Need Help on a PCB for a Lincoln Power Mig 180C

Thread Starter

diebog

Joined Mar 23, 2013
223
Hey guys, I bought this powermig 180c a ferw years ago and during a little welding project of mine it started to act up. While welding it was as if someone turned the current knob all the way down. I stop and mess with it some and try to weld on the highest amp setting (which I never go up to) and I couldn't weld 1/16" to save my life. Again just like someone turned the adjustment knob all the way down. Just not enough current to melt the metal.

I open it up and do a bunch of testing with the Power Mig 180 Service Manual and according to it I should have line voltage on the output of the pcb. I have line voltage (220v) going in, and when the trigger is pulled it says there should be the same going out. So problem with the board. I took it out and had a look, they actual use a silicone liquid potting on this board. Boy it was hard to remove it. I got most off after what seemed like hours, I find no burnt spots/traces and no components look damaged. I figured maybe the 10k pot went out on the current adjustment knob, also noticed a large triac so I ordered a replacement triac, new 10k pot, new electrolytic caps, and I replaced all the voltage regulators and 3 pin smaller transistor/triacs. Replaced all those componets and crossed my fingers, I hook it all back up but still the exact same! Wire feed works and adjusts fine, fan comes on, solenoid for gas turns on and off, there is just no current to weld with.

So I take a closer look at the board and schematic on page 110 of the manual above and I see a voltage control amp x7 ‎LM224N‎ which feeds an optocoupler IL4208‎ that controls the main triac src1. I had an LM3224N from another project and replaced it then removed the optocoupler from PCB and it tested fine. When I apply 5vdc to pins 1 & 2 I get different Ω readings so I figured its working. Try board again and it still does the same exact thing. Anyone got any idea where next to look? Am I missing something? Also on the schematic it shows 2 large 55a triacs SCR1 and SCR2 the info on the schematic drawing says the info doesn't apply to all models so I guess that second traic is on another model? Because mine only has the SCR1. The left side of the board is for wire feed and gas valve as well as a spoolgun/spot timer addon kit (which I don't have but want if I ever get this fixed). So the right side is all the current controls. Ive checked all the diodes and resistors (tested with them on the board) and nothing jumps out.

If anyone has a suggestion of something I should test, or to look at, please share. Is there another component on the board that would cause this issue? This issue literately happened while welding, no power outages or lightning strikes, wasn't dropped or got whet. Im out of ideas......This machine was farily new when this happened. I had welded maybe 1 or 2 other projects with it. It was a clean hardley used machine, I just don't get what went out. I can buy a replacement PCB at about $300 but this isnt about saving money or trying to be "cheap", I want to learn more about troubleshooting electronics. If I just went and bought another board for my house gen when it went out, I wouldn't of learned as much as I did when I fiugred out what went out and how to fix it. Ive fixed a family memebers handheld sonogram machine, and other various electronics and I always learn when I do this kind of stuff. If all else fails I guess Ill just buy the replacement board, but id rather spend twice as much time and money and fix this one. lol Im hoping someone sees something or has an idea.

Thanks,

Jeremy

Here are a few pics of the board.
IMG_6668.JPGIMG_6671.JPGIMG_6670.JPGIMG_6669.JPG
 

twohats

Joined Oct 28, 2015
126
Those circuit boards need a good clean. Disconnect from the mains power. Internet search, "How to clean PCB's".
That may well fix your problem.
Good luck...
 

drc_567

Joined Dec 29, 2008
844
... There is a disc like part ... brownish ... labeled MOV1 ... upper right corner of top picture. This would be a transient voltage protection part, if that is the case. Try desoldering that part, and check for high resistance ... if it is good. Possibly another MOV in the same picture, so check that one also.
 

Thread Starter

diebog

Joined Mar 23, 2013
223
Those MOV1 and MOV2 I thought were just for voltage surpression? I didnt replace them. What do these exactly do? Ill do what you said and report back. Thanks guys for the suggestions! And its near impossinble to get this stuff off the board. Its silicone and all the cleaning I did was via plastic scraper. I was able to get the actual schematic for this board and PCB layout/componet values and can post if someone thinks it will help
 

Thread Starter

diebog

Joined Mar 23, 2013
223
So I just removed them and using my Rigol desktop DMM I get no resistance reading on either of them. I have never replaced or delt with these before so I am learning here. Did I find my problem? If so that would be so awesome. So if this MOV was good I would have some sort of resistance reading?
 

drc_567

Joined Dec 29, 2008
844
... My understanding is that a low voltage placed across a varistor, such as a typical test voltage, allows only a very small current to flow through the device, giving a large resistance reading. As the applied voltage exceeds a certain value, there is a clamping effect, which prohibits greater voltage and passes a greater current magnitude. Varistors have a limited life cycle duration and can be damaged by excessive voltage events.
... So ... If you can obtain a similar varistor to the original, make another resistance reading and compare the numbers.
... Not completely sure, but switch test lead polarities and check to see what that value is ... It ought to be the same.
edit: ... some sources say just replace the MOV if there is any question or doubt.
 
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Thread Starter

diebog

Joined Mar 23, 2013
223
I bought the correct potting siicone and if I fix this I plan to repot the PCB. This board had like 1/4" of silcone across all components. Is it possible to remove all the old stuff? I want hoping just to fix the issues and repot.
 

drc_567

Joined Dec 29, 2008
844
... If replacing the faulty MOVs fixes the problem, be sure to keep any high voltage sources ... stun guns, etc. away from the machine. ...
 

jpanhalt

Joined Jan 18, 2008
9,426
I bought the correct potting siicone and if I fix this I plan to repot the PCB. This board had like 1/4" of silcone across all components. Is it possible to remove all the old stuff? I want hoping just to fix the issues and repot.
I have used a mixture of isopropyl alcohol (99%) and mineral spirits (about 2:1) to remove residual silicone from bath fixtures. It is still a physical removal (stiff brush). Nothing actually dissolves silicone, but it can be denatured.

IPA has an amazing ability to loosen stuff. Works great on hot melt too. Do not add mineral spirits for that. It doesn't dissolve the hot melt. Add a couple of drops and you can see it penetrate, then just pull off the hot melt.
 

Thread Starter

diebog

Joined Mar 23, 2013
223
Thanks that's good info! I use 99%IPA to clean flux from pcbs. I did try it on this stuff and it didn't budge. But I didn't try mineral spirits. Good tip!
 

Thread Starter

diebog

Joined Mar 23, 2013
223
Hey guys, so I bought new MOV's and replaced them but still no go. Any other ideas? Its really odd, everything else works on this board but the current control. I thought I have replaced everything that could be suspect but something is eluding me.
 

Thread Starter

diebog

Joined Mar 23, 2013
223
Anyone had a chance to view the schematic? Ive stared at it for along time studying it and not being an engineer, I really don't have a clue how the voltage control works on this. Like I mentioned I replaced the main SRC, I tested the octocoupler with it removed and with diode mode on my Rigol DMM and it seems to work fine. I also replaced the 10k pot. Lincoln has sense had a number of updated versions so my guess is there is something that is prone to failure that they fixed, I asked Lincoln but they wont say. Any suggestions are greatly appreciated. Thanks for your guys time. Jeremy
 

jpanhalt

Joined Jan 18, 2008
9,426
I looked at the schematic. It seems well labeled as for functions. But, we really aren't sure what the problem is. One is tempted to look at the voltage control (left side, just above TRIAC controls).

But I would take a step back to the very beginning of the problem. As background, my MIG welder is a Linde 225 (that was a popular model sold under a variety of names) bought new in about 1983. I do not have a spool-on-gun attachment. It was put into storage about 1993 and resurrected a few years ago. All of the problems encountered doing that were basically mechanical -- dirt, rust, loose connections. The old spool of wire had surface rust. The symptoms were things like a weak arc and slow or inconsistent feed. Exercising switches and controls on the control panel helped. Also, I brought it to life slowly so as to condition/reform the electrolytic capacitors. I replaced nothing. Since your's survived power up, that seems to have been done. I think we worry about those capacitors too much.

With that introduction, is your wire clean and free of rust? How does it feed? Does the feed change as expected? When you release, does the wire stop or does the drive coast. Have you checked all of the connections? Cleaned the drive rollers? Assuming all of those things are OK or done, then I would focus on the voltage control, and in particular the switch unit (cluster 2 and 3).

1590917427395.png

Can you test resistances R129 to R132. Is the switch working? Is R2 what you replaced? Op-Amps are pretty durable, and the 224N (LM224N) is pretty common. I would not replace it without good cause.

If it is not mechanical (I include dirty switches in that category), then it will require some circuit testing of suspect modules/clusters.

John
 

Thread Starter

diebog

Joined Mar 23, 2013
223
So the switch is working, I did replace R2. Wire is clean, this issue actually occurred while I was welding. Had been welding on a project some thin box tubing and it was as if someone cranked the voltage knob all the way down. I thought my friend was playing a trick on me as he has done this before, I looked up and no one was there. I tried to weld again and couldn’t. It just sputters and won’t melt the wire. I called Lincoln and asked them and they had me check voltage of the output from the board and it should have line voltage when trigger is pulled. But no matter where the dial is set there is no voltage preset. Maybe 1-2 volts for a second and then it goes away. The wire was brand new when this happened. All other functions of the welder work, it just can’t produce current from the board to run through the transformer. I have checked the resistors in that area but tested on the board and haven’t actually removed them and checked. Usually when a resistor fails it is noticeable. This board was also potted above all the components so dust can’t really get to anything.



Any suggestions of how to go about testing further? I have all the equipment needed to test components but I lack the knowledge of how this current control works. Also I did replace the 224N. I was ordering from Digikey so I just ordered all the main components in hopes it would fix this issues. But no go still.
 

Thread Starter

diebog

Joined Mar 23, 2013
223
Hey guys. Anyone have any ideas? I haven't spent much time on this as my 17 1/2 year old beagle Diesel just passed away and it's been very hard for me to deal with. Wife and I dont have actual kids but he was like one to us. He went everywhere with me. He helped me through many tuff/hard times in my life and I wouldn't be who I am if it wasn't for him, he will be greatly missed!

Anyway, just looking for something to do to get my mind off things for a bit. If anyone has further suggestions I'd be happy to hear them. Thanks Jeremy
 
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