[R-390] 60 Hz on Audio

[2002tii]

Jon wrote:

>WHAT WOULD REALLY HELP is to be able to correlate the 60 Hz hum
>levels I'm seeing with how other receivers perform.  I'm measuring 60
>Hz levels from 30 to 55 dB below a 30% modulated tone at the diode
>load output of the IF module, depending on mech filter
>selection.  This is a relative measurement that ideally should
>translate to the same measurement anywhere along the audio chain
>regardless of RF/IF path gain, etc.  If anybody has this kind of
>info, I'd be very appreciative to receive it.

Later, Jon wrote:

>Is this kind of behavior acceptable for these receivers?

Acceptable, no.  But I wouldn't be surprised to find that a lot of 
them work similarly.  But not all of them -- my 390A (missing name 
plate; I've never cataloged the provenance of the modules) has hum 
down more than 70 dB below a 30% modulated tone at the diode load 
output of the IF module, essentially independent of filter 
selection.  I can easily see the 60 Hz in your "16 kHz filter" photo, 
but not at my diode load output.

>If not acceptable, what could be causing the distinct difference of
>60 Hz level just by changing the BW setting?

Typically, this would be caused either by a ground loop or induced 
hum.  Ground loops are caused by 60 Hz current being drawn through a 
"ground" wire (or section of chassis, etc.) past a point where the 
input of a low-level stage is grounded (so the 60 Hz voltage 
developed across the finite "ground" resistance appears in series 
with the input to the low-level stage).  Induced hum is magnetically 
or capacitively coupled (yours looks like pretty pure 60 Hz, so I'd 
expect magnetic coupling) from some wire carrying 60 Hz (i.e., a 
heater wire) to another wire or circuit node.  I thought your problem 
might turn out to be the power transformer field inducing hum into 
the filters or their associated wiring, but using the test fixture 
has changed the relationship between the module and the power 
transformer, with no apparent change in the hum, so that isn't likely.

Ideally, all heater current would be carried in twisted pairs and no 
heater current would be returned via ground.  In this regard, the 
390A departs from good engineering practice.

Best regards,

Don