Mark 12D Issues (Troubleshooting Update)

I had posted an issue I was having, regarding my Narco radios, back
before Osh and got some valuable feedback:

http://groups.google.com/group/rec.a...09e45844bccf31


What I was lacking was some decent troubleshooting! This weekend, I
took my brother-in-law up in the plane and handed him a handheld, and
we tried several different things... I'd like to throw them out to see
if this narrows down the possibilities to those of you with far more
expertise in avionics/electronics than I!

1. We departed our local airport, both of us plugged into the intercom
jacks and I made a radio call (and heard others making calls on the
same frequency). I had side-tone in my transmissions and everything
seemed fine

2. 5 minutes after take-off, I noticed a constant scratchy reception
(continuous static, with something resembling radio calls through the
fuzz)... I tuned to a local ATC frequency and switched over, noticing
no difference in static.

3. I had my passenger plug into the same ATC frequency and indicate
whether or not he was hearing any transmissions between ATC and other
aircraft, as I was hearing static... He was in fact listenting to coms
between ATC and other aircraft (loud and clear)

4. 5 or so minutes after that, there was silence on my radio! I tuned
to the local frequency and tried to make a "radio-check" call, and
there was no side-tone. From my point of view, plugged into the
intercom, I had a completely dead radio (complete silence with no
side-tone on transmission)

5. I had my passenger tune to the same local frequency (on the
hand-held), and performed another "radio-check" call. The passenger
heard me loud and clear, yet I had no side-tone and still had silence
on the radio.

6. I then tuned to the local ATC frequency and had the passenger tune
to the same frequency. I unplugged my headsets from the intercom jacks
and plugged them straight into the aircraft jacks. I had the passenger
give me an indication if he was hearing coms between ATC and other
aircraft. He was hearing coms, yet I was still experiencing silence.

7. I then plugged back into the intercom jacks and made a "radio-check"
call to ATC. My passenger heard my call on the ATC frequency and also
heard ATC respond to me... "LOUD AND CLEAR"!!! ??? I still had
silence, and no side-tone on transmission.

So it appears as though I am transmitting fine (without side-tone
though), but I am receiving nothing. Sometimes everything will work
normally for 30-45 minutes before these symptoms start to appear. The
last few threads of the original post were leading me to believe that
this could be a COAX issue between the radio and the antenna... Does
this still seem like a possibility?

The radio has been run up on a bench for 4-5 hours and supposedly,
everything is within specifications. (I also threw a working radio in
my plane and was able to encounter a failure with it). The antenna is
brand new and was replaced during this troubleshooting process.

Does anybody have an idea of what the issue is? I would really like to
have it narrowed down to one or two possibilities before I go back to
the mechanic and start taking things apart. My semi-shotgun approach
has already cost me a fair amount of money and I am no better off today
than I was when we started this process.

Thanks for listening and thanks in advance for any input!

Best Regards,
Todd