[R-390] 60 Hz on Audio

[Jon Schlegel]

Tisha,

Actually I do have the module separated from the receiver.  Here's the setup.
<http://www.vecnav.com/R-390A_IFmodule/IFmodFixture-001-small.jpg>
<http://www.vecnav.com/R-390A_IFmodule/IFmodFixture-002-small.jpg>

This is turning into a mystery.  Here's the rundown so far.

(1)  B+ is clean.

(2)  Tightened chassis ground connections.

(3)  Replacing/Swapping tubes did not help.  60 Hz still present.

(4)  DC Heaters/Filaments eliminates the 60 Hz from recovered audio completely.

(5)  Thinking Heater return current in chassis was inducing hum 
somewhere, I removed heater returns from chassis at each socket, 
wired them together and brought the return out to connect to the 
heater supply externally.  This at least goes part way towards Don's 
twisted pair heater distribution.  To my surprise - NO CHANGE

(6)  Removed V502 and injected 455 kHz to pin 5 (plate) of 
socket.  60 Hz eliminated from recovered audio completely.

(7)  Removed V501 and injected 455 kHz to pin 5 (plate) of 
socket.  60 Hz present with level depending on BW selection.  See my 
post of today at entitled "[R-390] 60 Hz on Audio" at time 0330 EDT 
as an example of what I saw for this test..

(8)  Had previously removed and swept the four filters on a network 
analyzer and found that all showed excellent stopband attenuation.

(9)  Swept the entire module, Input to IF Test Point Output in all 
four mech filter positions and found stopband attenuation to be 
nearly as good as above.

(10)  I Questioned my wiring of the filter compartment, bottom side 
of chassis.  Rewired filter grounding to eliminate possible ground 
loops.  Followed an example photo on Chuck Rippel's page of correct 
wiring arrangement.  60 Hz still present.

(11)  Verified that input and output of unused filters are being 
grounded correctly at each position of the BW switch as Roger 
suggested.  All ok.

This all seems to focus on the mechanical filters but how?  Larry 
pointed out that the filters are magnetic devices but for this kind 
of behavior it would seem the filters would at least need some gross 
spurious response around 60 Hz.  It's hard to picture.  I was certain 
that getting the heater return off of the chassis would do something 
but no change at all.

Regards,
Jon



At 06:25 PM 6/9/2009 -0500, Tisha Hayes wrote:
>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
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