Ron Hardin wrote:

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



Maybe their cooling Wfans on the transmitter are noisy? ;-)

Maybe their cooling Wfans on the transmitter are noisy? ;-)

Or maybe their transformerless "All-American Five" transmitter has its
line cord plugged in the wrong way! :-)

(Casting an appreciative glance towards my Hallicrafters S-38C....)

In article ,
Robert Casey wrote:

Ron Hardin wrote:

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



Maybe their cooling Wfans on the transmitter are noisy? ;-)


When I tuned in tonight (Monday evening 1/13/04), I noticed an apparent
decrease in the "hum" level. At least the 120 Hz component of it appears
to have become attenuated and what I hear now are the upper remaining
harmonics - sounds sort of like a little roto rooter whirring in the
background.

I was having a good deal of difficulty hearing the hum during my
previous post last week when I identified the hum frequency as 125 Hz.
There was a Patriots football game in progress and there were precious
few moments to hear any hum at all (the Pats won the game and I lost the
hum). So I guess I was "out of bounds" on my frequency estimate and will
concede to Ron's measurements were he has identified the hum as being
composed of numerous 60 Hz harmonics. That whole bit about the stereo
exciter pilot tone was a bit of a stretch, anyway.

The spec sheet for the Harris 3DX-50 transmitter identifies the prime
power supply as a 12-pulse SCR controlled rectifier. This is exactly the
type of rectifier that is capable of generating the large number of 60
Hz harmonics noted in Ron's measurements. If the 3-phase ac input power
becomes unbalanced, then lower frequency 60 Hz harmonics will appear in
the ripple spectrum. There is no spec for incidental PM due to power
supply ripple, but the IQM spec for 95% 1 kHz modulation is -36 dB,
which is a lot higher than I would have expected, indicating that the
potential for phase mod definitely exists. Granted, they are using more
vintage DX-50s than the quoted model, but I would assume that the power
supply configurations are similar in nature.

I've listened to WCBS on 880 kHz (LSB), and don't hear any hum at all.
They are colocated with WFAN and are both using the same ac power line
feed, so why hum happens on transmitter and not the other is a mystery
to me.

T. Brown