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Old 05-02-2009, 12:53 AM   #1 (permalink)
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Anyone familiar with repairing welders?

I have a Century 130 mig welder, model 117-052. I bought it used from a weld shop a few years back, he said it worked great, he just had too many welders in the shop and wanted to sell off some. I run .030 flux core wire with the polarity set proper for the wire. I have run probably 10 lbs of wire through it the last few years with no problems. I have been welding the kammback/boattail frame out of 1/8 x 5/8 flatbar, so no serious duty cycle, haven't tripped the thermal breaker in years. I plug it direct into my 30A 120V RV outlet, so there should be no voltage drop problems.

The other day it just quit making an arc sometime after I finished a weld. It did not act weird before that, or even when it quit. To my surprise after I set up the next piece, it just would not arc. The fan still runs. The wire still feeds when I push the trigger button. I even tried welding direct to the ground clamp, to rule out the workpiece. You would think it is running normal, till you try and make an arc and there is no spark. I tried fiddling with the dials. I tried both the low amp and high amp setting of the switch. No go.

I used my multitester and both leads have continuity to the tip of the nozzle and through the wire. I checked the nozzle and it is not shorted.

I pulled the fuse and checked that for continuity and that tested good too.

I checked the thermal circuit breaker and it has continuity.

I took the circuit board out tonight and checked it. Right away it did not look very good. One burned up track on the end opposite the big connector. 2 spots where the diodes must have heated up at some time and melted the solder away. 1 resistor that was burned up.

I put in a good resistor of the same value (100 ohm) that I pulled from a scrap circuit board. It appeared to be 1/4 watt, not sure if the ones on the board were 1/4 or 1/8 watt. I resoldered the diodes and cleaned up and soldered the burned track.

Then I put it back together, plugged it in, and turned it on. Before touching anything or pulling the trigger, immediately I heard a small "poof" and smelled a little smoke. And I tried to weld and still no arc, but everything else still works, just like before.

I took the board out again and sure enough, part of the board had blown out again. It was the part where one of the white wires connects to the board. I soldered it, then hooked it back up again except I left the thermal breaker disconnected. And same thing, soon as I turned the welder on it did it again.

A board is like $200 or more, way too pricey, especially since I still don't know if it is the board or not. I wonder if something else could be making the board blow out. If anything else is shorting out, it seems it would blow out at the weakest point which would be the board. I don't know how to test the transformer or capacitor.

Supposedly the most common thing is one of the diodes on the board gone bad, but I don't know my symptoms are in line with bad diodes. If so, I can go through the process of drilling the rivets off, desoldering each one and checking it, then bolting them back to heat sink. I can solder small things, that is not a huge deal, I soldered 3 mpguinos.

Electrical has never been my strong point. I could use a hand if anyone is more familiar with this.

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Old 05-05-2009, 12:54 PM   #2 (permalink)
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Well I took the board out and disconnected and tested everything on it. The board is actually very simple, I could build a whole new one for probably $50. There are 8 diodes and they tested all over the map despite identical part number. 3 seemed ok, 1 tested sometimes ok and sometimes dead, 2 tested dead, 1 tested shorted across, 1 tested shorted across and to ground. After some investigation I finally found them at a reasonable price, they should be here by the end of the week. Then I'm pretty sure I'll be back in business and can get going again on the kammback/boattail.
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Old 05-07-2009, 11:37 PM   #3 (permalink)
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I got my parts today. I bought new diodes and this little optocoupler for another 60 cents since that was the only part on the board I could not test. First I put in the new diodes and tried it. Still no go, but at least it didn't blow the board this time. So then I took it apart again and soldered in the new optocoupler. Put it back together, tried it, and got a good bright arc. Then the thunderstorm started real quick so I took it back in the house. But it seems I have fixed it.
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Old 05-12-2009, 02:15 PM   #4 (permalink)
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Good to hear you got it sorted. We have a little gasless MIG that has the opposite problem -- it still arcs but the wire won't feed...

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