: 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?

Ah... you gotta be careful when you talk about intercoms. Sometimes they do
some funky relay switching when the XMIT is hit. What it's probably doing (even when
not in pilot ISO mode) is that it's disabling the intercom circuit. At the same time,
it's routing the pilot MIC to the radio, but routing the sidetone (that is generated
by the RADIO, not the intercom) out to all the pax's headsets. ... or at least that's
what's supposed to be happening.

As if that wasn't complicated enough, a flaky relay inside the intercom could
cause all sorts of spastic and seamingly unrelated failures between XMIT and non-XMIT.

: 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.

Not likely given the swap-out... radio is likely blameless.

: "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 audio output path from the radio is usually a "coax"... actually, a small
shielded aircraft wire... definately NOT an antenna-looking RG-58-type wire.

: "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? ;-)

Antenna likely vidicated before.

: 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! ;-)

The bitch of the avionics installs is getting the damn information you need.
With install manuals for everything, it's usually pretty straight-forward. Without
them, you're flying blind most of the time and things get mis-installed.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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