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Thread: Millermatic 350P issues

  1. #1
    Join Date
    Aug 2021
    Logan, Utah
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    Millermatic 350P issues

    I bought two Millermatic 350P welders from a local fabrication shop. They said they were "blown", but had been in storage long enough that nobody knew exactly what was wrong with them.

    I changed both to 240 VAC, "single phase". Both welders come up and I can go thru the menu and logic, and they appear to respond correctly. I reset both of them to factory default settings. Both had been sitting long enough they had no info on run time or # of arc starts. I borrowed a MIG gun and ground lead from a MM252 at work, just to test. I ran .035" flux core wire for testing.

    The first welder would not strike a real arc. When I pulled the trigger, the wire did feed, but did not burn away, although there was some sparking. I released the tension in the wire drive, and tried to scratch start an arc. During this testing, the voltage was set at about 20. When the wire was not touching ground, when the trigger was pulled, the panel indicated 87.5 volts, when grounded the panel indicated ~3 volts. It did burn the wire off a little at a time as I drug the wire around on the test piece, and watched the voltage fluctuate on the panel.

    I rigged the cables to the 2nd welder. It struck an arc and ran fine. I welded on a trailer loading ramp for 15-20 minutes, at which point it stopped responding to the gun trigger. I looked at the display, and it was blank. I tried turning the voltage and speed knobs, but no response. I pressed the setup button, and got a flicker across the display, but then it was dead again. I switched off the main power switch and switched it back on after about 5-10 seconds. No change.
    Switched it off for 1-2 minutes, switched it back on, and it came up, but had the previous voltage/wire feed settings. I adjusted the settings (Voltage/speed), it seemed to run fine, welded for about 15 minutes again, and it went dead like the first time. I switched it off for 1-2 minutes, it came back up, again with the former default values, I changed those to about 20 V and 240 speed, switched it off for 10 seconds, switched it back on, this time it held the settings.
    I welded again for about 10 minutes, at which point it seemed to go dead again. No response at the trigger, but the display was alive. I lifted the panel to watch the feed rollers to see if they were still working, and noticed I was out of wire... So that kind of concluded my testing.

    I called Miller Tech Support to see if they could shine any light on the situation for me. The gentleman helping me simply offered that the first machine seemed to have a control feedback issue, and the second machine may be going to a "brown out" state. He was kind enough to provide information on 3 service facilities that are located about 90 miles away. I did ask if these two machines are compatible enough for me to take parts from one and add to another. He said they might be, based on the serial numbers.

    The first machine is model 907300, serial LJ070492B
    The second machine is model 907300 serial LH460159B

    Both welders appeared to be quite dirty, with the usual weld shop grit and debris build up. I have wondered if I have a bad connection at one of the circuit board connectors, but haven't gone so far as to clean everything and try to clean the contacts and interface connectors. I have vacuumed and blown out a significant amount of dust and dirt.

    I bought them cheap enough that if I can just build one welder out of the two, I'd feel really good about it. If it is possible to repair them both for not too much money, that would be great.

    Might anyone be able to tell me what boards might be in question, or even what components might need to be replaced, or where I should be looking for issues?

    If you have time to share your insights, I'd greatly appreciate it.


  2. #2
    Join Date
    Jul 2018
    Cumbria, UK
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    Re: Millermatic 350P issues

    Back to complete basics: take the covers off, clean the boards with IPA and compressed air, and check all connections, look for damaged wires and connections first. Then look for signs of overheating on the boards, traces burned, components scorched etc.

    That's all free, and anyone can do it...
    Murphy's Golden Rule: Whoever has the gold, makes the rules.

  3. #3
    Join Date
    Sep 2016
    Detroit, Michigan
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    Re: Millermatic 350P issues

    This isn't much help, but something doesn't sound right with the shop's story. A running 350P will still bring $2,500-3,000 even if it's dirty and on the rough side. Shops don't just roll expensive machines like that into the corner and leave them if they break...they get them checked out. I'd be willing to bet the repairs required were going to cost as much as the machines were worth so they bought new machines.
    Check out my bench vise website:

    Miller Syncrowave 250DX
    Millermatic 350P with XR AlumaPro
    Miller Regency 200 with 22A feeder and Spoolmatic 3
    Hobart Champion Elite
    Everlast PowerTig 210EXT

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