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Burned up SCR in Millermatic 251

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  • Burned up SCR in Millermatic 251

    Quick backstory. I received this welder from work very cheap, they upgraded all of their systems to 350Ps and this poor 251 had been sitting in a corner and not touched for a year or two. I was told that the "whole thing had burned up" and that it was essentially scrap. I brought it home and tore it apart to find some obvious problems. The SCR was completely torched and it was going to need some new leads on the main transformer (see pictures). I don't see any "hotspots" on the motherboard and the rest of the unit is fairly clean.

    I have the new SCR on the way and have repaired the transformer. What should I start checking before I consider hooking this up to power? I plan to replace the thermistors with the SCR, but what could have caused this SCR failure? Should I just plan on a new motherboard right out of the gate?

    Appreciate your input!

  • #2
    You should be ok if you repaired the leads from the transformer that connect to the Thyristor. Thermistor probably is ok although I think the new Thyristor is larger and you might have to move the thermistor over a bit. The cause is aluminum connecting to non aluminum thyristor. Just makes a bad connection and eventually the bad connection and heat melts it down. I have replaced quite a few over the years. Did you order the 2 capacitors that connect to the thyristor?

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    • #3
      1997CST - I'm planning on ordering those here shortly. I believe you are talking about MM PN 031 689, right?

      Any idea what would cause the thyristor itself to fail and burn up like that? Click image for larger version

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      • #4
        It's the aluminum transformer leads to the metal Thyristor. The connection expands and contracts. It looses over time. All of your current goes through there and the loose connection causes it to burn up. I actually just repaired another one yesterday. but the Thyristor is the same size. It's the Thyristors on the XMT's that are bigger. You might have to move the Thyristor over closer to the transformer as your leads will now be shorter and move the mounts for the resistors to the other side. hope this helps.

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        • #5
          That is helpful, I appreciate your input.
          Anything else you would recommend I check before I plus this in?

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          • #6
            Clean the heat sink with brake cleaner and put a thin coat of grease, (supplied with the Tyristor) on the back of it. Don't over torque any of the bolts.
            You should be back welding in no time.

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            • #7
              Where the 2 capacitors that connect to the thyristor?

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              • #8

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                • #9
                  Does your cap bank look OK... any bumps or obvious distortion in the blue wraps on each... it may be wise to disconnect 1 lead (after checking for residual voltage) and just see from the middle strap if your meter to a side... if the meter makes them start taking a charge... the SCR connects to them.

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                  • #10
                    Sorry it a old post but i have the same problem .. cap bank look ok

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