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I presume you're referring to field voltage trim resistor R2 (tapped resistor) and not the Fine Current control rheostat R1 (variable resistor).
If it's R2 and not passing current, it could be corrosion under the tap strap which is not unusual. If this is the case, remove the strap and clean any rust off the inside of the tap, surface of the resistor winding and wire connections with sandpaper and reinstall. Adjust the position of the strap to achieve the proper OCV (Open Circuit Voltage) at the weld outputs with the Fine Current control set to 100%. Make any tap adjustment with the unit off.
The part Number is 145 780 which is R2. Appears a section of the resistor is burned out so have rigged it to bypass the burned out section. Not getting good welding performance from the unit. How would I adjust the position of the strap to get the correct OCV please?
The resistor should have two end terminals and the tap strap terminal. Field current obviously only passed thru the unit between the tap and end terminal used.
Where is the bad spot? Can you reconnect to the other end terminal and adjust the tap to approximately the dimension?
To adjust, with the unit not running, position the tap to result in a weld output OCV as close to spec as possible. It may take several adjustments to get it right and check it with the Fine Current control set to 100%.