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  1. #1
    Join Date
    Nov 2013
    Posts
    19

    Default Shopmaster blows breaker MIG only

    I have a Miller Shopmaster 300, s/n KD408963. My manual is January 1993.

    It has worked fine until recently. Now it will sometimes mig weld, sometimes it pops the breaker. The breaker pops right when I pull the trigger, before the wire strikes an arc. I looked at my liner, it had a couple of minor kinks so I replaced it, no change in symptoms. I was welding 1/8" steel, it will weld fine for a bit, pause, pull the trigger again and the breaker pops.

    Stick welding (reverse polarity, 170A) works fine.

    I am using a 50A breaker which has always worked for me. I changed to the 50A circuit for my plasma cutter (different breaker), same problem.

    Advice will be greatly appreciated.

    Steve

  2. #2
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,475

    Default

    Failed shorted capacitor on rail, replace em all

  3. #3
    Join Date
    Nov 2013
    Posts
    19

    Default

    You're talking about the ten caps below the fan? 30,000uF at 50V?

    They read like caps on my meter, but that's only a few volts. They aren't cheap, is there some way to confirm this is the problem?

    Thanks,
    Steve

  4. #4
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,475

    Default

    Probably going to find that one is loose and blown up. Though if one is loose, most of the others are loose as well. You can mark the +& - on the rails and meter (capacitance) on each one. Change out the ones that fail, but rest assured the ones you didn't replace will fail in a short time frame. Don't nessessarily have to buy them from Miller. Just have to buy the same style and same specs

  5. #5
    Join Date
    Nov 2013
    Posts
    19

    Default

    They all look fine, all are tight. I realize that's not saying a lot, I'm just trying to take this one step further before plunking down $300 for caps.

    Steve

  6. #6
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,475

    Default

    Still gotta pull and test them individually. not the rectifier to cause problems.

    Something surges, likely a cap. could be a failed gun as well. Trigger to conduit short may cause a problem
    Last edited by cruizer; 11-27-2013 at 06:09 PM.

  7. #7
    Join Date
    Nov 2013
    Posts
    19

    Default

    I don't follow what you mean by "trigger to conduit short". Are you saying the trigger wires could be shorting inside the gun/cable?

    I did take all the caps out (after marking the bus bars for +/-) and test them individually. I hooked them up to a 40V power supply (all I had) with 50 Ohm series resistor. I turned on the power and watched the voltage climb. All caps took about 10 seconds to reach 38V, which is pretty close.

    Next I could try putting them in the machine one at a time. I don't need to weld for the problem to occur, just pulling the trigger is enough.

    Steve

  8. #8
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,475

    Default

    don't know about the way you check capacitance, also don't know if there is a heat problem with one of your caps. a capacitance test would tell me everything I need to know. Now there are 2 trigger wires in the gun cable sheath and likely one spare. You will want to test them against the copper current carrying conduit. If one is shorted it may cause a surge

  9. #9
    Join Date
    Nov 2013
    Posts
    19

    Default

    The test I did checked the capacitance. I set up an R*C circuit, 50 Ohms and 30,000uF gives 1.5 seconds. I watched each cap charge for 10 seconds; they all got to about 38V (40V supply). I think they are all ok at 40V anyway.

    I don't think temperature comes into the situation, it will pop the breaker on the first attempt some times, stone cold welder.

    I also swapped the trigger wires to the spares. This did not help, unfortunately.

    Steve

  10. #10
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,475

    Default

    Ok, remove the feeder connector, and with a piece of mig wire jumper the amphenol on the machine a & b. Does the breaker blow?

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