?

05-03-2012, 08:12 PM
No, but, after rapping on the little relay on the circuit board, I was able to make the contactor close, then, the wire feed moter started cranking away.
I didn't mention that I found a break in the wire wind on the rheostat near the end of the wind, causing the rheostat to only work at the highest speed. By removing the leads from the rheostat and connecting to the wiper and the UNUSED lug on the rheostat, I had almost a full range of speed adjustment, in ALL power ranges. The only problem was...the dial worked in reverse, a temporay solution, but not something I want to use. I intend to replace the rheostat ASAP. But, aside from rapping on the resisistor on the circuit board to get things started, I was able to get the wire feed motor to work well.

05-07-2012, 12:28 AM
two things. this welder does not have output or wire drive in the fan only position.
You need the contactor to pull in before you can make the motor run. Motor uses weld voltage as the power source. Speed is controlled by the rheostat,
This voltage goes through the fuse on the pcb, This all the pcb has to do with the motor.
The pcb is a voltage sensor. after you pull the gun trigger the pcb allows the contactor to pull in. BUT if the pcb does not see weld voltage it drops the contactor right back off. takes about 1/2 second. I think miller no longer sells your pcb. but you should be able to get a new relay from an electronics supplier and solder it to the pcb. But first remove the pcb and look at all the solder connections under a magnifying glass. Look for cracks around the relay pins.
Just resolder all the connections and add a drop of fresh solder.
Get help if you have never done soldering.