[R-390] 60 Hz Hum Output from R-390A IF Module

[Jon Schlegel]

Hi All,

I've re-capped, tubed, etc the IF module of my 390A.  It seems to 
have a 60 Hz output component greater than I would expect.  Other 
than that it seems to be operating ok as best as I can tell.  Here's 
the basic setup.

SETUP
1) Module operating outside of radio on extender cable.
2) Audio Output taken from "Diode Load" output.  56k ohm to ground to 
simulate Limiter input circuitry.  Monitored with Scope.
3) RF Generator drives transformer whose center tapped output drives 
IF module inputs J513 and J518.
4) Spectrum Analyzer monitoring IF Test Point J514.
5) IF Gain control set at mid-rotation.


And the basic results.

RESULTS
1) With No RF input, audio out appears to be clear of hum.  Expected 
noise present on audio out.
2) As RF input level is increased, hum amplitude seems to increases linearly.
3) Hum amplitude increase stops at 600 mV p-p as module goes into 
gain reduction and carrier meter starts to read upscale.
4) For comparison, 30% modulation @ 1 kHZ provides 8.5 V p-p output 
-- Hum level is only 23 dB below 30% modulate tone.
5) 60 Hz sidebands easily visible at IF Test Point with Spec An.
6) Generator verified clean - no 60 Hz sidebands visible on Spec 
An.  As further verification, 2nd Generator also tried with same results.
7) Swapped the 6BA6's around with spares - no change.  Still need to 
swap 6AK6 V504.  Have no spare for this one.
8) 60 Hz ripple on B+ measured at about 200 mV p-p


OBSERVATIONS, THOUGHTS
I keep searching for ground loops, isolating the radio, test 
equipment, etc but with no real change.  To initially attempt to 
minimize this kind of problem, I peeled the Audio Out and Diode Load 
lines out of the extender cable as they come out of the connector to 
the module.  Since the hum appears present both at Diode Load and IF 
Test Point, it seems that point-of-entry would precede this common 
point.  Note that the Limiter circuit is not in the path.

In regard to ground loops, I can't help thinking about the reactive 
current flowing to ground via the power line input filter.  What part 
might this current play?

Regards,
Jon