[ARC5] Need Help with Variac Diagnosis

[Phillip Carpenter]

I have a General Radio W20MT3 autotransformer, this one handles 20 amps, or about 3,000 watts. It came without meters so I've added a Simpson AC voltmeter in parallel on the load side and a matching Simpson Ammeter in series with the reset beaker on the load side.

I'm having two issues: 1) the unit trips my 20A house breaker at no load. I believe this is caused by inrush current so I have ordered a 20A inrush current limiter to help solve that problem. 2) originally the voltmeter read 120 volts when the Autotransformer was rotated full, but now it only reads 117 volts. If I switch the output to 140 volts the voltmeter only reads 134 volts. Something has started acting like a resistor and dropping the voltage.

Does anyone have an idea what could be causing this? I plan to use this Autotransformer to drive my tube radios, such as the ARC-5s, as well as my Hallicrafters SR-2000 ( which is why I have the 20A version). The AC voltage here runs 120-121 volts, so I want to roll it back to 115-117 volts.

I can't figure why the output voltage dropped below 120 or 140, depending on the winding setting. Could there be some metal shaving has fallen into the winding that is shorting it?

Any thoughts from those with experience using a Variac is very much appreciated!

Phil

Sent from my iPhone