[Mondial]

John, have you tried grounding the chassis with a separate wire to the electrical outlet ground? With the line bypass cap connected to the hot side of the line and no external ground connection, the chassis is floating at 120 V AC. This can cause all sorts of hum effects.

You can try making an adapter so you can reverse the plug polarity and see if it makes a difference. In general, if you are using only one line cap and no external ground, the cap should be connected between chassis and neutral, so the chassis is at AC and RF ground potential.