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

three-eight-hotel wrote:
Thanks Ross,

With all of the symptoms I have been having and troubleshooting reports
I have provided, I'm surprised anyone can follow these posts, because
I'm having a hard enough time myself! ;-)

In a previous post, I reported that I had swapped out my radio with a
known working TKM replacement radio, and encountered a failure with
that radio as well. I also let my instructor throw my radio in another
plane, for a lesson she was giving, and she reported back that my radio
worked the whole time. I also took the radio in to a Narco dealer and
they ran it up on the bench for several hours and said that everything
was within specifications. This, however, after they made a
questionable repair only weeks earlier!

Based on the TKM's failing in my plane, and my radio working in another
plane, it seems unlikely that it is in the radio, but not impossible.
I could easily be experiencing one set of coincidences after another!

"The side tone is a product of the radio not the audio panel",
perplexes me though... Reception loss and sidetone loss at the same
time is the current prevolent symptom. At the occurence of failure, I
have reception loss and sidetone loss, I do have sidetone going
through the intercom though, as I can communicate with the passengers
fine. It is only on xmit where I lose sidetone... Plugging into the
aircraft jacks directly doesn't help because if I understand correctly,
you wouldn't get sidetone there anyways. Sidetone in that respect is a
product of the intercom, is it not? So... there are three components
(radio, intercom, audio panel) that are all interconnected somehow, and
that's where my eye's glaze over and I start to drool... I just don't
get it???

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.

If the sidetone is missing, either the audio path is broken or
the radio does not transmit properly. There is a slight possibility
that the sidetone receiving circuit is the culprit, but it's so
simple that the probability is tiny.

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.

HTH

--

Tauno Voipio (OH-PYM)
tauno voipio (at) iki fi

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

: 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 *
************************************************** ***********************

"Tauno Voipio" wrote:
...
If the sidetone is missing, either the audio path is
broken or the radio does not transmit properly. There
is a slight possibility that the sidetone receiving circuit is
the culprit, but it's so simple that the probability is tiny.

In the current Narco digital stuff, it isn't so simple at all. Can
be cantankerous.

Assuming that the radio behaves in another plane,

I wish I were clear on this, as I thought it works for a while then
doesn't. If heat is causing that, then no assumption can be made
that the stack gets as hot in the other plane. The probable
circuit fault if such the case draws zip, so it doesn't heat up.
It would be other heat, from within and/or external to the box.

Reg,
Fred F.

It is a long shot but consider the possibility that spilled defroster
heat is cooking your radio stack. We had a Narco Com that went south
everytime after a few minutues in flight. We finally found it by using
a shop vac to blow air in the defroster duct & looked for an air leak
under the panel. Sure enough it blew air directly on the radio. Duct
tape etc fixed it.

"Tauno Voipio" wrote in message
...

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.

That is not true. THe sidetone is generated by taking a sample of the
modulator's output. It is not an RF sniffer.




If the sidetone is missing, either the audio path is broken or
the radio does not transmit properly. There is a slight possibility
that the sidetone receiving circuit is the culprit, but it's so
simple that the probability is tiny.

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

- the power supply to the radio is flaky,

Possible, but if the radio worked fine in another aircraft, it would have
been flaky there also.

- the microphone signal path to the radio is flaky,

Would not explain loss of received signal.


- the audio output path from the radio is flaky,

Most probable.



or
- the antenna connection is broken / shorted.

Would not delete sidetone. I seem to recall in another post that a handheld
at a distance got carrier and not audio. If an antenna transmits, it
listens as well.

Jim