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Thread: XMT 304 CC/CV melting transformer coil

  1. #1
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    XMT 304 CC/CV melting transformer coil

    My XMT 304 has been working great until today. I was doing a little wire welding and smelled some smoke when I went to shut off the machine after a minute or two of welding. Quickly unplugged it and after taking the cover off found the large transformer at the rear of the machine to be smoking and looks like it started melting.

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    Any ideas? It was still welding fine when it happened. I don't see any other obviously damaged components but I haven't taken everything apart yet.

    Thanks
    Glen

    Hobart Champion 16
    Miller XMT 304 CC/CV
    Hobart Hefty CC/CV
    Lotos LTP5000D

  2. #2
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    Re: XMT 304 CC/CV melting transformer coil

    That is the aux power supply transformer. It supplies power to run the pcb and wire feeder, 115 volt plug if used. For it over heat like this, something is drawing way too much current. or it didn't link correctly. So before you turn it on again you will need to unplug the feeder and anything else plugged into the welder. with welder unplugged remove cover. Mark the wires connected to the interconnect pcb and remove it. You want to access the two contactors behind it. on the side of each contactor is an aux switch. this switch reconnects the transformer primary. between the two contactor is a device that only allows one contactor to pull in at a time.
    Check the switches for damage. Check the wiring harness from the transformer for damage. once you repair any damaged wires and reassembled the interconnect pcb you can attempt to power it up. Try to stand three feet away when you first power it up. If one of the buss capacitors fail they will blow up and can spray hot oil and pieces of plastic. you can use the diagram in the owner's manual or on the inside of the cover and test the transformer output voltages. or a quick test is measure at pins A and G at the 14 pin plug. should be 24vac if thing are now correct.
    If you can't find a cause of the transformer damage you better take it into a shop.
    It can be dangerous to work on this welders.

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  4. #3
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    Re: XMT 304 CC/CV melting transformer coil

    Thanks, I will try that this weekend when I get a chance to work on it. I appreciate your time and will let you know what I find.
    Glen

    Hobart Champion 16
    Miller XMT 304 CC/CV
    Hobart Hefty CC/CV
    Lotos LTP5000D

  5. #4
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    Re: XMT 304 CC/CV melting transformer coil

    Got a chance to look into it further and take some pictures. I can't find anything visibly wrong but maybe I'm looking in the wrong places.

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    Glen

    Hobart Champion 16
    Miller XMT 304 CC/CV
    Hobart Hefty CC/CV
    Lotos LTP5000D

  6. #5
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    Re: XMT 304 CC/CV melting transformer coil

    Looking at the picture again I am going to guess the your serial number is older than KG177168. The transformer has two black wires with a white stripe. That transformer is obsolete from Miller. To get a replacement you will need to find a donor welder that has blown its IGBTs which are also obsolete for these older models.
    The contactor on the right is the W2 contactor Pulls in when welder is connected to 230 volts. Left is W1 that pulls in on 460 volts.
    on W2 you can see small wires connected to the center pole and a piggy back aux switch on the right side. These wires reconnect the primary wire for the transformer.
    Most of the time if this malfunctions the transformer is toast. 460 gets put on the 230 windings and they are blown open. your transformer appears to be over heated by to much output current. what feeder are you using? Check it for shorts from the drive housing to the frame. It is possible you sent weld current from the drive through the feeder power line and then through the welder line cord and back to what you were welding on and then to the work clamp. welding project had to be connected to earth and electrical system ground. Check for damage in the feeder wiring and cords.

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  8. #6
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    Re: XMT 304 CC/CV melting transformer coil

    The serial number is KG247829.

    I have only ever used it with 230v single phase. I don't have 3 phase available.

    I'm using it with a v sense feeder, so only using the main output leads. Once, I forgot to change the switch on the feeder that switches from CC to CV. It welded poorly until I realized that it was wrong, but I can't imagine it would damage the welder?

    Thanks again for your help.
    Glen

    Hobart Champion 16
    Miller XMT 304 CC/CV
    Hobart Hefty CC/CV
    Lotos LTP5000D

  9. #7
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    Re: XMT 304 CC/CV melting transformer coil

    Going to have to unplug the transformer and test the wiring harness with an ohmmeter and find the short.

  10. #8
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    Re: XMT 304 CC/CV melting transformer coil

    So I tested the transformer coils with these results

    220v coil: 7.7 ohms blk/wht-blk/wht

    18v coil blu-gra: 0.8 ohms
    blu-blu 1.1 ohms

    115/24v coil vio/org 0.6 ohms
    Vio/yel 1.9 ohms
    org/yel 1.8 ohms

    Wht-blk 7.8 ohms

    Brn-red 8.1 ohms


    Machine side of connector
    Vio-gra 0.5 ohms
    Brn-blk 0.5 ohms
    All others open circuit

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    Last edited by ArcTan; 2 Weeks Ago at 01:08 PM.
    Glen

    Hobart Champion 16
    Miller XMT 304 CC/CV
    Hobart Hefty CC/CV
    Lotos LTP5000D

  11. #9
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    Re: XMT 304 CC/CV melting transformer coil

    Looks like gra RC2-5 feeds the coil for the remote contactor 115v...

  12. #10
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    Re: XMT 304 CC/CV melting transformer coil

    Brown to black you are seeing the W2 switch contact which is closed and you should see a short.
    Back to violet and grey, violet is connected to case ground, the grey wire is grounded through the pcb. Just went out and pulled the cover off mine and double checked your numbers. all the transformer wires measured the same except the black/ white wires. Mine measured 15.8 your was 7.7. Checked the wiring harness side and only two wires show they are connected to ground. wire 42 and 8.
    One test for you, pull the plug from RC2 it has 4 wires. lives in the front left corner on the control pcb. each pair should read 7.9 ohms.
    Since Miller pulled the pcb diagrams from the service manuals they are not on line anymore. But I have some old ones and will take a look and see if I can figure out what to look for in your welder. The transformer is no longer available from miller.
    To get one you may have to call around to different service centers and rental yards and try to find a dead welder that has a good transformer with the black / white coil wires. this coil was removed from later units, which is why they are no longer available. Haven't made them for years. Right now i think there is a problem with the circuit the black/white wires power. as you can see it is connected to both pcbs
    and it is possible that a short circuit of some type has caused this problem.
    be back later after i go through the archives.

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  14. #11
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    Re: XMT 304 CC/CV melting transformer coil

    Thanks again, you've been very detailed with your help!
    On my board it's labeled RC4. I'm getting 7.1 ohms between the pairs. I will see if I can find a replacement transformer.
    Glen

    Hobart Champion 16
    Miller XMT 304 CC/CV
    Hobart Hefty CC/CV
    Lotos LTP5000D

  15. #12
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    Re: XMT 304 CC/CV melting transformer coil

    Remember there are two different transformers. only one has the black and white leads and it is obsolete from Miller. So you may have to look for a donor machine Like I did, Call different repair shops and see if they have a junk machine with a good transformer. If the IGBT's fail this older welder it can't be repaired.
    Miller did a redesign and changed the transformer and went to a different IGBT.
    They stopped making the older parts.

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