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Thread: Millermatic 200 Troubles.

  1. #1
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    Millermatic 200 Troubles.

    A few months back i posted that my Millermatic 200 was giving me some troubles, in the way that I noticed a louder buzzing almost rattling of sheet metal when welding. I had some suggestions about the Controller contactor which I replaced and connections of the secondary wires which I checked and they were tight.

    The welder seemed a little better but it wasn't a huge improvement and it still welded what seemed to be very cold.

    Today I decided to pull all 4 of the diodes out of the fan housing and check them. I used my Fluke 88 meter in Diode check mode and found all 4 to be open in both directions.

    These are 275 amp 300 volt diodes and it seems that all 4 went out all at once because it was welding fine one day then the next day the first weld was cold.

    I stared by pulling each diode out of the frame and testing it that way rather than trying to test them ion the frame. All diodes were tight in the frame and had thermal paste under them.

    I think that one diode had been replaced earlier in the life of the welder because there was a different nut on it. I ended up having to pull the frame from the welder in order to get the last diode out. when i went to remove the wires from the thermostat NO one of the connectors pulled out from the thermostat. When operating the welder the fan would operated normally coming on when needed and then turning off when cooled off.

    What would cause all 4 Diodes to go bad? I checked the power coming out of the wall and it's 240 volts. There are only 3 pins for the voltage setting and the jumper strap on the welder is on the center and far right.

    any help would be appreciated thanks is advance.
    Doug
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  2. #2
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    Re: Millermatic 200 Troubles.

    I'm not fluent in electrical to say why they died.

    But if you need cheaper ones than "Miller" ones, I found replacements online from a DC windmill alternative power store on Ebay when I needed some.
    Dave J.

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  3. #3
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    Re: Millermatic 200 Troubles.

    It's not that unusual for one diode of a given polarity to go open and then its mate go shortly after. It very well could be that one or more have been open for some time and you just weren't aware of it.

    The NO thermostat is for the fan on demand circuit and without it the fan will never come on. You'll either need to replace it or just connect the two leads together so the fan runs all the time the same as the original MM200 and earlier MM35 does. I personally don't like the fan on demand system and believe that the reason older machines with constant run fans have run for decades is because they never get a chance to heat up and heat as we all know is the enemy of electrical components.

    The NC thermostat is there in case the NO malfunctions and fails to close to turn the fan on. When the rectifier assembly exceeds the NO t-stat setting and reaches the NC t-stat setting the NC opens to prevent the contactor from energizing thus preventing weld output and additional heat build up.
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  4. #4
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    Re: Millermatic 200 Troubles.

    Quote Originally Posted by MinnesotaDave View Post
    I'm not fluent in electrical to say why they died.

    But if you need cheaper ones than "Miller" ones, I found replacements online from a DC windmill alternative power store on Ebay when I needed some.
    Hey Dave,
    Thanks for the information I'll check them out.

    Quote Originally Posted by duaneb55 View Post
    It's not that unusual for one diode of a given polarity to go open and then its mate go shortly after. It very well could be that one or more have been open for some time and you just weren't aware of it.

    The NO thermostat is for the fan on demand circuit and without it the fan will never come on. You'll either need to replace it or just connect the two leads together so the fan runs all the time the same as the original MM200 and earlier MM35 does. I personally don't like the fan on demand system and believe that the reason older machines with constant run fans have run for decades is because they never get a chance to heat up and heat as we all know is the enemy of electrical components.

    The NC thermostat is there in case the NO malfunctions and fails to close to turn the fan on. When the rectifier assembly exceeds the NO t-stat setting and reaches the NC t-stat setting the NC opens to prevent the contactor from energizing thus preventing weld output and additional heat build up.
    Hi Duane,
    Thanks,I'll bet that's the case. I have just used the welder a hand full of times before it failed. I don't mind spending the money to replace the diodes but i just want to make sure that the rest of the welder is in good shape so that they don't fail soon after I replace them.

    my fan seem to come on after i welded for awhile and then turn off after sitting idle for awhile. I just used my fingers to try to pull the wires off the thermostat when the whole terminal came out. I'd like to replace it if it's still available. I haven't pulled the thermostat from the frame so I don't have any numbers off of it yet.

    Thanks for your help.

    Doug

  5. #5
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    Re: Millermatic 200 Troubles.

    That t-stat is PN 086323, still available from Miller and they are very proud of it.

    http://miller4less.com/advanced_sear...x=0&search.y=0

    Should be able to match it up from Mouser, Allied, etc. based on the spec in the description. The text is cut off so the "open" temperature doesn't show but based on the "1" that can be seen it's safe to say the open temp is at least 100F.
    MM200 w/Spoolmatic 1
    Syncrowave 180SD
    Bobcat 225G Plus - LP/NG
    MUTT Suitcase Wirefeeder
    WC-1S/Spoolmatic 1
    HF-251D-1
    PakMaster 100XL
    '68 Red Face Code #6633 project
    Star Jet 21-110

    Save Second Base!

  6. #6
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    Re: Millermatic 200 Troubles.

    Quote Originally Posted by duaneb55 View Post
    That t-stat is PN 086323, still available from Miller and they are very proud of it.

    http://miller4less.com/advanced_sear...x=0&search.y=0

    Should be able to match it up from Mouser, Allied, etc. based on the spec in the description. The text is cut off so the "open" temperature doesn't show but based on the "1" that can be seen it's safe to say the open temp is at least 100F.
    Hi Duane,
    thanks for the link. I see that Miller is very proud of that part. I checked a couple other sites and they have the same price or close to it. I did find that the thermostat closes at 125f and opens at 105f.

    I'll check Mouser and Allied.

    Thanks again, Doug

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