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Millermatic 200 - Big spark without trigger being pulled

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  • Millermatic 200 - Big spark without trigger being pulled

    Friend of mine has a Millermatic 200 from the late 80's (JH254133) - it's a great welder, but has received minimal love and attention, it just works every time.

    The issue that I have with it is that when you're using it, about half the time, if you touch the wire to the work, you'll get a big spark. I'm not talking a little static kind of thing where you touch the door knob and see a little spark, but a full blown arc, spatter flies in all directions, blinded by the light, etc. 20 years ago it didn't do this. I've welded on a dozen other wire welders and they don't do this - you might get a little spark plug sized spark but that's it.

    I looked at the schematic and figured that there must be a bunch of energy stored up in the low side transformer coil and maybe Capacitor 19 and Resistor 16 (both in the lower left) are responsible for dissipating this energy once you let off the trigger?

    Has anyone seen this before? Is it a common problem? Am I looking in the right direction?

    Any help or suggestions are greatly appreciated!

    Thanks!

    Click image for larger version

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  • #2
    Does it arc once and stop, or will it keep on if you let it? My guess is sometimes the contacts on contactor W are sticking; they are the ones just above the diodes in the lower left side of the schematic. It's also possible the trigger switch in the gun is sticking, or there are worn wires in the gun that are making contact sometimes and causing the welder to think the trigger is pulled. If it's just a "once and done", it could be that the bleeder resistor across the output caps has a bad connection (intermittent). That's R16 I think, (not very clear) but I think that's really a long shot, and I'm not convinced that would cause your problem. The contactor sticking, or a short in the gun wiring, is much more likely. If you take the cover off the machine, contactor W is looking you right in the face. It's on the right side top of the center divider panel in the welder as you look from the front. You can see the contacts work. Open it up, release the pressure on the wire feeder, and work the trigger. The three sets of contacts are all in parallel to carry the current, and I would guess one of them is not releasing now and then when you let go of the trigger. If they are all staying closed sometimes, the short in the gun or it's associated wiring inside the welder is more likely the problem. The "associated wiring inside the welder' also includes relays CR1 and CR2 ; either of them sticking closed would cause your problem. Just have to start with W and work backwards. First, though, I'd try jiggling the cord on the gun around and see if any relays click inside.

    Another thought--you could eliminate the gun/cable question by unplugging the trigger wire from the front panel each time you quit welding (right after you release the trigger--probably need a second guy) and see if it still occurs. If it does, the problem is inside the machine, probably a relay. If that stops the problem, you have a bad gun/gun cable.
    Last edited by Aeronca41; 07-09-2019, 07:13 PM. Reason: Another thought.

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    • #3
      Aeronca41 - thanks for the insight. It's a single arc/discharge, it won't sustain.

      I had thought about the trigger but there's no wire feed or gas flow when this happens - just simply touch the exposed wire to the workpiece and "poof" it goes (sometimes). Per the schematic, if the trigger is intermittent, I think it would engage the wire or gas sometimes, but it has never done that uncommanded.

      The sticky contactor makes sense with the random nature of it - a failed capacitor or resistor would typically do it all the time (though there are 8 capacitors so it's possible that a few of them are intermittent). Now that you mention contactor W, it also sometimes has a "hard start" where it will not weld very well initially - typically this is chalked up to a bad ground because you're welding on rusty junk most of the time but if one of the sets of contacts are not making or breaking cleanly, this could cause this as well.

      Thanks again for your thoughts and opinions - next time I'm there I'll take a look at "W" and see what I can find and then report back.

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      • #4
        Great! Anxious to hear what you find. agree; if the wire isn't feeding and no gas, it can't be the gun....I should have thought through that a bit better.
        I love my MM200;' there's just nothing like it. I do hate the old clunky GA20 gun, so since I never use it at full capacity, I got a real buy on a Bernard Q150 on a closeout at Cyberweld, and haven't looked back.

        If you do need a new W contactor, take the old one to an electrical supply house--it's a standard part, and it will cost way less than buying from a welder repair place. I think CR2 is only used if you have the "spot weld panel", but CR1 is the relay that is actuated by the gun trigger. It has multiple sets of contacts, and it is possible the set that actuates W is sticking while the others are OK, but W is a pretty common failure. I don't know for sure, never tried it, but some welder repair guys here have said don't waste your time trying to dress the contacts on W; it lasts for only a very, very short time--just buy a new one.

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        • #5
          It might be your heeberhober, but that’s really hard to check. It also could be the doofloppy. You should and make sure your doofloppy is unchooching when it’s supposed to unchooch.

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          • #6
            Originally posted by ryanjones2150 View Post
            It might be your heeberhober, but that’s really hard to check. It also could be the doofloppy. You should and make sure your doofloppy is unchooching when it’s supposed to unchooch.
            I have already checked the thingimajigger and doohickey for proper function, but hadn't considered the others that you recommended. I'll add them to the list of things to check .

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            • #7
              Yeah, you really need to be sure the doofloppy is ok....and don't forget about the transmognifier.

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              • #8
                make sure the plastic insulators where the lead plugs in isn't melted and touching the case. Common problem in our shop 35 years ago when we were running them..Bob
                Bob Wright

                Spool Gun conversion. How To Do It. Below.
                http://www.millerwelds.com/resources...php?albumid=48

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                • #9
                  Bob - do you mean the part circled below?

                  Click image for larger version

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                  • #10
                    I second what Bob said and reply yes.

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                    • #11
                      Looks like a late generation 200 built by Miller Electric after they heard about the hollow wire full of dust needing to change polarity to run.

                      Look at the caps for a bleed resistor that wore out and blew up.
                      The arc is just the caps discharging via the gun & wire.

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                      • #12
                        I had a chance to dig into this machine this weekend - Found that the 25 ohm 25 watt wire wound resistor (R16 - Miller 079 781) was open. These seem to be available in 50 watt versions all over the place for $5-$30 so I'll get one coming to replace it.

                        I also removed and disassembled the 75A, 3 Pole 120vAC Coil contactor (W - Miller 085 002). The contacts were well used and had lots of flash hanging from them.

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                        So, I took them all out, cleaned them up on a bench grinder and then re-assembled. It worked like before (random sparking and all) so I didn't do too much damage

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                        This particular make/model is discontinued from Miller - I found a few similar ones from Square D, Eaton, etc, but they seem to want $1500-$2000 for something like this which is just ridiculous. There's still some "hard face" left on these contacts, but will probably need to be replaced at some point.

                        Any suggestions on a good reasonably priced replacement for something like this?

                        Thanks for everyone's help so far - I'll update again when I get the resistor installed and attempt to prove the logically impossible (that it does not spark anymore) .

                        Attached Files

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                        • #13
                          Begin by understanding what you call "hardface" on those contacts was once silver, now it's an alloy called WhoKnoz.

                          Cleaning them up as you did will give you more hours, BUT be aware the machine is now in Limp Mode.

                          You also need to know Miller Electric did not manufacture that contactor. At the time the machine was built the contactor was an off the shelf item from Square D or another manufacturer of electrical components. My hunch is it may not have been rated for DC use, and a quick glance at pics tells me that contactor was switching DC.

                          You can keep grinding contact surfaces, BUT you need to pay particular attention to contact mating when it is pulled in or you're in the fast lane for future problems.
                          It is not necessary to replace with an exact duplicate, only needs to have 3 sets of contacts and the correct coil voltage. Unfortunately thanks to the Lead Free numbskulls much of what was available in the surplus market went to China for remelt so finding one might be fun.

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                          • #14
                            Yep

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                            • #15
                              That part number 085 002 shows as a stocked item at miller4less. There’s also a bunch of abbreviations next to it that tell me nothing but be a clue to you guys about it being the correct part. Might not even be what you’re looking for, but worth a try.

                              Also, some old components can have an updated p/n from miller. In the past I’ve obtained that information by calling miller tech support and/or parts department. This could also be worth a shot.

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