three-eight-hotel wrote:
: The weird symptom that goes along with the not receiving is the loss of
: sidetone, when I attempt to xmit. That's the part that confuses me,
: and leads me to question whether I can receive or not... Maybe I am
: receiving, but just can't hear it???

Sidetone is, by definition, NOT the audio you hear when you receive. That's
the "received audio." Sidetone is you getting to hear yourself when you transmit.

I don't completely understand how
: the radio, intercom and audio panel all tie together, so I'm confused
: at the various symptoms I am encountering. i.e. a) clear side-tone
: when talking over the intercom (no depression of the PTT) and able to
: talk and listen to passengers.

The "sidetone" there is actually just the intercom functioning normally.

b) obviously clear transmission
: (pressing PTT) but no sidetone and passenger doesn't hear me either.
: (I say obviously clear because ATC acknowledged my transmission, which
: I was able to confirm over a hand-held)

So it has always transmitted properly?

c) Not hearing radio calls
: (confirmed by listening and hearing them on a hand-held).

: Sidetone is a function of the intercom, is it not? Meaning, if I were
: to bypass the intercom and plug directly into the aircraft jacks (which
: I have tried) I would not hear sidetone, when I attempted to transmit.
: Either way, I was unable to hear known ATC coms while plugged directly
: into the aircraft jacks.

Sidetone can be done in the intercom, the radio, or both. All it's doing is
looping back your mic signal into your own headset when the transmit button is down.
It just depends on how far up the chain you go before it loops back. When I installed
my intercom and coms, the intercom was initially wired to produce the sidetone.
I reconfigured the setup so that each radio provided its own sidetone. That way I
could ensure that all the signals were going all the way to the radios (and back to
me).

Here's something to think about based on my experience. Before we finished
the interior, we had a couple of intercom jacks in the back seats just zip-tied up,
but with bare 0.250/0.208 jacks wired in. Every once in awhile, they would move so
that in-flight the jacks would touch something aluminum. That would cause a really
ugly static, intermittent signal on the intercom if it hit a MIC wire. Between that
and the possibility of the intercom sharing multiple mics together, see if that adds
something to your debug equation.


: It seems the first time it fails, it tends to fade out with
: scratchiness. The last time I flew, I was able to turn the radio off
: for a few seconds and turn it back on, to find it working for a minute
: or two. This worked a few times, but failed to work, at all, the last
: two times I tried it. The real ****er is that once I get it on the
: ground, it seems to work fine!

"Fade out with scratchiness" sounds like it could be internal to the radio.

: It sounds like a thermal fault in the squelch
: circuitry. Narco uses a large fancy squelch circuit, squelching in
: 2 different ways and works on a hair trigger. Bad thermal behavior
: of a component could cause grief. My Narco comm 120 does similar
: and obviously a thermal, w/o the annunciating scratchiness.
: Similar circuit; no time to pull and fix in such nice weather yet.

: I just had the radio in for some questionable repairs at an avionics
: shop that is a certified Narco dealer. They ran it up on the bench for
: 4 or 5 hours, and said that everything was within specifications... Is
: this something a normal bench runup would be able to detect?

Neither the thermal stresses nor the vibration are adequately simulated by
letting the thing warm on the bench for 5 hours. Do you have active cooling and is it
in good shape? i.e. a fan or ram-air blowing on it?

Just a thought... not too likely.

: At some point, before my last two flights, I crawled into that
: wonderful position with my head between the rudder pedals and reached
: my arm up behind the radio to see if I could feel anything out of the
: ordinary (like I would know what ordinary felt like!). I grabbed at
: the cables and wires and performed a wiggle-and-seat manuver for
: everything I could blindly grab... My following two flights, each
: nearly two hours, resulted in no radio failures. I was convinced the
: problem was gone (okay, I was praying that the problem was gone).
: However, the last two times I flew, it was back... This is when I
: decided that I might have temporarily fixed something, but that
: vibration had caught back up with me and undid what I fixed. My hope
: was that removing and cleaning all connection points and making sure
: everything is seated snugly when reinstalled, would fix my problem once
: and for all. The worst case scenario is $20 of coax cable and pulling
: back bloody stumps when I try to retrieve my arms from behind the
: panel. If I take it to an avionics shop at this point, I will be
: looking at a minimum of $300 to troubleshoot, and I have already dumped
: nearly $600 for a questionable radio repair and a new antenna, while
: shotgun troubleshooting.

I hear you. Troubleshooting intermittent problems sucks. Another possibility
is wrong spacers on one or more trays. My mechanic had troubles with his transponder
and intermittent connections on an encoder line or two. After a bunch of rewires,
checking, etc, we discovered that some small nuts/washers were installed on the wrong
side of the connector on the back of the tray. It was preventing the connector from
seating an additional 1/8" and causing intermittent failures on a few pins.

: I haven't tried that, but I did fly with a TKM slide-out loaner and
: encountered a similar failure. I also put my radio in another plane
: and the pilot reported that it did not fail during a nearly 3 hour
: flight.

From that right there, it sounds like you can rule out your radio. Don't
discount multiple *different* failures in the debug equation. Maybe it's your audio
panel or intercom?

: Above my radio is the audio panel, and below it is an ADF. The ADF is
: inop, so I could remove it... Does the radio require the audio panel
: to be useable in the airplane?
Doesn't *need* it, but it's probably wired to use it. If you've got a set of
direct-to-radio heaset plugs, those should still work with the ADF and audio panel
out. It also likely doesn't change anything whether they're in or not, though.

Could I pull out the audio panel and
: ADF, leave the radio in and plug my headsets into the aircraft (non
: intercom) jacks and be able to xmit/receive? I'm willing to try
: anything I can, to avoid throwing good money after bad! It would be
: one thing if I could explain a set of symptoms to an avionics shop and
: get an estimate to put this issue to bed, but I can't reproduce the
: issue at will, unless a tech is willing to go flying with me! :-(

: Per other post, RG-400 will cure anything, nor do much
: performance-wise at VHF.

: I don't completely follow this one??? RG-400 is or isn't necessary, as
: opposed to RG-58?

: Thanks for taking the time to respond!

: Todd

RG-400 shouldn't really be necessary at VHF frequencies. The materials are
better, and it has better high-frequency characteristics (e.g. transponder or DME).
At VHF it primarily serves to be more of a PITA to work with (much more rigid). It
also screwed with my compass once because it had a copper/tin-clad steel center
conductor.

-Cory

--

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