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Miller Bobcat 225 NT F1 Fuse

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  • Miller Bobcat 225 NT F1 Fuse

    Here is a little brief history. Purchased this unit used knowing it was not welding. The motor had recently been rebuilt and gone over so I thought it would be a good deal.
    The unit's manual is here http://www.millerwelds.com/om/o405e_mil.pdf . I am sorry but I don't have the serial number as its at my shop right now and forgot to get it. The unit has a Onan motor.
    I have been researching and looking at posts over here. I found a couple things to check.

    First off, the unit kept blowing the f1 fuse. It happens as soon as the motor gets up to speed. This prevents any output to receptacles and leads.
    I tested the SR2, and D4 diode as mentioned before. The SR2 checked ok, but the D4 diode checked bad. I replaced D4 but am still having the same issue.

    So far, I have checked the slip rings by using a multimeter. First, I disconnected both wires leading to the brushes on the rings. With each probe on each ring, I am getting around 29 OHMs. With the red probe to each ring and the black probe to ground, I am getting a reading of 1 on my meter for both of the slip rings.
    I disconnected the 33 wire on the capacitor and checked it using the 20k setting. I achieved a brief negative reading if .-07 going to a positive .02 reading. I reversed the probes, and I achieved the same thing.
    Now, here comes the strange part I think. When I replaced the D4, I bench tested it before it was installed. On the bench, the new part tested fine. On the machine, with the 33 wires from the capacitor disconnected and the f1 fuse removed, I am getting a reading of .850 both ways now. I removed D4 and bench tested it, and it tests fine..!!
    I then retested the SR2, and it tests fine again.
    I am at a lost here now. Maybe somebody can help me out here..

    Thanks
    Ross

  • #2
    Brushes have zero to do with F1 blowing. F1 protects the excitation circuit. so might want to pull and check #29 & #28 off SR2. for continuity and those wires to frame ground.

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