edechazal@comcast.net wrote:
> I've become irritated with the crackling noise my transponder (King 76C)...

 Ed,
Some quick thoughts:
1)Make sure its not direct RF by first putting a 50 ohm load instead of
the blade antenna on the end of the coax.  At that point there should be
no TX pulses generated and no VSWR on the coax to couple into other
equipment.
2) You mention the ground plane for the blade antenna.  Even if the
ground plane was bad and causing VSWR, you shouldn't hear "crackling."
As the transponder gets interrogated, there would be a buzz, which stops
while the radar antenna scans the rest of the circle.  So the buzz would
repeat every 5 seconds or so in the case of longer distance radar.
Airport surveillance radars would repeat faster.
3) Maybe the transponder is randomly self-triggering at a high rate
which then causes it to draw current on the battery bus, which might get
through the audio amplifiers.
4) I'm not familiar with the King 76C, but if it's a tube type, then
there is a high voltage supply, in the multi-hundreds of volts.  If
there is a leakage path externally on the Hi-V bus, or if a filter
capacitor is breaking down, the "sparking" would give rise to wide band
interference that might be picked up by audio circuits or even in the RF
receivers.  
5) If you tap the panel, does the crackling change, i.e., is it
microphonic at all?

As a troubleshooting technique, take a small, cheap, plastic case
(non-shielded in other words) AM radio, tune it to an empty channel, and
hold it near the King when it's on.  If the AM receiver hears the same
noise, you're radiating it from the King.  You could check the RF/rx
amplifier path by turning off the NAV/COMs and see if the crackling is
still there.  Also turning off the intercom amp would be another point
to check.

Regards,
Charles R. Patton