Millermatic 250 wont arc

[kah1130]

Serial #- KG244779
Stock #- 903291
I hope someone can help. I picked up a used Millermatic 250 that was supposed to be in working condition. The M25 gun was a bit ugly with tape on the cable and trigger not fully set into the handle and such but within 10min I had it looking much better and hooked it up to the machine. The cable fed nice and easy and the trigger works to feed the wire. The problem that I am having is that the wire will not arc, just sort of spark a bit. If I drag it across the metal with the trigger depressed it will continue to spark like a sparkler but if left in one spot it just does it once and nothing else.

I have read other posts and been checking to see if I have the correct voltage and the usual places (plg1/2, T1, ext..) and it all seems to check out. Also the DCV across the ground clamp and wire align with the voltage the knob is set to.

I pulled the gun apart a bit more last night and there is a white wire that was cut about a foot from the gun end that I cannot find where it would go to. I have been looking at a Bernard Q300 but if that is not the problem I would rather spend the money to make sure the machine gets up and running. Thank you for any help you can provide.

[duaneb55]

Could be due to any one of the following:

1) Output rectifier temperature switch (TP1) failed open
2) Failed SRCs in weld output rectifier assembly SR1
3) Failed SRC driver circuit on control board PC1
4) Bad connection at a harness or PC1 plug(s)
5) Bad voltage control potentiometer R3
6) ????

[kah1130]

I did get a new TP1 but have not put it in yet.

To test
1) if I unplug RC4 and test across the leads and have voltage the old one is still good.
2) If I unplug RC2 and test across the leads it is still good
3) With RC2 unplugged test slots 2&3 and 1&4 on the board for continuity
4) I will look at the connections again
5) When checking amps across the ground clamp and welding wire I do get the same as the dial is set to

[ccawgc]

You check volts, from work clamp to gun tip or at the polarity studs under the drive.
Load or no load the output should adjust from 10 to 32 volts. Smoothly
gun trigger has 15 vdc, control pot has 10 vdc. wiper should adjust from 0 to 10vdc.
18 vac from negative stud to either SCR input and 36vac across.

[kah1130]

I actually got a new CR1, not a TP1.


Testing from the ground clamp stud to the connection from the transformer to the rectifier I got 30.9vac on the outside connection and 30.5vac on the inside. Across the two I got 61.5vac.

Across the trigger connections I get 17.6vdc and then 0vdc when it is triggered

Across the gun to the ground clamp the vdc matches the voltage setting on the machine but I do not get any amps.

[ccawgc]

Connect your meter to the polarity studs under the drive. and try to weld. See if the voltage stays up or drops. If it drops real low, you have a bad connection inside the welder.
If it stays up, then your miggun may have a loose connection or broken weld lead inside the miggun. Check both ends, see it the weld conductor has been pulled out of the power blocks.

[kah1130]

So it was all the gun. The weld lead was not connected to the working end of the gun. I had to shorten the gun by about a foot but now I am welding. Thank you for the help that you provided.