Bobcat 250 blowing f1 fuse

[Posty23]

So I thought I had my welder problems all figured out, but when I went to strike an arch and run something off the act power I found both had about 2/3 of the normal output. While running with the settings maxed out I only get 92v at 120 plug and 140v at 250 plug. My welding power seems to be cut in half. Had to crank it all the way up to run a 7018 1/8 rod. Here is what I have checked, double and triple checked: f1 blows on startup, f2 is good, sr2 & sr3 check good, capacitor checks good, resistance between slip rings is about 20ohms, no short to ground either, D4 diode checks good, just replaced reohstat with new cause it was getting worn out. Also unit does not want to go into low idle when switched, don't know if that's in the switch or something else. Any help would be greatly appreciated. This thing is making me pull my hair out!

[Aeronca41]

Please post your serial number.

[Posty23]

Lh005511

[Aeronca41]

Has it ever worked since you replaced the rheostat? Any possibility you might have mixed up wires or gotten one mechanically crimped or caught somewhere that might be shorting out? Not questioning your abilities, but any time I have a problem immediately after replacing a part, i always make sure it wasn't something I did.

There are welder experts here; I'm not one of 'em, just an old retired systems engineer who fixed/fixes a lot of stuff. The fact that F1 is blowing says there is a short somewhere. Only a limited number of parts could be causing that, and I suspect it's one of the parts you think is good. How are you verifying that SR2 & SR3 are good? My money is on a bad SR2; someone with more direct Bobcat experience may have a better guess. I would try disconnecting and tying off/insulating (so they don't touch anything) wires 42 & 110 from SR2 and see if the fuse still blows.

Do do you have a way to verify engine RPM? Does your DMM have the ability to measure frequency? I set my Trailblazer RPM to provide for 62.5 hz per Cruizer's direction; don't know for sure but I assume your Bobcat should be the same. If your meter won't measure freq, a Kill-A-Watt meter is a great investment for anyone working on a welder. It will give you freq as well as voltage and is under $30.

We should be able to get this figured out.

[Posty23]

I work on electric forklifts for a living, and there are a lot of the same type of components. I checked sr2 & sr3 with my meter in diode test mode, same with D4. All checked good. The unit was acting up before I replaced the rheostat, and had all the same symptoms. Don't know if it was blowing f1 before I replaced rheostat only because the fuse that was in there was a white slow blow fuse and I replaced it with a clear glass fuse and could see it light up and blow seconds after starting. I will try removing those wires like you said and see if the fuse blows. Also I don't have a way of measuring rom, my fluke is a basic model. Thanks

[duaneb55]

Resistance across slip rings should be only 3.7ohms +/-10%.

Was your reading with brushes lifted?

[Posty23]

My reading was putting my leads on the wire connection points that go to the brushes. I will take one on the rings with brushes lifted.

[Posty23]

I'm getting 7 ohms across slip rings.

[duaneb55]

Some VOM meters aren't that accurate down low but try cleaning the slip rings with some 320grit sand paper and recheck.

Go to Home Depot, Lowes or Harbor Freight and pick up a Kill-A-Watt meter that you can use to check the frequency of the auxiliary power output. Should have 62-62.5Hz for the correct no-load RPM. Adjust governor speed spring as needed.

[Posty23]

Ok so I went through the wiring again at the diodes and traced all the paths per the wiring diagram. Found that I had actually crossed two wires in the diode blocks, now no more blown fuses. I cleaned the slip rings with coarse then fine grit. One of then was a lot rougher than the other. Once those were good I was getting almost 4 ohms between the two. Then I got a tachometer app on my phone. Measured the engine speed and it was way off. Adjusted to the speed the manual recommends and checked my voltages and low and behold I'm getting my correct voltages at the outlets again! Thanks for all your help, I'm great full that the welding community is willing and able to help others.

[Aeronca41]

Fantastic! Success is good!

[duaneb55]

Awesome job on the troubleshooting and fix.

Thanks for reporting back.


A tachometer app on your phone? Strobe? Have to look into that.

[Posty23]

Yah it's pretty cool.

[ryanjones2150]

What's the app called?

[Posty23]

Strobe tachometer/ timing light.