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

: The weird symptom that goes along with the not receiving is the
loss of
: sidetone, when I attempt to xmit.

I'm gettting a bit lost in facts and symptoms here. Anyway, we
must bypass intercom to isolate to which box. If the Narco, it
does not know receive audio from sidetone in the circuit area of
interest, if both those signals are absent.

There's audio leveling circuitry in there to which both mic audio
and receive audio are fed. The nature of that circuit is such that
component drift through heat upsets a delicate balance, and there's
many analog swith ICs in there to turn things on and off. The fact
that you can xmit w/o hearing mic audio I believe may isolate the
fault to a certain area of that circuit.

If true that it works for a while and then stops, that's gotta be
thermal. A wire or coax connection can theoretically behave like
that, but airplanes vibrate. The thermal expansion of metal is
like what in comparison? The only wire connection for your
symptoms should be phone audio out from the comm.

The reason I'm leaning in this direction is that there are simpler
circuits in comm boxes where all your symptoms could not happen.
In the Narco design it can; it's an electronic Rube Goldberg
machine. And the only company on earth who still write code for
the MK3870 CPU chip.

Fred F.

I will state again, I had very similar problems with my MK12D and it
went back to the factory. It is fixed and working fine. The side tone is
a product of the radio not the audio panel. How I isolated it was
swapping the two MK12Ds I had and the problem followed the radio.
Therefore not in the trays, coax, audio panel, etc.


-------------
Regards, Ross
C-172F 180HP
KSWI


three-eight-hotel wrote:
I've read your symptoms per your orig post, and you have one odd
collection of seemingly mutually-exclusive symptoms. If you can
xmit, but not receive, it's not the coax.


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??? 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. 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) 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.


Scratchiness, followed
by silence, isn't likely the connector, nor coax. Is that what it
still does?


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!


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?


You can't pull the tray without dealing with the wires at the
connector. If you can do that, you can just visually inspect the
stuff for integrity.


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.


For thermal, did you try flying w/o any box above and below the bad
one? That's how I know my 120's a thermal glitch.


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.

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? 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

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???

Are there any recommended readings out there, that anyone could
recommend to at least bring my up to a third grade level on this stuff?
A search for "avionics for dummies", came up goose eggs!

Thanks again for your responses!

Todd

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

: 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!

Sounds like you've pretty much removed the radio from the equation.

: "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.
"Sidetone" generally refers to hearing what you are transmitting on the radio,
not what you are saying on the intercom. One could argue it's a pedantic difference,
but for what you are troublshooting, the difference is important.

It is only on xmit where I lose sidetone...

Correction: It's only when you transmit that you should *HAVE* sidetone.
Any other time your "sidetone" is simply the intercom intercom'ing.

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?
It could be on the intercom, on the radio, or *both*, depending on how
whomever installed it decided to wire it up.

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???

: Are there any recommended readings out there, that anyone could
: recommend to at least bring my up to a third grade level on this stuff?
: A search for "avionics for dummies", came up goose eggs!

Trouble is that there is no "one true way" to install the stuff. There are
generally enough connections included in all of the devices that there are different
ways to accomplish the same thing.

Judging by your statement he
: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.

I'd say you've got a bad connection between the "audio out" of the radio and
the COM[12] on the audio panel, or between the audio panel and the intercom. I would
also speculate that the intercom is not producing sidetone, but the radio is... thus
the loss of sidetone upon transmit, but the intercom remains functional.

I don't specifically remember other details that may contradict this idea, but
I'm sure they'll be pointed out to me by someone...

-Cory

--

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

Ross Richardson wrote:
: went back to the factory. It is fixed and working fine. The side tone is
: a product of the radio not the audio panel. How I isolated it was

The sidetone can usually be set up either way (radio or intercom).

--

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

TaxSrv wrote:
: "three-eight-hotel" wrote:
: I've decided that I'm going to pull the radio and the connector
: tray and run new coax from the radio to the antenna,

: I've read your symptoms per your orig post, and you have one odd
: collection of seemingly mutually-exclusive symptoms. If you can
: xmit, but not receive, it's not the coax.

Oh, and I forgot on my other post that I concur it's most likely NOT the coax.
That doesn't really cause scratchiness.... you either have signal or you don't.
Scratchiness is caused by the signal to from your mics or to your headset (or from
within the audio panel, intercom, or radio, which is pretty much ruled out at this
point, too).

-Cory

--

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

UPDATE:

Last night, I went up to the airport and pulled out the radio, and
pulled the connector tray as far out/forward as I could. I performed a
close visual inspection and noticed something that I hadn't noticed
before... The radio and connector union seems to be made through two
24 pin connectors and two BNC connectors. The BNC connectors are an
opposing male and female (one each on the tray, and one each opposing
on the radio). What I noticed (forgive any improper terminology) is,
that the male connector on the tray seemed to be missing one tooth (for
lack of a better word) on the outer ring of the connector and the
radio, seemed to be lacking two teeth, on the outer ring of it's male
BNC connector (that's almost half!)

Aside from not being a good thing, is this a likely suspect for some of
the symptoms I have been experiencing? Replacing those two male BNC
connectors would seem like good common sense, but is this just an
unfortunate coincidence of something else being found while
troubleshooting another problem?

Look... I'm admittedly a couple of bricks short of a fireplace, when
it comes to talking circuitry and componentry, so no piling on! ;-)
I've been paying attention to all of the posts, but alot of it seems to
be going over my head. It's not like I'm not listening... I promise,
I'm not going to go pulling any wire!

I may, however, take you up, Jim, on a trip to Grass Valley to borrow
your constant signal device or bird watt meter you mentioned in a
previous post. One step at a time, though, I guess...

Thanks,
Todd

"three-eight-hotel" wrote:
... the male connector on the tray seemed to be missing one
tooth (for lack of a better word) on the outer ring of the
connector and the radio, seemed to be lacking two teeth, on
the outer ring of it's male BNC connector (that's almost half!)

Aside from not being a good thing, is this a likely suspect for
some of the symptoms I have been experiencing?
....

Whatever's goin' on there will not cause a loss of sidetone.

Fred F.

Yeah... that makes sense. Thanks!

I pulled everything from the stack this weekend and visually inspected
everything I could see... As mentioned in a previous response, every
single screw, holding in every tray, had a nut on the back of it, and
half of them became part of the permanent W/B! I took an air
compressor with me, along with the component/connector cleaner and
cleaned every connection I could. I'm going to hold off on putting the
trays back in and borrow the tools from my mechanic to do it right,
rather than figure out how I'm going to hold nuts on the back of trays
for each screw.

While I was working on this, I ran into someone up at the Ghost-town
(err, I mean Georgetown) airport that I haven't seen in a while. He
offered to let me swap out and try his audio panel, to see if that
might be the culprit! As soon as I get the trays back in place and can
reseat all of the avionics devices, I'll give it a quick check flight
to see if I can get it to fail again, then try the replacement audio
panel.

Thanks for all of the responses, and I'll provide an update when I can.

Best Regards,
Todd