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Miller Syncrowave 250 KA850249

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  • Miller Syncrowave 250 KA850249

    I have a Miller Syncrowave 250 KA850249 (1990 ) . The problem is when you push down on the pedal control it goes to full amperage and blows holes in the material we are tigging so I sent in to have repaired and was told the Board and the Thyristors were bad . I have since had the Board repaired and Thyristors replaced and I am still having the same issue . Has anyone else run into this issue and point me in the right direction ?

  • #2
    So they repaired it and said it was good? Did you send the foot pedal with it?
    Try this. Get a rod and try to stick weld with it. If it has control your problem may be in the pedal or remote circuit.
    Maybe the board repair didnt work. I'm surprised they replaces SCR's if you had out put but that hind sight.

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    • #3
      Originally posted by mudpup98 View Post
      I have a Miller Syncrowave 250 KA850249 (1990 ) . The problem is when you push down on the pedal control it goes to full amperage and blows holes in the material we are tigging so I sent in to have repaired and was told the Board and the Thyristors were bad . I have since had the Board repaired and Thyristors replaced and I am still having the same issue . Has anyone else run into this issue and point me in the right direction ?
      I'm not an expert, but I had that happen on a similar age machine, so I know a couple of things to look for. First off, it could be the pedal, and it's pretty easy to check if you have a multimeter....I'll cover that later.

      Assuming your machine hasn't been updated, it's the earlier style which has a shunt on the back panel just in front of, and below the cooling fan motor. It's an aluminum bar held with a couple of red rubber bushings. In your manual, figure 10-7, item number 6. There will be a wire on each end of the shunt that goes to a square, silver box that is on the back side of the middle panel...that's a filter, figure 10-4, item 22.

      The two wires coming off the bottom of the shunt going to the filter are part of the circuit controlling the amperage. If the wire comes off one side the machine will be either minimum or maximum amperage as you described. If the other wire comes off you won't get any arc. I found that the wires are held in place by some kind of soldering or tack welding process and can get knocked off....which is what happened to me when I was cleaning out the inside of the machine. I think I have a couple of pictures saved I can upload to help point you in the right direction....that might take me a few minutes.

      You'll definitely want to pull the side panels and top off the machine so you can see clearly.

      To test the pedal you can disconnect it from the machine and check continuity and resistance across the five pins on the plug.

      The pins should be A,B,C,D, and E. A and B close when you push the pedal (only continuity matters). The resistance between C and E should start at 1k ohm and go to zero as you press the pedal down. The resistance between D and E should be the opposite (start at 0, go to 1k ohm). That should rule out pedal issues.

      I'll see if I can find the pictures I took when I had the wire fall off.

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      • #4
        Here's a pic of the shunt....shiny aluminum bar under the red insulators. You can see the wire that had fallen off. You can also see most of the filter in the upper right side as well.



        Click image for larger version  Name:	IMG_9729.jpg Views:	0 Size:	3.01 MB ID:	614274

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        • #5
          No they did not repair . That was what they told me was wrong . I had to send the unit out of town because we no longer have a service tech here . I did try a different foot pedal with the same results . I sent the board to LyraTronix in Texas and they replaced the Caps and looks like a chip . I bought all new Thyristors and replaced them all just to be safe . I will take a look at the shunt this morning .
          Last edited by mudpup98; 03-31-2021, 08:18 AM.

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          • #6

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            • #7
              Did you try to stick weld with it?

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              • #8
                When I hear that any machine is putting out more than what you are expecting I always go to feedback first when troubleshooting. If any of these units lose feedback, it doesn't know how much it is making, thinking it isn't making enough output and then makes even more, sometimes maxing out the output.

                That being said, I would start with verifying voltage and current feedback to the control board.

                Current feedback should be on RC55 pin 1 and 2 and should measure some milli volt signal from the shunt connections.

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