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

[Jon Schlegel]

Hi All,

It turns out that the bulk of the 60 Hz hum is caused by the finite 
ground impedance extender cable itself despite the extender ground 
being a pair of #16 wires.  Not surprising considering 20-20 hind 
site.  The bulk of this appears to be the hum impressed Cathode Bias 
Line.  Grounding this line locally at the IF module drops the hum 
level 10 to 15 dB.  Touching a short piece of heavy ground strap 
between the radio chassis and the IF module reduces the hum further 
so something is still sensitive...

Regards,
Jon


At 09:39 AM 5/2/2009 -0400, Jon Schlegel wrote:
>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
>
>
>______________________________________________________________
>R-390 mailing list
>Home: http://mailman.qth.net/mailman/listinfo/r-390
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:R-390 at mailman.qth.net
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html