Problems with a Snap On MW120 MIG Welder

[RandyW]

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have exhausted all avenues as I have replaced the diodes, power transformer, tried a different capacitor (the original is obsolete) printed circuit board, and torch and tip with no change.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!

[CAVEMANN]

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!

[ccawgc]

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.

[Chasjohn]

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!

[Chasjohn]

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, & 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!

[Chasjohn]

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

[tapwelder]

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

[RandyW]

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, & 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.

[r8183594]

Hello forum members! I have a Snapon MW120 that has no weld power. It only clicks the gas solenoid, and unspools wire. I have sent the board into Innovat who changed 3 components and sent back, but still no change. I have the original manual with the schematic in the back, BUT ALSO have the board level schematic, ..and will share it with whomever would find it useful working on theirs. I have had the welder to 3 different, VERY reputable shops, and theyve all been stumped. This model has a very simplistic board, with few components, compared to the newer inverter based welders sold currently, so I cannot beleive this cannot be repaired. One thing that is for certain is that the point of weld power failure IS the lack of threshold voltage at my ssr. It requires 3 to 30 vdc, and I'm only getting 1.8, sometimes 2 vdc. If memory serves me correctly, this is being fed from U1, which is new, and been tested by Inovat. Is there anybody else working on an mw120, or, can offer guidance?

[RandyW]

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, & 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 have a manual for the MW120 but it does not have the any technical data or PCB schematic. I do have two spare PCBs and I'm not sure if they will work. They may be prototype models I salvaged from the scrap during the design testing, but it may be worth a try to see if this would solve your problem. I'm not sure how to proceed, if I have your contact information, I could send you a board and you could try it. They are just collecting dust now. My email: truenorthtech@paulbunyan.net

[r8183594]

Good afternoon, reply to Randy sent!

[whtbaron]

Hello forum members! I have a Snapon MW120 that has no weld power. It only clicks the gas solenoid, and unspools wire. I have sent the board into Innovat who changed 3 components and sent back, but still no change. I have the original manual with the schematic in the back, BUT ALSO have the board level schematic, ..and will share it with whomever would find it useful working on theirs. I have had the welder to 3 different, VERY reputable shops, and theyve all been stumped. This model has a very simplistic board, with few components, compared to the newer inverter based welders sold currently, so I cannot beleive this cannot be repaired. One thing that is for certain is that the point of weld power failure IS the lack of threshold voltage at my ssr. It requires 3 to 30 vdc, and I'm only getting 1.8, sometimes 2 vdc. If memory serves me correctly, this is being fed from U1, which is new, and been tested by Inovat. Is there anybody else working on an mw120, or, can offer guidance?

It looks like the detailed board level schematic is getting hard to find... if you have both it might be a good idea to post pdf's of them both on here for future references.

[smithdoor]

If post the schematic that would help. Like whtbaron said

Most welders can not read schematics but I can.

Dave

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!

It looks like the detailed board level schematic is getting hard to find... if you have both it might be a good idea to post pdf's of them both on here for future references.

[r8183594]

Here's the two Ive been working with. One is the one out of the manual, the other, the board schematic

[r8183594]

Around the perimeter of the board schematic are the notes a good friend and I documented as we confirmed functionality to the welder peripherals.
My point of failure is the lack of 3 volts on the brown, and lt blue wires to the ssr. If anyone else has everything BUT weld power, check there. Can anyone post a picture of their ssr? Mine is not the original, and perhaps an original with part numbers could be cross referenced. My research has determined 99% of this power level of ssr are rated at 3-30 volts on the control side of the relay.

[smithdoor]

I would check D1 and D2 to they working.
Then check voltage out transformer under load.
It can be K1 and S1 they both check until power is apply.

Dave

It looks like the detailed board level schematic is getting hard to find... if you have both it might be a good idea to post pdf's of them both on here for future references.

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!

Here's the two Ive been working with. One is the one out of the manual, the other, the board schematic

[r8183594]

Has anyone made any headway, or discoveries regarding their MW120 welder? I have bookmarked Mr. Smithdoor's troubleshooting suggestions, but haven't had the opportunity to pull the welder project back out front and center to resume yet. Winter is coming, and I look forward to getting back to it!

[CAVEMANN]

I don't know about your welder but my inverter machine started doing really weird stuff like that & it turned out that one of the dinse connectors had come unseated and was loos. It would shoot out wire at high speed until it touched something grounded then the wire got red hod & burned in two. The machine was fairly new & I thought sure something had failed,been doing great ever since that happened.

[Rareformdesigns]

I have a Snap On MW120 that I bought new off the truck in 1991. The welder worked great until about 2 months ago when it started to pop the breaker once in awhile and now it trips the breaker as soon as you pull the trigger. If anyone needs parts let me know and I'll save them for you. I'm going to toss this in the dumpster, but would hate to waste the working spare parts if someone could use them. If I had more space I would try to fix it sometime in the future, but I need the space that this thing takes up. I live in New Jersey just so you know.

[Runt h]

I know this is old thread but I have snap on mw 120 welder everything work except for creating an arc .was only getting 2volts to the ssr so it would not turn on.i have been messing with this thing forever.finally figure it out .it was the heat control potentiometer the whole time.i had check it before and it would control the voltage smoothly from 2 volts to zero but it was causing the input voltage to be only 2volts .hope this makes sense.just wanted to post because I have used the diagram and other knowledge from this forum.so thank you all and finally fixed after 3 years. Ron from cabazon calif