Sidetone is, by definition, NOT the audio you hear when you receive. That's
the "received audio." Sidetone is you getting to hear yourself when you transmit.

The "sidetone" there is actually just the intercom functioning normally.

That all pretty much makes sense to me...

So it has always transmitted properly?

I'm pretty sure it has... I recall one of my first failures, while
shooting ILS approaches at SAC. After minutes of trying to figure out
why ATC wasn't responding to me, I grabbed my handheld to find out that
they had been hearing me all along. However, at that time, I was
hearing sidetone while xmitting, just not hearing audio back.

Here's something to think about based on my experience. Before we finished
the interior, we had a couple of intercom jacks in the back seats just zip-tied up,
but with bare 0.250/0.208 jacks wired in. Every once in awhile, they would move so
that in-flight the jacks would touch something aluminum. That would cause a really
ugly static, intermittent signal on the intercom if it hit a MIC wire. Between that
and the possibility of the intercom sharing multiple mics together, see if that adds
something to your debug equation.

I'm considering opening all of the intercom jacks (two in front and two
in back) and visually inspecting the connectors and wires there too,
just for grins. I'll pull new wire for them, if I have to as well...

"Fade out with scratchiness" sounds like it could be internal to the radio.

That's a tough one, because I don't even know if I'm describing this
symptom well... It's very quick and is simply an indication that the
failure is occurring. It's not a long drawn out fade, and the
scratchiness is really more of a quick (but barely audible) static that
I hear, when the radio fails.

Neither the thermal stresses nor the vibration are adequately simulated by
letting the thing warm on the bench for 5 hours. Do you have active cooling and is it
in good shape? i.e. a fan or ram-air blowing on it?

Not that I am aware of???

I hear you. Troubleshooting intermittent problems sucks. Another possibility
is wrong spacers on one or more trays. My mechanic had troubles with his transponder
and intermittent connections on an encoder line or two. After a bunch of rewires,
checking, etc, we discovered that some small nuts/washers were installed on the wrong
side of the connector on the back of the tray. It was preventing the connector from
seating an additional 1/8" and causing intermittent failures on a few pins.

That 1/8" issue is one of the top things in my mind... I'm thinking
the whole exercise of pulling, cleaning and reinstalling may enable me
to get a proper seating, that may not be occurring now???

From that right there, it sounds like you can rule out your radio. Don't
discount multiple *different* failures in the debug equation. Maybe it's your audio
panel or intercom?

Possibly???

RG-400 shouldn't really be necessary at VHF frequencies. The materials are
better, and it has better high-frequency characteristics (e.g. transponder or DME).
At VHF it primarily serves to be more of a PITA to work with (much more rigid). It
also screwed with my compass once because it had a copper/tin-clad steel center
conductor.

That's good to know... The RG-58 is much cheaper!

Thanks to you, and to all for your input! I feel like Chevy Chase, in
Fletch, when I'm looking at this stuff! I'm pretty darn sure the whole
problem is in the Fetzer Valve... Now if I can just round up some
gauze and some ball-bearings, I think I might be able to get this
problem licked! ;-) Ohhh, and I'll have to clean the windows too,
because they've got "filth muck" all over them!

Todd