Hi Mike!
That is a possibility but, unless we're given more information on this power source like a schematic posted, model number and/or serial number of this specific power source as well as what type of power source Ray is even talking about, then we could all drive ourselves crazy guessing for naught!!! :) :) :)
Hey Ray? Could you please give us some more specifics on this old L-Tec power source and if possible, could you post up a schematic of the power source so we could troubleshoot which circuit would interact in such a manner to enable the AC balance control to influence the intensity of the hum...
By doing so, we could then see if there's a component that's normally closed to prevent that from occurring which obviously is now open so that one can effectively alter the intensity of the hum via either increasing or decreasing the resistance, or voltage, or current on that specific circuit which may be one of many, many circuits which make up the power source electronics.
So there's something that has obviously failed for this to occur... The only thing that's left is to isolate the component, or components that are malfunctioning which is the cause of the problem and replace it! Btw, Ray! You didn't answer my question regarding whether or not you're getting the output you should be getting with respect to the power source setting you have chosen to weld with... Could you tell us what your output current is while welding, and is it within reason what you set your power source to?
Respectfully,
Henry