In article ,
Ron Hardin wrote:

Since Thursday morning I've been hearing 120Hz hum on WFAN (NYC) 660,
does anybody else?

The R8B hears it except with DSB synch detection, so there's some
relation between I and Q channels that cancels the hum.

In article
Ron Hardin wrote:

It's still there at 4:30am EST. It's not audible at all if you're hearing
both sidebands but obvious in quiet phone call pauses (no music, not fan
noise). It's some difference between the upper sideband and lower sideband
that gets modulated by hum; so if you don't hear them equally the hum
starts to creep in, and at some point becomes noticeable. (So propagation
at night can make the hum come and go if you're hearing both sidebands, by
depressing one and raising the other.)

WFAN's transmitter is diplexed into a common antenna with WCBS's (880
kHz) transmitter. Part of the diplexing arrangement includes impedance
matching networks that are supposed to equalize the antenna load
impedance reflected towards the transmitter output over the sideband
bandwidth. I suppose they could be slightly out of adjustment, causing
asymmetrical sideband response. One side effect of asymmetrical response
would be the generation of AM-to-PM.

At the joint transmitter site are older Continental transmitters that
function as backup for the regular Harris DX50 transmitters. Maybe they
have one of the Conti's on occasionally?

Most of these high power transmitters run off of 480V 3-phase power.
Ripple frequency with a straight 3-phase rectifier would be 180 Hz, not
120 Hz. A popular supply arrangement is the 3-phase 6-pulse rectifier
which outputs a 360 Hz ripple frequency. 120 Hz ripple frequency would
most likely originate in either an audio or low level RF stage powered
from a conventional 2-phase rectifier.

I'll take another listen tonight.