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87 Millermatic weak initial arc

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  • 87 Millermatic weak initial arc

    Bought an old mm200 off a body guy who said machine worked. New gun, ground clamp and roll of wire and it hates to get initial arc going. Once you do get it to strike(rare) it welds like a dream. Cleaned the contacts on the voltage switch and checked machine on off switch and alc getting 230 in and out. Jumper for 208/230 seems to be in correct position according to diagram behind wire spool. Just slowly piles wire up on Material. Won’t even nip excess wore off tip if you try to clip it on the work clamp. Tried a different m25 gun with the same results. Bad capacitors maybe? Or diodes? Hoping you guys can point me in the right direction. Any help is appreciated

  • #2
    Welcome! You found a treasure!

    May need to clean the contacts on the "W" contactor--or see if they're worn out or have loose connections. First thing I would check. It is at or near the top, on the right side of the center divider when you are looking inside the machine from the front. Has three contacts, all tied together in parallel by a little "three-fingered" looking piece of brass sheet. That thing clicks closed/open every time you pull/release the trigger, and if it's been welding for years, it might be worn. I was fortunate to find one that was old but didn't have a lot of use and the contacts looked like new. Just cleaned it up and started welding. Fantastic machine!

    After checking the contacts (welder UNPLUGGED), have someone try to weld and watch the contactor. See if it looks like the contacts are pulling in tightly.

    If it welds like a dream once it gets started, I would doubt you have bad diodes or caps, but I suppose there is a possibility of caps since they would provide a "surge" of power to help get the arc started. What is your electrical experience? Caps can hold a nasty bite for some time after the machine is turned off. Be careful. If you need info on discharging caps, let us know. Saw a guy cut a deep gash a foot long long on his arm when a cap surprised him and he caught the pointy end of a sheet metal screw while jerking his arm away. Lots of blood.

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    • #3
      I will check that contactor out in the morning. My electrical knowledge is basic. Understand switch operation and low voltage motor control but I am not farmiliar with capacitors or the correct way to bleed them or test them out. They all seem visually ok. No signs of oil leaking anywhere.

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      • #4
        Checked the contacts. They were a little boogered up so I took them down with a file to clean brass. Seem to be pulling in correctly but still not welding right. I notice two other small relays up and to the right of the w contact. The higher of the two relays clicks when you pull the trigger on the gun but the one just below it does not. Is it supposed to be?

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        • #5
          You need full power through that contactor. You really can't clean the points. You should be able to match that contactor at an electrical supplier. 60 amp.
          One other thing you may look at is the contacts on your main switch.

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          • #6
            Ok I will try the local supply house and see what I can come up with

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            • #7
              The main switch is also a good thought. Even though you read voltage coming through it, sometimes if the internal contacts are worn out, they will intermittently not pass enough power for whatever is downstream to work properly. You could check that easily, if my memory is correct that the terminals are screws (haven't had the case off of mine in several years). Just unplug the machine and move the wires from the downstream end of the switch to the same side terminal on the other end, putting both the wires to the welder internals and the power line under the same screw, bypassing the switch completely; plug it in and see how it works.

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              • #8
                Good idea. I will bypass on off switch and see if that helps then do the same with w contactor. Free time is right lately so il check as soon as I get a minute. Thanks for the help

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                • #9
                  Good idea. I will bypass on off switch and see if that helps then do the same with w contactor. Free time is right lately so il check as soon as I get a minute. Thanks for the help

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                  • #10
                    Aha! We’re back in business. I bypassed the contactor as You suggested and the machine now welds great. Now just to hunt down a 3 pole 120v 60amp contactor. I am a plumber/hibachi tech by trade so I’m sure I can find something at the supply house reasonably priced. Thank you guys for your help. This is the first welder I’ve personally owned although I’ve been welding for about 15 years just for automotive hobby stuff. I’m excited to get this thing cleaned up and into service

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                    • #11
                      Hvac tech. Not hibachi, although that would be interesting

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                      • #12
                        Well I darn sure wouldn't hire you to work on my AC if you were a hibachi. Unless I was hungry. We all do strange things when we're hungry.

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                        • #13
                          That was one fine autocorrect! With HVAC background this fix ought to be a no brainer for you! Hibachi background, not so much.

                          Glad you found such a simple fix. One of the great things about these old machines is almost everything is a simple fix if you're patient. You will absolutely love that machine. Just be very kind to the wire feed motor; virtually impossible to find a replacement, I'm told. I think most anything else in there can be fixed; Miller even provides a circuit diagram for the PC board for that machine, which they usually don't do, so I'm reasonably confident I can fix whatever might go wrong with mine unless the transformer toasts. I might even try rewinding that if it failed, but transformers generally outlast their owners, and at my age, that's probably a pretty good bet.

                          1997CST, thanks for the words on (not) cleaning the points on the contactor. I am an electronics systems guy, not a welder guy (or a hibachi guy, for that matter), and would have guessed that cleaning them up would work. Thanks for passing the benefits of your experience on to the rest of us!
                          Last edited by Aeronca41; 03-02-2019, 07:38 PM.

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                          • #14
                            Miller 2 different contactors in that model. If you have trouble finding a contactor post the serial #. They are still available.
                            Aeronca41. No worries. I like helping when I can.

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