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  • Xmt 300 cc/cv

    Hoping someone can provide some insight here or a few things I should check. I picked up a used xmt 300 cv/cc that wouldn’t maintain an arc and the display was showing erratic values. Based on some other posts here I was able to get almost everything working on it after replacing the three front selector switches. The only issue that now remains is the display does not show the preset amp value when not welding in the cc,panel, contactor on mode. It does display the actual metered when I make contact and goes up and down with the front knob. When I break the connection it goes back to displaying -005 or -006. Click image for larger version

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ID:	607909 If I flip the meter to voltage it displays 77.7 Volts and drops down when I make connection. Click image for larger version

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    In all the other modes it displays the preset and metered and I tested the 14 pin remote and it functions correctly. Part of me is thinking of just letting it be as is it’s functioning and welds but the other part of me is wondering it there is something else simple I could check.
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    Last edited by gallenbt; 04-24-2020, 02:43 PM.

  • #2
    So, just to feed you back what I understand from your message, the panel is displaying 5-6 amps when an arc is not struck, and the correct amperage when an arc is struck?

    If that's the case, then I would recommend you clean the output hall device connection on both sides. It sounds like it is picking up about 5-6 amps when there is zero amps running through it. I have had this problem before on a welder that spent too much time near the ocean and had corrosion on the hall device connection (black or blue box near your output connector).

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    • #3
      Thanks I’ll give that a try, it’s reading negative 5-6 not sure if that matters.

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      • #4
        I don't think it would matter either way, but I haven't looked over the XMT 300CC/CV panel board to know for sure. But it does sound like you have some low signal noise in your current feedback.
        Last edited by jjohn76; 04-27-2020, 06:58 AM.

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        • #5
          Click image for larger version

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ID:	607962 I did notice what looked like a resistor cut out of the board...definitely intentional. This wouldn’t come from the factory like this would it?

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          • #6
            No telling. That looks like the primary current sensor. Isn't there a secondary current sensor near the output?

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            • #7
              There was a diode in that spot not a resistor

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              • #8
                From the many lems in my used but works

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                • #9

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                  • #10
                    Well got around to check out the blue coil and tracked the two wires back to the board disconnected and reconnected...now I no longer hear the buzzing when I switch to contactor on mode and the leads are dead...I’ll see see if I can track down what else is going on. Thanks for the help so far.

                    Update: also tried jumping The contactor pins on the remote 14 input with no effect, this used work...
                    Last edited by gallenbt; 05-08-2020, 07:52 PM.

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                    • #11
                      Click image for larger version

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ID:	611205 Thought I would post an update, finally got around to testing the IGBTs removed one and as soon as the screws were out I started to see my problem Click image for larger version

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                      • #12
                        That's not an uncommon problem, but you likely have more to fix than just the IGBT. Any signs of overheating/blister/fluid on the capacitors? Did this all just happen (you would have heard it, pretty loud pop)?

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                        • #13
                          That looks like PM2 (left side IGBT kit?), with the input rectifier right behind it. What diode drop do you get C2/E1 to C1 and E2. Any shorts, and you likely burned other components. It's worth checking the diodes on the rectifier - diode drop tests from each line input to the + and - terminals on the rectifier, SR1. Also, check if the snubber resistors or capacitors have failed too. If your input rectifier is showing any shorts across the diode, you will need to replace both IGBTs, possibly their snubber resistors/capacitors, all four of the 4000uf 250V capacitors, the diode rectifier, and maybe the switch. Hopefully that isn't the case. I wouldn't think the machine was working with that IGBT blown like that, but who knows... If it's just the IGBT, check the gate drives, which should be right near plug 18, given away by the gate drive transformers (small transformers on the board, probably 4 nearby). You should be able to backtrack from the plug and find the clamping diodes (do a diode check), and there are likely some burned gate drive resistors, plus a 10k-~100k resistor pulled to the gate common (hope that makes sense). Check for any charred areas around the resistors (or charred resistors) and check the diodes with your multimeter.

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                          • #14
                            Interesting, I was thinking maybe it was bad and limping along causing my issues and then fully died...not sure if that’s even possible. Checked the rectifier and caps prior to this was planning on testing the IGBT out of the unit when I saw it was in pieces. I got some other feedback as well that that I should check the gate drivers. Might be a while machine is low on my list but just cant seem to part with it just yet. Found a replacement IGBT for $20 should a couple weeks till it arrives.

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