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