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  • Another M250X Problem

    Hi - Somewhat new to welding and new to forum. There is a wealth of information on here but I am struggling with a recent purchase of a 250X. Unfortunately I had not read all of the wonderful reviews out there on the 250x before bringing it home. Got it for $100 so I figured it was a steal, even if I had to fix up a few things. The individual I purchased it from said it needed a component soldered to the board. Did that and cleaned everything up and figured hey I will power it up and just see. Here is what happens when I do so:
    - The drive motor keeps running and running (even after unplugging the power cable)
    - The fan is running immediately
    - I noticed a plastic burn smell and turned it off immediately
    - Of course no display on the display

    Took me a bit to locate the burning smell. There is a small clear plastic part that has wires plugged into it towards the rear on the side with the board. It has several wires running to the bottom of it and connected below it. I believe it is the CR-1 Relay from the Part diagram. Just below this there is two wires coming out and connected to each other with a small blue disk (resistor?). This is what is getting hot and "burning".

    Anyway I am thinking the board is toast (PN 196456) and will require a rebuild. Looks like these boards are not cheap. I may be way over my head in trouble shooting it.

    Is there a guide out there that directs methods to trouble shoot the board or determine which components may need to be replaced?

    Or am I looking at shooting this off to someone that can run diagnostics and repair it if need be?

    I went through a bunch of topics and there are similar problems but they didn't exactly explain where to go from here. I very much appreciate any assistance all the gurus on this site can provide.

    Thank you ahead of time!!

  • #2
    At least you got cheap enough to get out before it cost to much to fix. Hope you find something easy to fix it.

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    • #3
      You're asking about a Millermatic 250X correct? The clear part is CR1 and the blue disk is a varistor. The varistor is shorted which bypasses the relay and causes the fan to run all the time. That's often a sign of a welder that either was subjected to a voltage spike from a lighting strike, or it was set for 230V and plugged into 460V (or similar). The varistor is cheap...I had to replace one on a MM251 (similar part) a while back and they were just a couple of bucks, but I had to buy a pack of 4 or 5 as I recall. I'm not sure about the drive motor running.

      The guide you would need would be the service manual, but they don't publish or sell them to the public. There are a few guys here who might be able to walk you through troubleshooting the board, but it sounds like there are multiple issues, and the display board might be toast as well.

      Around here a MM250X still sells for $1000-1500 pretty easily even though they aren't the most popular model, so it still might be worth pursuing.
      Last edited by G-ManBart; 06-14-2020, 10:00 AM.

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      • #4
        Thanks guys for your help. I agree it still might be worth checking into it. Other than this issue it looks pretty clean but if I had to guess it must have had a spike hit it or like you said someone plugged it into the wrong phased outlet. I will at least start with the thermosistor. Although without knowing what caused it I am sure it will toast it again.

        If anyone can't help with a service manual or help troubleshooting the board does anyone have a recomendation on a rebuild house? I wouldn't mind paying for a copy of the manual that I need, Sounds like they are hard to come by though.

        Without radio shack anymore I guess I will have to order parts through the net somewhere.

        Thanks again for the information.

        Comment


        • #5
          Look for parts at DigiKey, Mouser, Allied, TEDSS. I buy mostly from Digikey or Mouser.

          Comment


          • #6
            When you go to buy it, it will be called by its more technically correct name, a varistor, or MOV, Metal Oxide Varistor. In the industry, we generally referred to them as MOVs. Just find one rated for the input voltage as specified in your user manual; more wattage/current is not a bad thing as you choose one, so long as you don't get carried away to big high-amperage ones. As G-man said, they have some thermistor properties, but are not quite the same in that they are designed to sacrifice themselves and provide a short across the line when activated, to prevent surges from getting on downstream and damaging other parts. Thermistors are simply resistors that vary their resistance as their temperature changes. Fuses blow open, MOVs short. Good circuit design often places a fuse upstream of the MOV so it will blow when the MOV shorts and prevent what you have happening now. Design shortcut saved a few cents on a fuse. They're so cheap you will no doubt have to pay more for shipping than for the part.

            Comment


            • #7
              Originally posted by Aeronca41 View Post
              When you go to buy it, it will be called by its more technically correct name, a varistor, or MOV, Metal Oxide Varistor. In the industry, we generally referred to them as MOVs. Just find one rated for the input voltage as specified in your user manual; more wattage/current is not a bad thing as you choose one, so long as you don't get carried away to big high-amperage ones. As G-man said, they have some thermistor properties, but are not quite the same in that they are designed to sacrifice themselves and provide a short across the line when activated, to prevent surges from getting on downstream and damaging other parts. Thermistors are simply resistors that vary their resistance as their temperature changes. Fuses blow open, MOVs short. Good circuit design often places a fuse upstream of the MOV so it will blow when the MOV shorts and prevent what you have happening now. Design shortcut saved a few cents on a fuse. They're so cheap you will no doubt have to pay more for shipping than for the part.
              Thanks for correcting my error. I said thermistor when I meant varistor. I had a machine that I replaced one of each on and was confused. I think I have a couple of the varistor's left from a MM251...likely the same part. I'm going to check and post back.

              Comment


              • #8
                Originally posted by G-ManBart View Post

                Thanks for correcting my error. I said thermistor when I meant varistor. I had a machine that I replaced one of each on and was confused. I think I have a couple of the varistor's left from a MM251...likely the same part. I'm going to check and post back.
                Don’t feel bad—it took me five minutes to remember the word varistor this morning! :-). I could think of MOV but couldn’t for the life of me say varistor.

                Comment


                • #9
                  The motor running sounds like a board problem. How long after you unplug it does the motor keep running? Could you vary the speed at all? Was the mig gun connected?

                  ​​​​​​

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                  • #10
                    Thank you guys for the information on the MOV. I can barely read the one that is currently in there so if possible I will at least buy that replacement and maybe a few others around that same design.

                    Yeah I am thinking the board has issues. I can check how long it keeps running later but I cannot vary the speed and will keep running even when the gun is disconnected at the small plug in front. I did trace wires to the plug on the board and when I pull that it does stop.

                    Comment


                    • #11
                      Originally posted by jtimm1975 View Post
                      Thank you guys for the information on the MOV. I can barely read the one that is currently in there so if possible I will at least buy that replacement and maybe a few others around that same design.

                      Yeah I am thinking the board has issues. I can check how long it keeps running later but I cannot vary the speed and will keep running even when the gun is disconnected at the small plug in front. I did trace wires to the plug on the board and when I pull that it does stop.
                      I found the leftover MOVs that I used on my MM251. It's hard to read, but the top line says "H S2" and the bottom line says "275L10".

                      I'm not sure if it's the same part for a 250X, but I wouldn't be surprised. I got this MOV part because I happened to have two MM251s at the time and the MOV was perfect on one machine and I could read the part number clearly. This was a cross-reference I found online. I think I posted a thread here about it...I'll see if I can find it. I might have listed the Miller part numbers in that.

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