[EdHolland]

Interesting, and similar to an experience I had recently. A while back, I bought a nice Yaesu shortwave receiver. It suffered from terrible hum on some AM stations, but going up in frequency to the shortwave section, this diminished and went away.

I was convinced there must be an issue in the PSU, so replaced the caps, and bridge rectifier. No change. One day I had run the car down into the garage with the radio running, and noticed a hum that had not been present while driving. The garage is right beside my radio room. Eventually, I traced the hum to a USB charger I was using as a power supply for another radio project. Grounding the negative lead of the charger cured the hum - presumably a modulation hum of some kind on both radios.

So, I think you are on the right track with your detective work. With my case, certain stations in the AM broadcast range were affected more than others. I would suggest testing your transmitter at several different frequencies if possible. Also, check general reception of broadcast stations to see if there is any 60 Hz or 120 Hz present - and are any stations affected when the transmitter is running.

Good hunting!

Ed