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Carl wrote: “…If I had been right about the hum in my C.Crane FM transmitter being an audio ground loop, then turning the input level control all the way off should close the hum path and bring silence…”
If the source of the hum is in the audio chain and is other than a ground loop then this test will zero out the hum so this doesn’t differentiate between the ground loop and another audio chain source.
A very common source of hum in an FM signal is the RF getting into the power supply and/or the transmitter circuitry. I have ALWAYS had hum with all of my FM transmitters unless they are in a metal box with a ferrite clamp-on around the power cord. This hum is usually, but not always, present when the audio cable is unplugged so this is something to try. This hum would still be present with the tx. audio input gain set to zero.
Let us know what you find so we can learn.
Neil
