I'm Going In... Radio Saga Continued...

I've read your symptoms per your orig post, and you have one odd
collection of seemingly mutually-exclusive symptoms. If you can
xmit, but not receive, it's not the coax.

The weird symptom that goes along with the not receiving is the loss of
sidetone, when I attempt to xmit. That's the part that confuses me,
and leads me to question whether I can receive or not... Maybe I am
receiving, but just can't hear it??? I don't completely understand how
the radio, intercom and audio panel all tie together, so I'm confused
at the various symptoms I am encountering. i.e. a) clear side-tone
when talking over the intercom (no depression of the PTT) and able to
talk and listen to passengers. b) obviously clear transmission
(pressing PTT) but no sidetone and passenger doesn't hear me either.
(I say obviously clear because ATC acknowledged my transmission, which
I was able to confirm over a hand-held) c) Not hearing radio calls
(confirmed by listening and hearing them on a hand-held).

Sidetone is a function of the intercom, is it not? Meaning, if I were
to bypass the intercom and plug directly into the aircraft jacks (which
I have tried) I would not hear sidetone, when I attempted to transmit.
Either way, I was unable to hear known ATC coms while plugged directly
into the aircraft jacks.

Scratchiness, followed
by silence, isn't likely the connector, nor coax. Is that what it
still does?

It seems the first time it fails, it tends to fade out with
scratchiness. The last time I flew, I was able to turn the radio off
for a few seconds and turn it back on, to find it working for a minute
or two. This worked a few times, but failed to work, at all, the last
two times I tried it. The real ****er is that once I get it on the
ground, it seems to work fine!

It sounds like a thermal fault in the squelch
circuitry. Narco uses a large fancy squelch circuit, squelching in
2 different ways and works on a hair trigger. Bad thermal behavior
of a component could cause grief. My Narco comm 120 does similar
and obviously a thermal, w/o the annunciating scratchiness.
Similar circuit; no time to pull and fix in such nice weather yet.

I just had the radio in for some questionable repairs at an avionics
shop that is a certified Narco dealer. They ran it up on the bench for
4 or 5 hours, and said that everything was within specifications... Is
this something a normal bench runup would be able to detect?

You can't pull the tray without dealing with the wires at the
connector. If you can do that, you can just visually inspect the
stuff for integrity.

At some point, before my last two flights, I crawled into that
wonderful position with my head between the rudder pedals and reached
my arm up behind the radio to see if I could feel anything out of the
ordinary (like I would know what ordinary felt like!). I grabbed at
the cables and wires and performed a wiggle-and-seat manuver for
everything I could blindly grab... My following two flights, each
nearly two hours, resulted in no radio failures. I was convinced the
problem was gone (okay, I was praying that the problem was gone).
However, the last two times I flew, it was back... This is when I
decided that I might have temporarily fixed something, but that
vibration had caught back up with me and undid what I fixed. My hope
was that removing and cleaning all connection points and making sure
everything is seated snugly when reinstalled, would fix my problem once
and for all. The worst case scenario is $20 of coax cable and pulling
back bloody stumps when I try to retrieve my arms from behind the
panel. If I take it to an avionics shop at this point, I will be
looking at a minimum of $300 to troubleshoot, and I have already dumped
nearly $600 for a questionable radio repair and a new antenna, while
shotgun troubleshooting.

For thermal, did you try flying w/o any box above and below the bad
one? That's how I know my 120's a thermal glitch.

I haven't tried that, but I did fly with a TKM slide-out loaner and
encountered a similar failure. I also put my radio in another plane
and the pilot reported that it did not fail during a nearly 3 hour
flight.

Above my radio is the audio panel, and below it is an ADF. The ADF is
inop, so I could remove it... Does the radio require the audio panel
to be useable in the airplane? Could I pull out the audio panel and
ADF, leave the radio in and plug my headsets into the aircraft (non
intercom) jacks and be able to xmit/receive? I'm willing to try
anything I can, to avoid throwing good money after bad! It would be
one thing if I could explain a set of symptoms to an avionics shop and
get an estimate to put this issue to bed, but I can't reproduce the
issue at will, unless a tech is willing to go flying with me! :-(

Per other post, RG-400 will cure anything, nor do much
performance-wise at VHF.

I don't completely follow this one??? RG-400 is or isn't necessary, as
opposed to RG-58?

Thanks for taking the time to respond!

Todd