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  • MM251 Question

    Hi all, Iím a new member here but have been lurking a long time. This forum is great and has learned quite a bit in the past. I'm no great welder but can do it and sometimes my welds even look half decent. I finally bought my first welder but am having a few issues with it.

    So I knee jerked and bought a Millermatic 251 at auction last month sight unseen. Only had 2 pictures to go by and they weren't great. The price ($400) was right for what I thought was a complete welder. Turns out there is no gun but that is fine I'll get one when I confirm everything functions as per. Wired into my shop and everything powers up, so far so good. Borrow the gun from work, a MM 252, I have no wire feed. Gas solenoid clicks and the shed lights dim a little, I feel good about the transformer and having an output without actually measuring it. I take the covers off and look around a bit. I found Q1 on the main board melted. Replaced but still doesn't work. I'm still not getting any power output to the drive motor. Does anyone know of other common faults caused by or because of a fried Q1 transistor?

    Other observation, the output shaft of the wire feed motor seems to have alot of play compared to the 252 at work which has no play at all. I this common or cause for concern?

    Unit S/N: LG190362B

  • #2
    Unfortunately, there isn't much to look at in the manual. All hidden in the circuit board block. Maybe a miller tech will respond.

    Comment


    • #3
      I've found a tech manual online with some extra info and the circuit diagram. It seems like the Q1 transistor keeps getting damaged.

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      • #4
        Something's taking it out. Is Q1 in the motor drive circuit ?

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        • #5
          Yeah. Q1 appears to be the main power transistor for the drive motor.

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          • #6
            Originally posted by Ykmajik View Post
            ...I have no wire feed. Gas solenoid clicks and the shed lights dim a little, I feel good about the transformer and having an output without actually measuring it. I take the covers off and look around a bit. I found Q1 on the main board melted. Replaced but still doesn't work. I'm still not getting any power output to the drive motor...
            Did you test for power output to the drive motor before you refried Q1? Could a failed motor be causing the blowing of Q1?

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            • #7
              Originally posted by MAC702 View Post

              Did you test for power output to the drive motor before you refried Q1? Could a failed motor be causing the blowing of Q1?
              I had not tested for power output before powering the motor. I did check continuity of the motor and got numbers I thought made sense. That was going to be my next check to do.
              Last edited by Ykmajik; 09-27-2017, 09:51 AM.

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              • #8
                I hooked up the drive motor to a dc power supply at work. It seems to function normally. speed is consistent and varies with voltage. At 24vdc the motor is drawing 850mA.

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                • #9
                  How about resistance to ground (motor case) from the motor windings? Could be shorted and may have worked ok lying on the bench not grounded to the welder frame, but a short blows Q1 when mounted? Wonder how much the motor is supposed to draw? What is the max current rating for Q1 from its spec sheet?

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                  • #10
                    Originally posted by Aeronca41 View Post
                    How about resistance to ground (motor case) from the motor windings? Could be shorted and may have worked ok lying on the bench not grounded to the welder frame, but a short blows Q1 when mounted? Wonder how much the motor is supposed to draw? What is the max current rating for Q1 from its spec sheet?
                    Windings to case are good. Checked with my megohmmeter.

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                    • #11
                      850 ma doesn't feel unreasonable for the motor draw but would love to know what it really should be. Do you have a vendor part number for Q1?

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                      • #12
                        How about the path from Q1 to the motor--chafed/shorted wire?

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                        • #13
                          Wiring looks good. Haven't tested it and can"t until I get home tonight. I don't have a part number from Miller for Q1, from emailing the techline I got this description "TRANSISTOR,PNP 10. A 100V TO-218AC *NONE" and I am using transistor part number TIP147.

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                          • #14
                            No load tests are fine, but you have an obvious load problem on that motor and its controller board ------- any number of things on their own (e.g. gummed up liner) can add loading and cause excessive current through that PCB (beyond design tolerances); couple that with another problem in the drive assembly and we're talking blown components.

                            I'd first work the mechanical path --- clean liner, tips, etc; investigate that output shaft issue (my 252, a close cousin, has absolutely no play whatsoever), then remember Q1 is not your problem, but rather the "evidence" of a problem elsewhere.

                            As for PCB repair, short of having an O-scope, signal generators, labscopes, etc it's becoming increasingly difficult to repair at the component level these days. Soldering (unless you're a NASA Certified Solderer) in and of itself can be problematic, dangerous to overall PCB functioning.
                            ESAB TIG 252 with Miller CoolMate
                            Spectrum 875
                            Diversion 180
                            Oxy-A (Harris, ESAB, Ox Weld)
                            Miller 252
                            MM 211
                            CST 280
                            Trailblazer - Kubota

                            http://www.blackdiamondblooms.com/

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                            • #15
                              It's taking a pretty hefty malfunction to pop a 10 amp transistor. Agree with checking mechanical loading. Did you try running it with no wire tension before it blew the second time?

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