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  • Need help bad 351 died

    Hey guys I recently purchased a used 351 machine very clean and nice was working great yesterday I went into the shop and it tripped my breaker on dcr and dcs the ac works ok.Well I blew it out looking for something obvious but don`t see any sign of arching but it trips the breaker in dcr and dcs.I`m an electrician by trade so I`m guessing since it trips only on dc its a scr or rectifier somewhere does anyone here know how to dianose such a problem? I`m only hobby welding so this isn`t a weld shop where I can afford a costly repair.My plant where I work offer this for aution and thats how I got such a big machine, IF I can`t fix it I`d be interested in a swap for a smaller welder.

  • #2
    Check the D1 diode on the main rectifier. My guess is that its shorted. Unhook the welding cable from the D1 and put polarity switch inbetween positions, then check with ohm meter. It should be open in one direction and closed in the other, if not its junk.
    Kirk Kopitzke
    Service Technician
    Miller Electric Mfg. Co.

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    • #3
      Thank you for your assistance. D1 is that the one with big bolt and nut.Not the ones sandwiched between the cooling plates correct?I`ll refer to my manual.Thanks again. Man my heart sank when it quit. I`ll post back tomorrow with the news.

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      • #4
        D1 looks like a regular diode with a pigtail on it, the ones sandwiched between the plates are SCR's.

        Disconnect the pig tail before you test it, perferably with a diode tester

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        • #5
          Thanks as soon as I get home I`ll get it out and check it.

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          • #6
            Sounds exactly like the D1 issue. D1 is a flyback diode used only in DC and it's purpose os to protect the main rectifier from the stabilizer when the arc is shut off. To test properly, disconnect one side of the diode out of the circuit.

            Good luck
            Andy

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            • #7
              Syncrowave 351

              If that is your only problem, those diodes are cheap. You can pick one up for around 20.00. weldersales

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              • #8
                Well the D1 is bad shorted. OK is there a root cause I should be looking into vs just replacing it. And also can someone send me the info Can`t beleive 20.00 part. Its a 037-956B looks like 275Amps 300v Made in mexico.Please send me info for a replacement Thanks in advance,You guys are ok with me nomatter what the lincoln boys say

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                • #9
                  Originally posted by Reel-Break View Post
                  Well the D1 is bad shorted. OK is there a root cause I should be looking into vs just replacing it. And also can someone send me the info Can`t beleive 20.00 part. Its a 037-956B looks like 275Amps 300v Made in mexico.Please send me info for a replacement Thanks in advance,You guys are ok with me nomatter what the lincoln boys say
                  List price from Miller is $62, If your LWS doesn't have a repair shop, the price for this part may be somewhat higher

                  This is also a common part on Ebay, I bought a bunch of new ones for like $5 each

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                  • #10
                    ok replaced the diode that was shorted still trips as soon as I turn machine on dcs or dcr I removed the scrs and none are shorted 1,2,3,4 Can someone share some troubleshooting info. Thanks

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                    • #11
                      Originally posted by Reel-Break View Post
                      ok replaced the diode that was shorted still trips as soon as I turn machine on dcs or dcr I removed the scrs and none are shorted 1,2,3,4 Can someone share some troubleshooting info. Thanks
                      Ahhh, how did you check the SCR's, alot easyer in the circuit with an SCR checker. or a tracker, out of the circuit they must be tightly held in a vice

                      Now I failed to recollect that the machine "trips" the breaker as soon as you start it, This may be an indication of a failed Primary transformer.

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                      • #12
                        Need help bad 351 died

                        The SCR's need to be checked while in place as they require pressure to ensure internal contact. A shorted SCR can check open if it is just lying on a table. If all are ok, you can disconnect them from the control board by unplugging the white Molex connector with the twisted red/white leads - that will keep the units from firing in case that is your problem. weldersales
                        BTW, for 20.00 diodes, PM us anytime.

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                        • #13
                          OK I`m welding again my thanks to all who helped.The problem was the diode I replaced wasn`t shorted it was made backwards from the original as far as the diode symbol on the component. My thanks to Andy for answering my e-mails.Now back to learning how to tig.

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                          • #14
                            Need help bad 351 died

                            I am confused. Are you saying someone gave you the wrong polarity diode? In our industry, forward and reverse diodes are commonly used, allowing the use of fewer heat sinks. They look exactly alike, the diode symbol being the only external difference. weldersales

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