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  • XMT 304 Board Issues

    Hello, new to the forum, have been reading for a while and would like to see if the group could offer some advice. I just purchased a 4 pack of XMT 304's at an auction hoping to get a couple that worked for light shop use/farm repair work. The welders were production units with signs of heavy use, but upon removing the covers have been maintained somewhat (new boards hear and there, new caps, etc.). I've been able to troubleshoot and swap enough parts out to get one machine that powers up with no error codes. I wanted to ask the folks here if they might be able to point me in the right direction of how to diagnose the problems on the PC boards of the machines, I have attached pics of the non-working machine pc boards. Single phase power 239V measured input power to the machines, no clear signs of bad caps, IGBTs appear ok but haven't tested them yet. They present as follows:
    1. Board 1 - Delivers Help 0 message.
    2. Board 2 - Delivers Help 7 message. I understand that most of the time this error is an output board resistor issue, however when I transfer this PC-1 board into the machine that powers up, I get the same Help 7 error in that machine.
    3. Board 3 - No errors delivered, but voltage reads 0.2 to 0.4 volts in DC stick mode.
    When I swap the working PC-1 board into two of the 3 non-working machines (1 machine had the negative terminal melted so I need to do some other repairs before I try to power it up), the 2 machines power up with no errors. I have not welded with any of them yet, just ran them long enough in DC stick mode to run through 1 fan cycle. Since a good board powers up 3 units when swapped in, then I have assumed I have PC-1 board issues, but maybe I am coming to the wrong conclusion? Can anyone see anything obviously wrong with the boards in these pics? I have some basic skills here, I've repaired industrial boards for my CNC machines in the past, so I some understanding of what to look for but don't see any obvious problems here. Thanks in advance and apologies for the long post.
    Attached Files

  • #2
    Unfortunately, most failures aren't visible... But a couple minutes with a garden hose and a soft bristled paint brush will make those boards a whole lot easier to work on!

    Start with all the pre-power checks in this manual: https://www.red-d-arc.com/pdf/EX300%...e%20Manual.pdf

    Be warned the big caps store high voltages and may store them for a long time after the machine is unplugged, and treat the entire unit as live 460V mains until you've verified the caps are discharged each and every time you work on it.

    Comment


    • #3
      Thanks, I have been looking for this manual but hadn't found it yet. I'll start working through the checks as best I can and come back.

      Comment


      • #4
        Yeah, miller doesn't publish their tech manuals, but the red-d-arc model is just a rebranded xmt304, and they do publish them. Note that there were a lot of changes to the boards over the years, more than just the major revisions shown with different schematics in the manual, so always verify the schematic matches what you're looking at.

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        • #5
          I hear you on the revisions comment. The working board is a 213664, which I believe is an earlier version, while the 3 suspect boards are all 240570B versions. I understand those to be later revisions. Will see if I can get more specific on the individual issues and come back. Thanks again.

          Comment


          • #6
            I started working through the pre-power checklist on one of the units. I have a couple of readings on the Input Rectifier SR1 that don't make sense. I've attached some pics so you can see the readings, but the SR1 on this unit actually looks new, as do all of the other boards. The diode test checked out ok, however I get some large resistance numbers when measuring SCR- to positive terminal (depending on which multimeter I used 3.6M ohms or 1.7M ohms). I checked R7 and it measures ~200 ohms as it should. When I connect the jumper between the gate and positive terminal, I get 0.1 ohms.

            I'm not sure what to conclude from these measurements since they don't seem to fit the tech manual specs. Any thoughts?
            Attached Files

            Comment


            • #7
              That's a pretty impressive burnt spot... that burned clean through the high-temp fiberglass. I would imagine things look new because the last ones looked like charred blobs!

              The readings are for with the board in place and the bolts in. With the board removed, the 200 ohm resistor on the board won't show up....

              The 0.1 ohm reading suggests the gate might be shorted in the SCR unless your probe slipped or such. Compare to your readings on other units. You could also rig up a scr/igbt tester. As a general rule, nothing should ever measure 0.1 ohms when testing semiconductors.

              A couple of notes... SR1 failures do often take out the power switch like the manual says, so consider it suspect. The oil around the tank capacitor (next step in the checklist) is normal - every single xmt304 I've ever seen has it. In addition to unscrewing the pcb and sticking a shim behind it to measure the capacitors directly, the two capacitors under PC1 should be given complete visual inspections, preferably by removing them. Any sign of bulging, wrinkling, etc and they should be tossed. Internal shorts in those capacitors, that sometimes clear themselves and don't show up on a measurement, are a common cause of SR1 failures. They often show up as slight bulges on the sides of the capacitors. See https://weldingweb.com/vbb/threads/2...MT-304-Problem for some pics of a particularly obvious case.
              Last edited by Bushytails; 04-25-2023, 09:20 PM.

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              • #8
                Grrr, my post got flagged a spam since I included a link in it. Since they often take a couple days to get approved, I'll re-post without the link so you can keep working.


                That's a pretty impressive burnt spot... that burned clean through the high-temp fiberglass. I would imagine things look new because the last ones looked like charred blobs!

                The readings are for with the board in place and the bolts in. With the board removed, the 200 ohm resistor on the board won't show up....

                The 0.1 ohm reading suggests the gate might be shorted in the SCR unless your probe slipped or such. Compare to your readings on other units. You could also rig up a scr/igbt tester. As a general rule, nothing should ever measure 0.1 ohms when testing semiconductors.

                A couple of notes... SR1 failures do often take out the power switch like the manual says, so consider it suspect. The oil around the tank capacitor (next step in the checklist) is normal - every single xmt304 I've ever seen has it. In addition to unscrewing the pcb and sticking a shim behind it to measure the capacitors directly, the two capacitors under PC1 should be given complete visual inspections, preferably by removing them. Any sign of bulging, wrinkling, etc and they should be tossed. Internal shorts in those capacitors, that sometimes clear themselves and don't show up on a measurement, are a common cause of SR1 failures. They often show up as slight bulges on the sides of the capacitors.

                Comment


                • #9
                  Thanks for the comment about taking the readings with the board in place, I guess I missed that point. With the board back in I still measure 3.7M ohms from SCR- to positive, and 0.1 ohms with the jumper from gate to positive. I measured the SCR- to positive on the one machine that is working and it reads 200 ohms. I check the other two machine that are not working and they give similar very high ohm readings from SCR- to positive. I'll order a new power supply module and see if that let's me move forward. Appreciate the help so far, I'll report back when the new module shows up and I'm able to test again.

                  Comment


                  • #10
                    Did you have the screws installed and tight for the 3.7M reading?

                    Comment


                    • #11
                      Yep, screws were in and tight. Similar measurements on the other non working units. Same measurement on the working unit reads 200 ohms. Checked multiple times to try to eliminate human error.

                      Comment


                      • #12
                        That probably means the little board is bad too. It may be repairable.

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                        • #13
                          Update. The new power supply module came today. I installed it and got similar pre-power checklist readings on the the SCR- to positive terminal ohm check, very high ohm readings. When I plug it in and turn the switch on, I get Help-6. Seems like this was not the problem. I'm inclined to swap the little board out of my working unit and see if next unit powers up properly. I guess then at least I would know if it is the little board. Any other thoughts?

                          Comment


                          • #14
                            If you're not seeing the 200 ohms, the resistor on the little board is bad, the little board is bad, or it's not making contact for some reason, like burnt pads.

                            Comment


                            • #15
                              Is the photo above the little board that's suspect?

                              Comment

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