1996 Miller Bobcat 225G Plus - Serial # KG052667. Should I have a full 120 volts at the aux power receptacles when the engine is at fast idle?

At slow idle I have about 30 A/C volts and at high idle I have about 90 A/C volts. Both idles are set correctly (per rpm - I can't measure freq.) and the fine tune control is at "10". Adding an aux A/C load does not change the voltage output. My concern is, if I don't have full volts at aux power, I won't have full volts in the welding leads?

If those voltages are low (and I suspect they are), what would cause this? And how do I test/fix it?

Any help is greatly appreciated.