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Thunderbolt DC short?

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  • Thunderbolt DC short?

    Hello all, I've come into a Miller Tbolt AC/DC Serial # JD-4 welder that I need some help with. With the hood off the unit I plug it in and the fan runs as it should but the machine sounds like its under load and trips the 50 amp breaker after about 1 minute, I don't have leads so no welding is being done. If I shut the machine off before it trips the breaker I get a loud bright flash at the switch.

    Ive tried to test the diodes after doing some research online but can't seem to figure out what I'm doing. After reading somewhere that you can disconnect the DC portion of the machine, I thought that I would give it a try to try to isolate a short. After I removed the power supply to the stabilizer and turn the machine on it hums along nicely and when I turn it off there is no bright arc at the switch.

    So I feel that I've got a short somewhere in the DC side of the machine. Will bad diodes cause this or is there another problem? I currently have removed the entire DC portion from the machine and have it on the bench for testing (or tossing out).


  • #2
    One or more of your diodes is pickled, post a serial#


    • #3
      Serial # JD-4


      • #4
        Not exactly the serial#, never the less, pull the pig tails out of the circuit and with a diode meter check them. You can also use a simple resistance meter and check them in both directions.

        Likely only have one thats shorted in both directions. It will have a Miller # and order it


        • #5
          Well it looks like one of the reverse current diodes are bad. Looking at the diodes at millers4less it think it's part# 087 999. That's an 85 amp 1/4" stud. The part out of my machine has the numbers D1807 (diode symbol) 82??. The last two digits I can't make out.

          Would you think I have the correct part at millers4less?


          • #6
            Got the new diode in today and the DC is working great. Thx for the help.