[R-390] 60 Hz on Audio

[Tisha Hayes]

Now this is way out in left field:

How about magnetic coupling from the power transformer that is mounted
directly below the IF deck? If you have eliminated all 60 Hz sources on the
IF deck (using DC rectified filaments, etc...) you may be picking up
something inductively coupled.

The aluminum chassis will pass a magnetic field and when the IF deck is
mounted in the chassis it is really in close proximity to the power supply
(less than 1/2 of an inch away). You could have a power supply fault that is
generating a strong 60 Hz magnetic field.

Or maybe there is something in the wiring harness (ground returns that are
shorted together, creating a weird current loop). I have seen this happen in
process control instrumentation and have ended up chasing down ground loop
hum.

You may need to physically separate the IF deck from the radio to see if it
is magnetic coupling. Or you can get a piece of Mu-Metal and put it under
the IF deck to make sure there is not magnetic coupling.

Again, these suggestions are way out in left field but we are talking about
the amplification of extraordinarily low signal levels and trying to hunt
down something that is probably measured in micro-volts at the source.

-- 
Ms. Tisha Hayes

----------------
"I will not recant the truth. I am corn, not chaff; I will not be blown away
with the wind or burst by the flail. I will survive both."
-Walter Milne, 1558