Miller econotig

[Spencer15]

I have a miller econotig from 1995. As soon as you touch the pedal, it’s full amperage. No control at all with foot pedal. Tested foot pedal, and getting 1k ohms released, and 1 ohm fully pressed down. And it moves smoothly through the range. Replaced sup1 and sup2, still does the same exact thing. Disconnect rc4 or rc5 and it welds at the minimum amperage. No idea where to go from here

[Franz©]

Check the panel/remote switch and verify the switch is functioning as it should.

[Spencer15]

Check the panel/remote switch and verify the switch is functioning as it should.

Seems to be functioning properly. Flip it to gtaw and the foot pedal works. Flip it the other way and scratch start tig works. I’ll try stick welding with it tomorrow and see if I have amperage control with the knob

[jjohn76]

Did you replace SUP1 and SUP2 because the board showed short circuit across RC4 and RC5 without the shunt winding connected?

[Spencer15]

Did you replace SUP1 and SUP2 because the board showed short circuit across RC4 and RC5 without the shunt winding connected?

Replaced them based on research on the no amperage control concerns most people have. Measure between rc4 and rc5 and getting OL for resistance.

[jjohn76]

Ok, since it's open loop, your shunt drive SCR (Q13) is probably still good. It looks like optocoupler OC2 should also be ok, or you would see some resistance across RC4 and RC5. What resistance do you measure from RC9-4 to board ground (checks if Q12 is ok)?

[jjohn76]

Another possibility is C35 could be shorting out at voltages higher than your multimeter uses to measure resistance. Either way, it seems either your shunt driver circuit is shorting out without being triggered, or your trigger circuit is turning on Q13 (shunt SCR) all the time.

[Spencer15]

Ok, since it's open loop, your shunt drive SCR (Q13) is probably still good. It looks like optocoupler OC2 should also be ok, or you would see some resistance across RC4 and RC5. What resistance do you measure from RC9-4 to board ground (checks if Q12 is ok)?

From rc9-4 to board ground, showing OL

[jjohn76]

Ok, so it doesn't seem like any of the trigger transistors are blown. The reset transistor (Q10) seems to be ok (showing OL). Does your machine work properly in SMAW?

[Spencer15]

Ok, so it doesn't seem like any of the trigger transistors are blown. The reset transistor (Q10) seems to be ok (showing OL). Does your machine work properly in SMAW?

Full power no matter what the amperage knob is put on when trying to stick weld

[jjohn76]

Ok, there is a voltage pinout setting in the tech manual, have you measured those voltages against the specs listed in the technical manual?