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Need help with bobcat 250nt very low output

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  • Need help with bobcat 250nt very low output

    Miller bobcat 250Nt
    Serial: LC136994
    Kohler 20hp motor


    I have been trying to repair my welder that has a very low output on:
    - auxilluary ac when voltage is set to 10
    - all weld setting for all polarities

    Here is what I've done and tested so far:
    Aux. Ac frequency is 60hz at high idle
    System does not idle up or down when attempting to weld.
    Inspected the brushes they are in good condition and move freely
    Polished slip rings to shine
    Test F1, F2 and D4 all good
    Resistance between slip rings 3.8 ohms
    Voltage between slip ring and ground is 13.6vdc
    No continuity between slip rings and shaft

    Measured resistance of rheostat and cleaned measures from 14 ohms to 0.25 ohms
    Measured resistance of large ceramic resistor 4 ohms

    Engine sound changes when voltage is turned to 10.
    After running with voltage on 10 for about 15 minutes coild smell developed and there was a very small amount of smoke from stator.

    Measuring voltage between the lugs I get the following
    Polarity, amps, voltage, reading
    AC, 100-250, 10, 0.85 VAC
    AC, 100-250, 10, 0.49 VAC
    AC, 40-100, 10 0.2 VAC
    Stick+, 100-250, 10, 14.3 VDC
    Stick+,100-250, 5, 8.5VDC

    I get sparkles when welding on stick+ and get nothinf while welding on other polarities.

    I am at my limit of what I can do to fix this thing. If I can't then it will be the second 250nt I've scrapped in a few months.

    Thanks in advance for any suggestions.

  • #2
    Have you checked the big output diodes?


    • #3
      Sorry I'm not sure which part you are referring to as big output diodes. I've tested the d4 flashing diodes and both of the bridge rectifier modules. The main rectifier is a Miller proprietary thing and I cannot find any procedures describing how to test it which is the main reason I hoped that I could get help at this forum.

      Does anyone know what the resistance reading through the stabilizer be? Maybe im wrong but Im assuming stabilizer is another name for a choke/inductor, the other chokes I have access to (much smaller) all have continuity the one in the Bobcat has resistance in the mega ohms.



      • #4
        According to the manual, it should be something like this:

        You should be able to test it like any other set of output diodes - it has two AC inputs and two DC outputs. From the picture, I think (haven't worked on one) the two studs are for mounting, the AC inputs are the second and fourth heatsinks, and the front and rear are negative and the middle is positive, from the stickers, but just look for the ones the wires connect to. The wires should run to the output polarity switch.

        With the output switch in AC, you should measure a diode drop (0.4v-0.7v using the diode check scale on your multimeter) from each of the ac inputs on that rectifier module to the positive output, and from the negative output to each of the ac inputs. In particular, none of those four tests should read a dead short, and all should read open in the opposite direction than they measure a diode drop.

        The main reason I suspect the diodes is that not much else could make the stator start smoking, as that, combined with the engine loading up when you turn the power up, suggests a shorted output. Could also be a shorted aux output dragging the whole unit down, but there's a lot fewer things that can randomly short on that side of the unit. Another option might be a majorly broken mode switch, or rubbed through wires or other mechanical short circuit. I don't think a failure of either inductor can short the output.

        There's two big stabilizers/inductors, one used for all modes that's connected to the range switch, and one only used for dc output that's connected to the polarity switch. All resistance readings between any taps on them should read pretty close to a dead short as far as any typical multimeter can measure, as they're just coils of wire, and of wire heavy enough to carry the weld current. But, since they don't look like they're placed such that they could cause the stator to smoke, I'd investigate them after checking the diodes.
        Last edited by Bushytails; 05-06-2020, 10:18 PM.


        • #5
          Obviously you have a short if you turn the rheostat to 10 and the engine senses it. Output bridge is a good starting place. If its not the bridge it's the stator. You could try un plugging the Lem but normally it won't create the short you are experiencing.


          • #6
            Good morning. I sincerely appreciate your responses. I have a little experience with repairing machines, but primarily CNC machines, this system is unfamiliar to me. Can you please clarify the following?

            Is the output bridge that you referred to the component labeled SR1 in the wiring diagram?
            Is there a procedure available to test the stator for shorts?


            • #7
              Yes, it's the part labeled SR1 on the wiring diagram.

              Testing the stator for shorts isn't easy. Rather than trying to test it directly, I'd disconnect everything from it and measure its output with a battery supplying the excitation winding. However, I've seen plenty of failed diodes, and no failed stators, so I wouldn't look for the problem there first. Checking the output bridge should only take a couple minutes. Other than the first and fifth heatsink, which are bolted together, there should only be diode drops between the various heatsinks, no shorts.


              • #8
                Stator shorts are rare but I've seen a few in 24 years. He could disconnect the bridge, switch to AC and that eliminates the stabilizer also. Normally a stabilizer opens on the DC coil. If you have Wire output and no DC you can nail it down to open winding in the stabilizer, but I digress.


                • #9
                  I know that this thread is old history but I wanted to circle back with what was going on in case it can help someone else later on.

                  In short the generator stator was shorted and it was beyond anything that could be repaired short of a stator replacement or rewind. Given that the engine (onan/suburu) had a hogged out race surface on the rear main bearing and it could not hold an oil seal correctly it was not worth the cost of repair so I gave away whatever parts could be used by people locally and the remainder was recycled.

                  It wasnt pleasant but I learned an important lesson about trusting strangers in equipment swaps/trades/deals etc. Well by looking at my other active post about the XMT I just got maybe I have not learned that lesson after all. Although the experiences sure did teach me a whole lot about welder repair and troubleshooting which is worth more than a stack of welders in my opinion.