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Millermatic 250 Power Supply

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  • Millermatic 250 Power Supply

    I have an older Millermatic 250 that was purchased from a school district. When I received the machine, the wire feed was working and did not have an arc to weld with. Gas relay was clicking in sync with the trigger.

    Opened up the machine and while looking for obvious burn marks or arcing, I noticed that the VR-1 varistor on the fan relay was completely gone. Took a few voltage measurements and did not have any DC voltage from the diodes (hockey puck type). Replaced both of them along with the varistor.

    The status now is I have DC output from only one diode and no arc. Varistor is in tact.

    Question: Did I do something wrong during installation of the diode ? Is it possible the diode was defective when received ?

    I'll order another diode, but before I fire the machine back up, I'd like some input from the forum just in case I've overlooked the obvious.

    Thanks
    Last edited by sancobg; 09-26-2017, 06:22 PM. Reason: sp
    Executive Director
    USMC Veterans Organization of America
    Enlistment: 1965-1967
    WIA Vietnam 11 May 1965
    Website USMCVOA.org

  • #2
    Please post your serial number so we can look at the correct diagrams.

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    • #3
      The Ser # is KC327877 200/230V
      Executive Director
      USMC Veterans Organization of America
      Enlistment: 1965-1967
      WIA Vietnam 11 May 1965
      Website USMCVOA.org

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      • #4
        I'm a bit confused by your description of symptoms. Since the cathodes of both diodes (SCRs, actually) are connected together, I'm not sure how you know you have DC from one and not the other . Are you looking at the output with a scope? It could be you are not getting the gating pulses from the circuit board and your old diodes were OK, just not being turned on. You really need to scope the gating pulses to determine if the problem is the board or the diodes. Do you or anyone you know have an oscilloscope?
        Last edited by Aeronca41; 09-28-2017, 06:14 AM.

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        • #5
          Rc2-1&4... Rc2-2&3 Red and white wires...unplugged...Should measure 18 ohms.. The hockey pucks have a torque on the bolts that need observed (The odd thingy on the bolt ends has a gauge there, don't know what it should be)...
          I'm with Aeronca41, what is meant by "have DC output from only one diode"

          Do you have them installed like this?

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