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Rehab of a very tired Millermatic 200..

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  • #16
    Ok.. no worries.. I’ve got plenty of sand paper and will use that.. I didn’t know it was conductive.. interesting.. Thx!!

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    • #17
      That seems pretty hard to believe, since emery is mostly aluminum oxide, the same as sandpaper!

      The biggest issue I've found, with any abrasive, is leaving behind particles that mechanically hold apart connections. I like scotchbrite pads for this reason. If you do use any abrasive, use a rag, cardboard, etc for a final pass to remove particles. But, my usual rule is not to fix things that don't look like they need fixing, lest you end up with them actually needing fixing due to your efforts...

      The capacitors in welders like that are secondary voltage (weld output voltage), and thus are a lot safer than, say, the 400V bus caps in an inverter welder. They also tend to be varieties that self-discharge a lot faster. I usually don't worry about them.

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      • #18
        Originally posted by Bushytails View Post
        That seems pretty hard to believe, since emery is mostly aluminum oxide, the same as sandpaper!

        The biggest issue I've found, with any abrasive, is leaving behind particles that mechanically hold apart connections. I like scotchbrite pads for this reason. If you do use any abrasive, use a rag, cardboard, etc for a final pass to remove particles. But, my usual rule is not to fix things that don't look like they need fixing, lest you end up with them actually needing fixing due to your efforts...

        The capacitors in welders like that are secondary voltage (weld output voltage), and thus are a lot safer than, say, the 400V bus caps in an inverter welder. They also tend to be varieties that self-discharge a lot faster. I usually don't worry about them.
        Agree completely on the scotchbrite pads--I always worry about the abrasive particles left behind.

        On the emery cloth, it is absolutely true that the abrasive is aluminum oxide, which is an insulator. However, in the manufacturing process for emery cloth, they use iron oxide powder to improve the bond with the flexible backing, and that is what gives emery cloth its characteristic dark gray/black color as opposed to the lighter color of aluminum oxide paper. Thus, the dust is somewhat conductive. I've never measured it, and am certainly no expert in abrasives manufacturing, but that's what I was told. If you look up the properties of Fe3O4, it is an electrical conductor.

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        • #19
          Man, that machine is in way better shape than you made it sound. I wish I had taken pictures of this old MM250 when I saved it, you would’ve been shocked. Barring any major electronic issues, that should be a good salvage project. I’d clean it up and put some power to it. You should be able to hear the relays click when you pull the trigger and and you’ll be able to measure the output voltage and WFS with a DMM. I think you’ll be able to save it no problem.

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