Thanks Tauno!

The intercom and sidetone go different ways:

- the intercom connects the amplified mic signal directly to the
all the headphones,


- the sidetone is created inside the COM radio by receiving
the transmitted signal with a simple detector and feeding
the signal to the headphone line.

So, feeding the signal to the headphone line would automatically send
the signal to all headphones on the intercom unless the intercom was in
Pilot ISO mode, if I understand correctly? I'm assuming this, since
the passengers can typically hear the radio calls I make. When I lose
sidetone (and reception), the passengers can't hear my xmissions
either, indicating, based on your response, that the signal is never
being fed out of the COM radio (or possibly the signal isn't even
getting to the COM radio). Am I catching on?

Assuming that the radio behaves in another plane, I see the
possible causes:

- the power supply to the radio is flaky,
- the microphone signal path to the radio is flaky,
- the audio output path from the radio is flaky, or
- the antenna connection is broken / shorted.

The simultaneous loss of reception and sidetone kind of
drop the microphone from the cause list above, the others
are still relevant.

If the power supply to the radio was an issue, would that be evident in
the radio itself, such as led's dimming or loss of power? I never seem
to notice a visible issue with power, and if it's relevant, the NAV
side of the radio never seems to fail at all. I always have constant
indication of a tuned in VOR, regardless of issues I am having on the
COM side.

This loss of reception and sidetone is ALWAYS simultaneous, so I'll
skip the microphone issue.

"The audio output path from the radio"... Would this be a coax cable
from the radio to the intercom? I recall seeing a box (2"x4" or so),
separate from the intercom, with coax from the radio to it... I tried
to trace where the coax went from there, but it seemed to be buried
deep inside a nest of other wires, so I gave up on trying to follow it.
If the radio behaves in another plane, I am assuming you are talking
about an audio output path that is not a part of the radio. So...
somewhere between the radio and the intercom then???

"The antenna connection is broken/shorted" - I'm a little confused at
the likelyhood of this, because what I seem to be hearing in this and
previous posts is that the loss of sidetone likely rules out a
connection between the radio and the antenna. Maybe I'm not catching
on? ;-)

Slowly but surely, I think I'm getting it, but I still think one of you
guys could make a lot of money on an Avionics for Dummies book! I'd
buy it! ;-)

Thanks for your feedback!
Todd