Sealey mightymig 150 keeps blowing the 13A plug fuse

[disco_monkey79]

Hi, please may I ask your advice. I bought this machine a couple of years ago and have been using it regularly for car body repairs.

All was fine until yesterday when the fuse in the plug blew. Changed it, and as soon as I pulled the trigger it blew again.

A quick Google suggested crud in the shroud causing a short. I had a replacement torch so replaced the whole thing (torch, tip, shroud - the lot).

Put it all back together, pulled the trigger and... the fuse blew.

So as it has a brand new torch that has done zero work, can anyone suggest an alternate culprit?

Many thanks

 

[BrokenBiker]

They are pretty simple machines but sounds like and internal fault so whipping the case off and sticking some pictures up may help.

 

[eddie49]

Does the plug fuse blow when pulling the trigger at all settings of the power select switches?

The most common reason for a fault like this is a short-circuit diode in the bridge rectifier.
You could check this by unbolting one of the two connections between the heavy solid wires of the main transformer secondary and the input tags of the finned aluminium rectifier. Open the bolt and move the wire away so that it is not touching anything, then power-up and pull the trigger. The welder should come on, providing low-voltage AC out of the transformer but not feeding the rectifier. It will not weld and there will be no wirefeed, but if the fuse does not blow it indicates that the rectifier has failed.

 

[BrokenBiker]

Does the plug fuse blow when pulling the trigger at all settings of the power select switches?

The most common reason for a fault like this is a short-circuit diode in the bridge rectifier.
You could check this by unbolting one of the two connections between the heavy solid wires of the main transformer secondary and the input tags of the finned aluminium rectifier. Open the bolt and move the wire away so that it is not touching anything, then power-up and pull the trigger. The welder should come on, providing low-voltage AC out of the transformer but not feeding the rectifier. It will not weld and there will be no wirefeed, but if the fuse does not blow it indicates that the rectifier has failed.

That sounds similar to what my mig did yesterday...cooling fan works but no feed or arc...but mine wasn't blowing fuses

 

[disco_monkey79]

Thanks. I need to get some more fuses, and i will test the above.

I didn't try changing the power setting, so your suggestion may well be correct.

Thanks again

 

[disco_monkey79]

Just to check (pic 1) this is what you meant me to do? If so then same result - fuse pops as soon as the trigger is pulled.

I can't see anything obviously wrong. One of the contacts on the main power switch was a bit loose, but tightening it hasn't changed anything.

Thanks

 

[eddie49]

I think that the open connection at the rectifier is the correct one....

There will be two AC input wires coming up from the low-voltage secondary of the main transformer - the lower and larger laminated metal-cored item. It looks like this winding is flat aluminium or copper strip. One end needs to be opened, and I think you have done that.

One of the two DC outputs of the rectifier goes to the inductor or choke - the smaller laminated item, on top of the main transformer. I think that the winding is round wire. The inductor only has one winding, with the input coming from the rectifier and the output going either to the work return cable ( "Earth" clamp ) or to the MIG torch, via the "Polarity Swap" terminals.
The second DC output from the rectifier goes straight to the "Polarity Swap" terminals, and then on to the torch or work clamp.

If you could trace these wires at the rectifier then it would confirm that you have opened the correct connection, and so the fault is not due to a shorted rectifier. If so, it could be the primary of the main transformer, which would be very rare, and uneconomic to repair.

I have not found a circuit diagram for the Mightymig 150, but maybe the gas solenoid valve is also a 230v mains-operated component, and it could be shorted. The valve may be labelled with it's operating voltage. You could try pulling off one of the two power leads at the valve and check if the fuse blows.

 

[Hutcho]

You said you replaced the torch ? Wa sit like for like - was they same amount of wires as the old one

 

[disco_monkey79]

I think that the open connection at the rectifier is the correct one....

There will be two AC input wires coming up from the low-voltage secondary of the main transformer - the lower and larger laminated metal-cored item. It looks like this winding is flat aluminium or copper strip. One end needs to be opened, and I think you have done that.

One of the two DC outputs of the rectifier goes to the inductor or choke - the smaller laminated item, on top of the main transformer. I think that the winding is round wire. The inductor only has one winding, with the input coming from the rectifier and the output going either to the work return cable ( "Earth" clamp ) or to the MIG torch, via the "Polarity Swap" terminals.
The second DC output from the rectifier goes straight to the "Polarity Swap" terminals, and then on to the torch or work clamp.

If you could trace these wires at the rectifier then it would confirm that you have opened the correct connection, and so the fault is not due to a shorted rectifier. If so, it could be the primary of the main transformer, which would be very rare, and uneconomic to repair.

I have not found a circuit diagram for the Mightymig 150, but maybe the gas solenoid valve is also a 230v mains-operated component, and it could be shorted. The valve may be labelled with it's operating voltage. You could try pulling off one of the two power leads at the valve and check if the fuse blows.

Thanks for the very detailed reply - i will have another look tomorrow

 

[disco_monkey79]

You said you replaced the torch ? Wa sit like for like - was they same amount of wires as the old one

Yes, just two wires to close the circuit when the trigger is pulled. I had to solder on the connector but other than that it was like for like. Thanks

 

[minimutly]

You need to get a multimeter on the individual diodes in the rectifier, sounds like one, or more likely two are short circuit. If your lucky they can be replaced.
Edit to add - simple test is to disconnect the big wires (only one of them) that feed the rectifier, if it then doesn't blow the fuse it will be the rectifier.

 

[disco_monkey79]

Hi,

Yes, I disconnected a wire to the rectifier, and it continued to pop fuses. I can't find a short anywhere else. I have therefore ordered a replacement machine. A shame, but there it is.

Thanks all for yuor help