Shopmaster blows breaker MIG only

[sajurcaju]

I have a Miller Shopmaster 300, s/n KD408963. My manual is January 1993.

It has worked fine until recently. Now it will sometimes mig weld, sometimes it pops the breaker. The breaker pops right when I pull the trigger, before the wire strikes an arc. I looked at my liner, it had a couple of minor kinks so I replaced it, no change in symptoms. I was welding 1/8" steel, it will weld fine for a bit, pause, pull the trigger again and the breaker pops.

Stick welding (reverse polarity, 170A) works fine.

I am using a 50A breaker which has always worked for me. I changed to the 50A circuit for my plasma cutter (different breaker), same problem.

Advice will be greatly appreciated.

Steve

[cruizer]

Failed shorted capacitor on rail, replace em all

[sajurcaju]

You're talking about the ten caps below the fan? 30,000uF at 50V?

They read like caps on my meter, but that's only a few volts. They aren't cheap, is there some way to confirm this is the problem?

Thanks,
Steve

[cruizer]

Probably going to find that one is loose and blown up. Though if one is loose, most of the others are loose as well. You can mark the +& - on the rails and meter (capacitance) on each one. Change out the ones that fail, but rest assured the ones you didn't replace will fail in a short time frame. Don't nessessarily have to buy them from Miller. Just have to buy the same style and same specs

[sajurcaju]

They all look fine, all are tight. I realize that's not saying a lot, I'm just trying to take this one step further before plunking down $300 for caps.

Steve

[cruizer]

Still gotta pull and test them individually. not the rectifier to cause problems.

Something surges, likely a cap. could be a failed gun as well. Trigger to conduit short may cause a problem

[sajurcaju]

I don't follow what you mean by "trigger to conduit short". Are you saying the trigger wires could be shorting inside the gun/cable?

I did take all the caps out (after marking the bus bars for +/-) and test them individually. I hooked them up to a 40V power supply (all I had) with 50 Ohm series resistor. I turned on the power and watched the voltage climb. All caps took about 10 seconds to reach 38V, which is pretty close.

Next I could try putting them in the machine one at a time. I don't need to weld for the problem to occur, just pulling the trigger is enough.

Steve

[cruizer]

don't know about the way you check capacitance, also don't know if there is a heat problem with one of your caps. a capacitance test would tell me everything I need to know. Now there are 2 trigger wires in the gun cable sheath and likely one spare. You will want to test them against the copper current carrying conduit. If one is shorted it may cause a surge

[sajurcaju]

The test I did checked the capacitance. I set up an R*C circuit, 50 Ohms and 30,000uF gives 1.5 seconds. I watched each cap charge for 10 seconds; they all got to about 38V (40V supply). I think they are all ok at 40V anyway.

I don't think temperature comes into the situation, it will pop the breaker on the first attempt some times, stone cold welder.

I also swapped the trigger wires to the spares. This did not help, unfortunately.

Steve

[cruizer]

Ok, remove the feeder connector, and with a piece of mig wire jumper the amphenol on the machine a & b. Does the breaker blow?

[sajurcaju]

I pulled the connector to the gun cable and triggered it with a piece of wire. Popped the breaker first try. I guess that eliminates the gun assembly. Is there anything else besides the caps?

Steve

[cruizer]

NO, I said the MACHINE, ie the shop master There is no A & B on the trigger terminals...

[sajurcaju]

Maybe I wasn't clear.
I pulled the connecter that goes from the wirefeeder to the machine end of the gun assembly. The connector is marked Tweco, not Amphenol. It has no A/B labels.

I shorted the wirefeeder end of the connector, leaving the gun assembly end open. This blew the breaker.

If this is not what you meant, I need a little more detail, like where the connector is located.

Steve

[cruizer]

And the control cable that links the feeder to the machine? If you were to remove that from the machine and jumper A & B. We can eliminate the feeder as being the problem.

[sajurcaju]

When I jumper A and B on the machine side of the connector, nothing happens. Breaker doesn't pop, no wire feed. Should the wire feed?

Steve