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Thread: Problems with a Snap On MW120 MIG Welder

  1. #1
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    Problems with a Snap On MW120 MIG Welder

    Hello,
    I am new to this site and joined to see if any members have some insight to a problem I am having with a MW120 Snap on welder I have. A little history on the welder, I worked for Snap on as a Manufacturing Engineer in the East Troy , Wisconsin plant that built this welder and purchased this unit in the late 1980's. I had the opportunity to procure many of the spare parts for this welder ex: transformer, PC Board, etc. The welder is long since been obsoleted and parts are hard to find. This welder is just like new and even though it is only one of many different welders I own it is the only 120v unit I have and I would like to get it running again.
    The problem with the unit is that it doesn't seem to have the ability to generate an arc, the wire comes out touches the work and turns red hot, bird nests and doesn't melt. The only time it will even generate an poor arc which lays on the surface of the material, is when the wire speed is turned way down and the voltage way up.
    When I first got the welder it performed very well but after pulling it out of storage it has had this issue. I have exhausted all avenues as I have replaced the diodes, power transformer, tried a different capacitor (the original is obsolete) printed circuit board, and torch and tip with no change. The wire feeds fine and the gas solenoid is working correctly.
    I have reached out to Snap on and they directed me to a couple of sites, one doesn't do repairs on these units and the other was Lincoln Electric, they purchased Century which is very similar to the Snap on. Actually, the consulting Design Engineers that helped design the Snap on unit were retired Century people. I reached out to Lincoln via phone and email with no response. I took it to a local welding supply that had diagnostic equipment and they are stumped with what's wrong. My hope is to find a technical resource that may have history with this or a similar welder.
    Sorry for the long dissertation but I wanted to get as much included as I could so a possible contact would have some insight to my dilemma.
    Thank you!

  2. #2
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    Re: Problems with a Snap On MW120 MIG Welder

    Welcome! There's manufacturer forums/threads further down in the forum, you'll probably have better results there, this is a great bunch on here & if anyone can help they'll chime in. Best wishes!
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  3. #3
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    Re: Problems with a Snap On MW120 MIG Welder

    Give me the weld a load bank and a meter. should be able to find the bad connection in a matter of minutes. You should be able to do the same thing by running out 2 feet of weld wire and clamp the work lead on the end. careful the wire will get red hot.
    The weld circuit is a circle so map out a test of each part in the loop.
    connect a voltmeter across each part one by one and press the trigger just long enough to get a meter reading. If you see near the full open circuit voltage you have found the bad part or connection. Check the miggun first then look at the power block and the weld lead connection to it. Small welders with the smaller migguns have a high failure rate of the miggun. the weld connector breaks strand by strand until there are nit enough left to carry the weld current.

  4. #4
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    Re: Problems with a Snap On MW120 MIG Welder

    Quote Originally Posted by RandyW View Post
    Hello,
    I am new to this site and joined to see if any members have some insight to a problem I am having with a MW120 Snap on welder I have. A little history on the welder, I worked for Snap on as a Manufacturing Engineer in the East Troy , Wisconsin plant that built this welder and purchased this unit in the late 1980's. I had the opportunity to procure many of the spare parts for this welder ex: transformer, PC Board, etc. The welder is long since been obsoleted and parts are hard to find. This welder is just like new and even though it is only one of many different welders I own it is the only 120v unit I have and I would like to get it running again.
    The problem with the unit is that it doesn't seem to have the ability to generate an arc, the wire comes out touches the work and turns red hot, bird nests and doesn't melt. The only time it will even generate an poor arc which lays on the surface of the material, is when the wire speed is turned way down and the voltage way up.
    When I first got the welder it performed very well but after pulling it out of storage it has had this issue. I have exhausted all avenues as I have replaced the diodes, power transformer, tried a different capacitor (the original is obsolete) printed circuit board, and torch and tip with no change. The wire feeds fine and the gas solenoid is working correctly.
    I have reached out to Snap on and they directed me to a couple of sites, one doesn't do repairs on these units and the other was Lincoln Electric, they purchased Century which is very similar to the Snap on. Actually, the consulting Design Engineers that helped design the Snap on unit were retired Century people. I reached out to Lincoln via phone and email with no response. I took it to a local welding supply that had diagnostic equipment and they are stumped with what's wrong. My hope is to find a technical resource that may have history with this or a similar welder.
    Sorry for the long dissertation but I wanted to get as much included as I could so a possible contact would have some insight to my dilemma.
    Thank you!
    Hello Mr. Randy W,
    I have a MW120 and it began spewing wire several years ago. I’m hoping you might have a schematic of the printed circuit board for the MW120 Welder. I’ve looked extensively on the web and have not found one at this time. If you do not have a schematic, would you consider turning loose one of your printed circuit boards. I have tested all of the resistors, capacitors, diode’s, & opto isolator, but cannot seem to find the problem. I changed the control transistor (TO-3) mounted on the heat sink thats external to the circuitboard, and also replaced the solid-state relay. The welder now clicks when you press the trigger, but I think the click is the gas solenoid. Right now I only have a doorstop with a full tank of gas!


    Last edited by Chasjohn; 07-09-2021 at 12:15 PM. Reason: Pic

  5. #5
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    Re: Problems with a Snap On MW120 MIG Welder

    Quote Originally Posted by RandyW View Post
    Hello,
    I am new to this site and joined to see if any members have some insight to a problem I am having with a MW120 Snap on welder I have. A little history on the welder, I worked for Snap on as a Manufacturing Engineer in the East Troy , Wisconsin plant that built this welder and purchased this unit in the late 1980's. I had the opportunity to procure many of the spare parts for this welder ex: transformer, PC Board, etc. The welder is long since been obsoleted and parts are hard to find. This welder is just like new and even though it is only one of many different welders I own it is the only 120v unit I have and I would like to get it running again.
    The problem with the unit is that it doesn't seem to have the ability to generate an arc, the wire comes out touches the work and turns red hot, bird nests and doesn't melt. The only time it will even generate an poor arc which lays on the surface of the material, is when the wire speed is turned way down and the voltage way up.
    When I first got the welder it performed very well but after pulling it out of storage it has had this issue. I have exhausted all avenues as I have replaced the diodes, power transformer, tried a different capacitor (the original is obsolete) printed circuit board, and torch and tip with no change. The wire feeds fine and the gas solenoid is working correctly.
    I have reached out to Snap on and they directed me to a couple of sites, one doesn't do repairs on these units and the other was Lincoln Electric, they purchased Century which is very similar to the Snap on. Actually, the consulting Design Engineers that helped design the Snap on unit were retired Century people. I reached out to Lincoln via phone and email with no response. I took it to a local welding supply that had diagnostic equipment and they are stumped with what's wrong. My hope is to find a technical resource that may have history with this or a similar welder.
    Sorry for the long dissertation but I wanted to get as much included as I could so a possible contact would have some insight to my dilemma.
    Thank you!
    hello mr Randy W,
    I have a MW120 and it began spewing wire several years ago. Iím hoping you might have a schematic of the printed circuit board for the MW120 Welder. Iíve looked extensively on the web and have not found one at this time. If you do not have a schematic, would you consider turning loose one of your printed circuit boards. I have tested all of the resistors, capacitors, diodeís, & the opto isolator, but cannot seem to find the problem. I changed the control transistor on the heat sink thats external to the circuitboard in the solid-state relay. The welder now clicks when you press the trigger, but I think the click is the gas solenoid.
    right now I only have a doorstop with a full tank of gas!

  6. #6
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    Re: Problems with a Snap On MW120 MIG Welder

    Looks like 6-11-91 mfg date on cut board.


  7. #7
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    Re: Problems with a Snap On MW120 MIG Welder

    dave's driveshaft nashville. Used to be a authorized century repair center. Not sure he could help, but may be worth a shot?

  8. #8
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    Re: Problems with a Snap On MW120 MIG Welder

    Quote Originally Posted by Chasjohn View Post
    hello mr Randy W,
    I have a MW120 and it began spewing wire several years ago. Iím hoping you might have a schematic of the printed circuit board for the MW120 Welder. Iíve looked extensively on the web and have not found one at this time. If you do not have a schematic, would you consider turning loose one of your printed circuit boards. I have tested all of the resistors, capacitors, diodeís, & the opto isolator, but cannot seem to find the problem. I changed the control transistor on the heat sink thats external to the circuitboard in the solid-state relay. The welder now clicks when you press the trigger, but I think the click is the gas solenoid.
    right now I only have a doorstop with a full tank of gas!
    Hello,
    I don't have a schematic of the PC board its self, I have the welder schematic in the owners manual but it is fairly basic. I have a spare board that I could part with. Let me know how to proceed.

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