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Help diagnosing XMT 350. Blown up like grenade inside

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  • Help diagnosing XMT 350. Blown up like grenade inside

    serial# LE124886 (2003)
    60M feeder or Spoolgun use with Optima pulser

    Running on 1 phase 240V power

    Back story.
    Bought it used about 1 year ago. I am pretty sure the thing was used industrially.

    Everything was working fine except I noticed the cooling fan wasnt coming on. First thought it wasn't warm enough until 6 months ago it was definitely warm enough after a bunch of welding...but the fan wouldn't come on. Welder never shut down or overheated but was definitely too warm.
    I was in a hurry to get the trailer job done so I just put an external fan blowing through the front of the machine. Suprisingly the miller fan turned on after a few minutes of the other fan running.

    A few days later I took the hood off the machine. Blew everything out. It was very dirty with clumps of dust. Visually checked everything out and tightened down anything I could... or at least I thought. (I did not tighten the capacitors)

    With everything back together and working fine except the fan still not working properly. Same thing added external fan... after a few minutes the machine fan would kick on....sometimes.
    So I used it like that for the last 6 months just propping an external fan against the front of the machine blowing through it. An everything was good....
    Until yesterday. I was running some wire on a trailer with the 60 m feeder. Everything was working fine (except the cooling fan).
    During some fit up time the machine was powered on just idling and I heard a click (electrical noise) a few times. Thought it was maybe the fan trying to turn on so I welded a couple more spots.
    Heard the click sound a few more times over a half hour of fit up and light welding.
    Then while welding the machine literally went BOOM! Like a grenade. It shook my ear drums it was so loud. And let out the white smokey ghost. Up until the explosion the welder was not hot or in heavy use.

    SORRY FOR SUCH A LONG POST. Just not sure if any of this is related.
    If you are still with me. THANK YOU!

    The hood on the welder is actually expanded like a balloon along the bottom side edges. With wrinkles blown outward in between the bolts of the hood. Never seen anything like it.

    Ok. Under the hood I see the -bus capacitor c12 blown out the end and its Bleeder Resistor R1 broke off the board on 1 side.

    Crazy enough all else looks good. Considering I thought it was going to burn down my barn. I actually unhooked everything in record time, wrestled the machine out from the bottom of the stupid miller cart and threw it outside. Lol.

    So, I did some research reading here.

    I got the service manual for the ex360 red-d-arc machine printed out and was going to try and run through the pre power check list to see if anything else is fried. However the serial #s listed are a little newer than my machine. The diagrams all look pretty least to my untrained eye.


    Is this red-d-arc ex360 manual going to work for me?ht
    (sorry, not sure how to put actual link)

    Should I fix this thing?
    Can I fix this thing for less than the cost of another used unit? Looks like the capacitors are $160, but I keep reading horor stories about the expensive miller proprietary boards or mods or whatever in these 350s.

    Does that clicking noise (pre explosion) give any insight at what else to look for in here?

    Now, 6 months ago I swear I tightened everything including (especially) the main input power wires. But when inspected yesterday after explosion the white wire is just barely a little loose but the red one is wiggling loose. Black was tight.
    I don't think the red wire matters as I'm running 1 phase power. (But IDK?)
    Could just that lightly loose main input white wire (or red wire) have caused this thing to grenade?

    Was the explosion from not torquing the capacitor bolts?.. (wish I would have tightened them.) I was just kind of jumpy around capacitors...still am. But at least I see how to discharge them now with the service manual. And I am now going to get a smaller torque wrench for inch pounds.

    Any tips would be greatly helpful....and appreciated!

    Here are pics of what it looks like under the hood.

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    Attached Files

  • #2
    Any tips would be greatly helpful....and appreciated!
    Replace the blown up bits and hope for the best? I'd probably send it in for repair and pay the ask instead swapping parts that might or might not fix it. That's just me. It might be cheaper to just replace it with another used unit then trying to repair it.


    • #3
      That is one heck of an explosion from that capacitor... It's worth going through the pre-power checks. The front mod may be damaged, but the checks will let you know for sure. There is a guy on ebay selling new old stock capacitors for ~$15. I am about 90% sure they are the right ones, but would ask you to compare part numbers/specs with yours. Search "Kemet 1800uF DC Electrolytic Capacitor 400-550v ? Rail Gun Pulse Coin Shrinker" on eBay and you should find them. I picked up 4 for a couple of my projects and all 4 are within 10% of each other and the other 1800uf cap I have. The bleed resistors are very different from the older ones and the newer replacement ones, so it's worth calling Miller tech for the replacement info - just give them the board/assembly number and the components number (R4 or whatever it actually is written on the board).
      That clicking was probably your SCR relay turning on and off as the capacitor was starting to fail and pulling the bus voltage down. I have fixed two with capacitor failures and no other problems, so you may be in luck.

      Hope this helps.


      • #4
        Also, post some pics of the back side of the interconnect board. The copper may have disintegrated and the board may need some hackery to remove the scorched board. Try not to move the power modules when you do remove that board (long after the pre-power checks) so you don't have to fiddle with the thermal paste. If you haven't picked up a torque wrench, the Wheeler wrench for mounting scopes is what I use and may be a good option for you.


        • #5
          Had similar blow-up of my XMT-350 cc/cv with serial number a little later in the LExxxxxx series than yours about 8 years ago. A technician-member here called cruizer helped me with advice. I identified the ruined board and capacitors. I bought the replacement parts for about $1200. Cruizer told me this damage usually occurs when after many heating/cooling cycles the capacitor connections loosen a bit and unless they're re-torqued the capacitors explode and take the board with them. A local Miller trained technician later told me the new replacement board is of a slightly different (improved) design and such damage is less likely in the future. Good luck.
          Last edited by Dmaxer; 09-20-2020, 07:26 PM.
          Miller XMT-350 CC/CV
          Miller S-22A wirefeeder
          Bernard 400A "Q" gun
          Miller 30-A Spoolmatic w/WC-24
          CK 210 & WP-18 GTAW torches
          Hypertherm Powermax 30
          O/A Rig, Enco 4x6 bandsaw, etc.


          • #6
            Thank you guys. I really appreciate all the help. I am feeling a bit more hopeful about this machine. Still going to check out a newer used xmt304 tomorrow and hopefully that goes well and I'll be back up and running...after I check the torque on all the connections inside of course.
            Then I can take my time to go through that pre power checklist. I'll also grab pics of the back of the interconnect board when I get back under the hood.
            Thanks for the tips on the cheaper parts. You guys are a lot of help.
            Hopefully I can pull this off. I am not an electric engineer by any means, but I am usually a quick study. It seems the red-e-arc manual lays everything out pretty nicely. Some things (a lot of things actually) are over my head but I am going to go for it. Guess thats what the internet is all about.


            • #7
              I took my blown up XMT 304 that is probably 16 years old to get repaired, I told him that I had a blown diode, I heard some loud pops ( he said probably capacitors ) He said its not worth fixing.


              • #8
                Jon. Thanks for the tip on the wheeler torque wrench. Looks pretty nice for the price. Got 1 ordered up. Kind of excited to play electrician when I get some time.


                • #9
                  I just use the good ol' arm-ometer torque wrench...


                  • #10
                    Originally posted by Bushytails View Post
                    I just use the good ol' arm-ometer torque wrench...
                    Yeah, I had one for my scopes - Too many issues overtorquing screws, and now since I have it...


                    • #11
                      It's easy not to overtorque screws... Tighten them until they strip, then back off a quarter turn.