Total posts : 45366
Good question, Carl, and I don’t know if I have a good answer regarding the FM situation since the problem could have been with something other than the RF getting back into the transmitter. It has been reported by some that wall warts and chargers can pick up the carrier and modulate and re-radiate it. This is consistent with moving the wires around to reduce the hum.
The transmitter was mounted in the metal box for shielding but, in general, a metal box is not as good a shield as it would appear. I learned this the hard way when I built a 2 meter repeater. I tested the shielding by keying my handheld next to the receiver box with the antenna shorted and it heard the signal loud and clear despite being in a closed metal box. Effective shielding required feed thru capacitors and silver plated braided “gaskets” at all joints with screws every inch along the covers. I finally achieved good shielding but it wasn’t easy.
Fortunately this effort is not needed for part 15 FM because of the low power involved but the first thing I would do (and did) with a hum problem is put the transmitter in a metal box.