: The weird symptom that goes along with the not receiving is the
loss of
: sidetone, when I attempt to xmit.

I'm gettting a bit lost in facts and symptoms here. Anyway, we
must bypass intercom to isolate to which box. If the Narco, it
does not know receive audio from sidetone in the circuit area of
interest, if both those signals are absent.

There's audio leveling circuitry in there to which both mic audio
and receive audio are fed. The nature of that circuit is such that
component drift through heat upsets a delicate balance, and there's
many analog swith ICs in there to turn things on and off. The fact
that you can xmit w/o hearing mic audio I believe may isolate the
fault to a certain area of that circuit.

If true that it works for a while and then stops, that's gotta be
thermal. A wire or coax connection can theoretically behave like
that, but airplanes vibrate. The thermal expansion of metal is
like what in comparison? The only wire connection for your
symptoms should be phone audio out from the comm.

The reason I'm leaning in this direction is that there are simpler
circuits in comm boxes where all your symptoms could not happen.
In the Narco design it can; it's an electronic Rube Goldberg
machine. And the only company on earth who still write code for
the MK3870 CPU chip.

Fred F.