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Thread: Miller 350/350P main contactor does not engage

  1. #1
    Join Date
    Jul 2019
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    Miller 350/350P main contactor does not engage

    I have a Miller 350. When I got it, R11 was crispy but still in spec, R12 was smoked. I worked through the whole prepower section of the tech manual, found one leg of the main contactor had around 23 megaohms resistance when manually pushed in, and another had even more. (All tests performed with a Fluke 114). I am running on, and linked for 230 volts single phase. New contactor shows .8 ohms when manually pushed in on all three legs.

    I replaced the contactor, and R12. I used a 50 Ohm 25 watt resistor because thats what I had. Originals are 40 ohms, probably 20 watt. On power on precharge the DC bus comes up to 321 volts with 237 volts AC across the two used inputs on SR1. The contactor does not pull in. No 24vac is present on the pins indicated in the tech manual. 24 vac is available to the control board. The DC bus voltage shows up on the connector specified on the control board.

    If I jumper the triac which controls the contactor, the contactor pulls in and DC bus voltage rises to 337 volts. Machine welds beautifully as long as the triac is jumpered. As soon as the jumper is removed, the contactor disengages. Bad things would happen if one tried to weld in this state (vaporization of the precharge resistors). Has anyone repaired the board for this? I tested the triac using the diode function on a Fluke 101 and it tested fine. I am guessing there is an optoisolator somewhere up the line from the triac. Machine does not throw up any error screens and other than the wire feed speed encoder needing a steady hand to smoothly increase the speed lays a bead like nobody's business.

    Serial number is LF046609. If I end up having to buy another board, what part numbers will work with my non pulse tach encoder board and front panel? I assume newer boards got better and less failure prone, so I would want the newest/best one that will plug into my chassis. I forgot to take a pic of my current control board but i am 99% sure it is a 220522.

  2. #2
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    Jul 2019
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    Re: Miller 350/350P main contactor does not engage

    Pulled the board and on the workbench it goes. Found an open 680 ohm, 3 watt resistor (R106), and a dead mosfet (q13). The transistor which drives the Mosfet tests good. The triac tests good. Parts are ordered.

  3. #3
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    Jul 2019
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    Re: Miller 350/350P main contactor does not engage

    Closing the loop on this. Replacement of r106 and q13 fixed the board and the machine is happy.

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  5. #4
    Join Date
    Oct 2015
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    NW New Jersey
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    Re: Miller 350/350P main contactor does not engage

    Thanks for posting the follow up , it may be helpful to someone else in the future. Maybe even yourself.
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