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Repairing Scratches on XMT Heatsink

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  • Repairing Scratches on XMT Heatsink

    Hi,

    I picked up a Miller XMT 304 with a failed input diode/SCR module and charred input protection board. All other pre-power checks passed and the buss capacitors' ESR match new ones. I have replacements for both, but found scratches on the heatsink when I removed the burnt input diode/SCR module. I can feel the scratches when I run my finger over them. There is one group near the top mounting hole that takes up about a 1/4 square inch, and a group of scratches in the bottom right corner that also takes up about a quarter square inch. Is it worth lapping the entire mounting surface for that diode module? I have a flat glass block that I could use with lapping compound or wet/dry paper up to 2000 grit. I also figured I could stone just the scratched areas to ensure they are at least lower than the rest of the mounting surface. Any thoughts on a better way to do it without complete disassembly?

    Thank you,
    Jon

  • #2
    Well Jon, while I would think the smoother the surface the tighter the surfaces contact, easier conduction occurs, I'm doubting in the bigger picture it's going to make much of a difference. At least that's how I see it with the picture I have rolling around in my head?

    Comment


    • #3
      Don't they make a mounting paste that fills voids and conducts to the heat sink?

      Comment


      • #4
        It wouldn't hurt to run it on the sheet of glass with some very fine sand paper. Why not do it, I mean how long is it going to take? And yes there is a thermal paste you can put on, and it can make a big difference. Does the factory put the paste on though? I kind of doupt it. But again for the minimal cost why not do it. A computer store or Amazon will cary the thermal paste.
        www.silvercreekwelding.com

        Miller Trailblazer 325 efi
        Miller extreme 12vs
        Thermal arc 186 ac/dc
        Lincoln power wave 455m/stt with 10m dual feeder

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        • #5
          I don't know if they do as mentioned add a thermal paste of some or any sort? But the reason they might not... would be particulate collection. Unless the product seal coated, excess would hold metallic dust. That would attract more and before long, zip, zap and fry something maybe? I'm still holding out for pictures.

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          • #6
            It would not surprise me to find out they do use thermal paste or oil. It was specified to be used on an SCR I replaced on our 200 kw melting furnace in a foundry setting. Just remove the excess after mounting.

            ---Meltedmetal
            ---Meltedmetal

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            • #7
              Thank you for the replies. I do have the TechSpray thermal paste and planned to use it either way. I already have it mounted without lapping and with the thermal paste, but am second guessing that solution. I will just check with a flat plate and some marking fluid how much is not in contact. I will post pictures when I get back to the shop.
              Thanks again!
              Jon

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              • #8
                So Jon... the $64 dollar question is, did it come with some from the factory?
                The way I see it, if it did you're good to go? If it didn't...I'd wonder why not? Maybe the risk out weighs the reward? Or maybe it will be an improvement? Live and learn.





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                • #9
                  Noel,
                  It did have some thermal paste present from the factory. The replacement I bought was the specific Sanrex module, not the marked up Miller replacement part (pictures of the Miller part showed a small tube of thermal compound). I won't be back until tomorrow night, but will try to post pics of the scratches then.
                  Thank you!
                  Jon

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                  • #10
                    Where is the Almond Farmer when we need him to tell us how to do this? Have you got six months and $4000 so you can pretend to improve it?

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                    • #11
                      Oh man, I just about forgot about that. He probably still installing his wall socket.

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                      • #12
                        Well Jon, sounds like you have things well under control. And with a promise of pictures, like Pavlov's dog I have a drool on as I wait. Good to know an answer to the thermal paste question. Take a few pictures if you can. I'm curious how deep you stuck your fingers in this XMT thing!
                        Thank you.

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                        • #13
                          As promised, here are a couple photos. My phone died just as I was about to take a picture of the new diode after I removed it and the scratches on the heatsink. I will try to get those when iI' back from work travel this weekend. Here are a couple pictures of the original diode modules, which show shorts across L1 and L2 legs (I am pretty sure this was only run on single phase). It looked like it had pretty good coverage from the thermal paste. The pictures of the input protection board are after I removed all of the charred PCB sections. The previous owner had his electronic buddy repair the board and add all new MOVs. I took out so much and didn't like the looks of the snubber resistor across the SCR terminal (little white one in the middle), so I have a replacement coming from Innovat. The picture of the heatsink with the thermal compound is after I pulled off the replacement module. I had the compound on there pretty thick, and there were still a couple spots that looked bare (will post the pic of the module later and the heatsink without compound later). I will only run this single phase, so the input module should not have to dissipate as much heat through the heatsink as designed (only two pairs of diodes conducting on single phase instead of three phase). The welder passed all other pre-power checks, and ESR through the buss capacitors measure .005 ohms at 10kHz. I won't be able to get to it until next weekend, but any thoughts on whether or not to lap the heatsink surface would be greatly appreciated!
                          Thank you again for all the responses!
                          Jon
                          Attached Files

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                          • #14
                            I don't want be the complainer with those pictures Jon, but a little more light shining on the subject is a good thing. Hard to tell if it's a boy or a girl in that light lol.
                            Looking at the pictures...I smell toast. Burnt toast. Good job Jon. If I was you I'd be feeling pretty good about things, b
                            rave is the man who gets it done.
                            Did I miss the part where someone had been in there before you? I must have? Getting pretty good taking it apart aren't you now!
                            If that picture is the heat sink, I'd polish it down. Yes I would. I have a piece of emery taped to a flat glass surface for just that kind of purpose.

                            I'm a little unsure what I see in picture #4? That's the paste residue and coverage? Kind of dark but I think I see it's mostly uniform? But assuming it's thin in some areas, easy to adjust for. I do appreciate the story and your explanation. Kudo's for getting it done.

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                            • #15
                              Noel, thank you for the comments and the help. I took a couple more pictures this morning, but will get some better lighting on it when I get back. The scratches are visible on this picture of the heatsink, and there was fairly uniform coverage on this diode as well. I will try the emery cloth and flat block, (1000 grit up to 3000 grit laying around). The previous owner did attempt to have it repaired, but said his friend only tried to repair the input protection board. I wouldn't think it came from the factory with the heatsink scratched like that, but maybe that was the cause for the input diode failure?

                              Thank you again!
                              Jon
                              Attached Files

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