craftsman mig troubleshooting

[steven p.]

hello forum members, i have a sears craftsman 85 amp mig welder that im having trouble with. it is model #934.20111 it has 4 heat settings and fan only setting on the rotary switch . when i plug it in and switch it on fan only it seems o k. but when i switch it on a heat setting it hums really bad like it has a dead short. i know this is not the greatest unit out there but ive used it rarely and havent abused it at all if anyone has any tips on what to check and how to go about diagnosing the problems would be appreciated. thank you steven p.

[Sberry]

It probably is shorted, seems like given enough time it would trip a breaker.

[Mike W]

Disconnect one lead on the transformer output. Put an AC voltmeter on the transformer output and see what you read or if you still get a loud hum.

[steven p.]

are you talking about the wire that runs to the mig gun trigger? also do i put the volt meter in line? thanks

[Mike W]

No, the big transformer that gives you the welding current. I assume that is the one with the loud hum. Put the voltmeter across the secondary leads that go to the capacitors and bridge rectifier. Disconnect the lead the goes to the capacitor + side.

[Sully2]

Disconnect one lead on the transformer output. Put an AC voltmeter on the transformer output and see what you read or if you still get a loud hum.

What sort of voltages would a guy be typically looking at??

[hankj]

Steven,

From your posts, I'm guessing you don't have clue one what is what inside the box. Be carefull! The capacitor bank is capable of holding a pretty good charge, and if you become part of the path that shorts it out, theres a nasty surprise in there for you!

If you have doubts, take it to a shop.

Hank

[steven p.]

hello i fired up the welder today with the side panels off and its the big transformer thats doing the humming i think hank has a good point about being careful thank you according to the schematic they call the smaller transformer a reactor.. is that the capasitor? im not in the mood to get knocked on my azz thanks

[Mike W]

The reactor also called an inductor is just a single winding of heavy wire. From the transformer you go to the rectifier diodes to convert AC to DC. Next is the capacitor bank to smooth the pulsating DC. Next is the reactor and out to the gun. The DC no load output on my Hobart can range from 16 volts to 42 volts but I have 27 taps I can select from. :D You may have a bad capacitor but we need some voltage readings to find the exact problem. I hate to be guessing.

[hankj]

hello i fired up the welder today with the side panels off and its the big transformer thats doing the humming i think hank has a good point about being careful thank you according to the schematic they call the smaller transformer a reactor.. is that the capasitor? im not in the mood to get knocked on my azz thanks

The capacitors are large, cylindrical units. There are two connections on each of them.

Hank

[malcolmz57]

this sears 934.20111 was made by century. the century model is 117-050. take the daisy wheels off the rectifier plate and check each diode with a battery and test light. they should flow current one way and not the other. the red band on the diode should face the daisy wheel. i think you will find at least one bad and probably burned. you can remove the bad ones and see if the wire feed will still work. if it will arc at the gun ,but no wire feed, the wire feed switch is fried. parts are avaible at 800-992-9898 and tech service at 866-236-0044 ext 22. they can e-mail a parts sheet and manual

[Some Creep]

Dude, you sure dug up an old post.

You can also do an OCV test to ground. Too much AC means bad rectifier too. Litt'ler bit quicker. I've had button diodes test OK on a meter and fail under load. Took longer finding which ones were the culprits.

[Enga]

Does anyone know the diode p/n?

I picked up a Century 83085 (p/n 117-50) awhile back at a garage sale. It works but the wire feed only starts in just below 5 on the dial and after 7, it is moving too fast for the heat setting. It was not pretty but I was able to do some welding with some patience and technique.

I stuck a volt meter in the wire feed motor quick connects. The wire feed dc motor gets about no voltage until about 4 on the dial, at 4-1/2 on the dial, I measured about 1 volt. After 5, the voltage increases quickly to about 10-14+ volts at 6 on the dial.

I took apart the rectifier plates and checked the diodes with a meter and they passed but I read in the prior post that they can fail under load. Is this a symptom of failing diodes? Or do problems lie elsewhere?

Another problem is on the heat settings, 1-3 work fine but on 4 there is nothing. If I go 1 past 4 (which is unlabeled) and I get another welding setting. Could someone have worked on it before and not put the quick connects back in correct order?

Thanks

- John