where exactly to put the noise filter

On Fri, 20 Feb 2004 17:57:10 GMT, "Jay Honeck"
wrote:

Hey Mike,

What's the cure for "bleed through" from one com radio to the other?

We usually keep "Com 2" tuned to our local AWOS frequency. When we're near
Iowa City, we can faintly hear it in the background through our
headphones -- even when "Com 2" or "Both" aren't selected on our audio
panel.

Of course, this isn't an issue without ANR headsets (couldn't hear it), or
when we're far enough away from the strong signal -- but it's one of those
little annoyances that always bugs us.

Jay - what make/model are the COM radios in your plane?

-nathan

Jay - what make/model are the COM radios in your plane?

Com 1 is a brand new Narco 810-R, digital. (It's the slide-in replacement
for the Com 120)

Com 2 is an old-but-perfect Narco 120.

Both work flawlessly.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

On Fri, 20 Feb 2004 23:11:27 GMT, "Jay Honeck"
wrote:

Jay - what make/model are the COM radios in your plane?

Com 1 is a brand new Narco 810-R, digital. (It's the slide-in replacement
for the Com 120)

Com 2 is an old-but-perfect Narco 120.

Hi Jay,

Take this with a caveat as I am a digital engineer who likes to learn
about RF stuff. I could be in left field here...

I asked because I was trying to determine if the radios utilized the
same IF frequency (which they would if it is the same radio). I don't
know enough about the Narco 810 or 120 to determine if they share the
same IF.

Where I was going with this...

The bleedthrough might be leakage from the IF section of one radio
coupling into the IF of the other radio. That would explain why the
audio is only noticeable as you are near the AWOS transmitter, where
the energy entering the IF stage on COM2 would be strongest.

An interesting test would be:

1. Shut off COM2 on the audio panel
2. Fly to a distant location where you can't hear the bleedthrough.
3. Turn on the audio for COM2
4. Can you hear the AWOS over the audio panel?

If you can hear the audio, then I suspect the audio problem is related
to the radio and not the audio panel (or audio wiring).

-Nathan

An interesting test would be:

1. Shut off COM2 on the audio panel
2. Fly to a distant location where you can't hear the bleedthrough.
3. Turn on the audio for COM2
4. Can you hear the AWOS over the audio panel?

If you can hear the audio, then I suspect the audio problem is related
to the radio and not the audio panel (or audio wiring).

I don't think I've got enough information here. What position would I have
the audio panel in for this test? Com 1, Com 2, or Both?

And, assuming Both, what do you mean when you say "Can I hear the AWOS over
the audio panel?" Do you mean on the speaker? Or in my headphones?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

On Sat, 21 Feb 2004 02:24:12 GMT, "Jay Honeck"
wrote:

An interesting test would be:

1. Shut off COM2 on the audio panel
2. Fly to a distant location where you can't hear the bleedthrough.
3. Turn on the audio for COM2
4. Can you hear the AWOS over the audio panel?

If you can hear the audio, then I suspect the audio problem is related
to the radio and not the audio panel (or audio wiring).

I don't think I've got enough information here. What position would I have
the audio panel in for this test? Com 1, Com 2, or Both?

And, assuming Both, what do you mean when you say "Can I hear the AWOS over
the audio panel?" Do you mean on the speaker? Or in my headphones?

Ahhh, not exactly clear given your audio panel. I was debugging this
thinking of the KMA24 in my plane. I meant headphones only.

Going back a few steps (to make sure we're debugging the correct
problem) - the initial problem was with ONLY COM1 selected, you hear
the station that COM2 is tuned (to) through the headset?

For step 1 above, listen ONLY to COM1, but leave COM2 on and tuned to
the AWOS. You should still hear the AWOS on the bleedthrough on COM1.

Once you get far enough away that you no longer hear the bleedthrough,
deselect COM1 and select COM2 to be played through the headset. See
if you can hear the AWOS.

If you can hear the AWOS, I suspect the problem is due to IF coupling
between the radios. The reasoning being that if COM2 is still
receving the AWOS and producing audio - but the bleedthrough is gone,
than it is not a wiring/audio panel problem.

Anyway, Mike's idea is a bit easier to check. Vary the volume on COM2
and see if the volume on the bleedthrough changes. If it does, you
probably have some combination of a wiring/audio panel problem.

If it doesn't try my test.

Aren't avionics in 30 year old planes fun?

-Nathan

Going back a few steps (to make sure we're debugging the correct
problem) - the initial problem was with ONLY COM1 selected, you hear
the station that COM2 is tuned (to) through the headset?

Yes, very faintly. It's only noticeable on (or over) the field, with the
AWOS transmitter nearby.

Once you get far enough away that you no longer hear the bleedthrough,
deselect COM1 and select COM2 to be played through the headset. See
if you can hear the AWOS.

I already know the answer is "yes." The bleedthrough is only audible when
we're close to the transmitter.

If you can hear the AWOS, I suspect the problem is due to IF coupling
between the radios. The reasoning being that if COM2 is still
receving the AWOS and producing audio - but the bleedthrough is gone,
than it is not a wiring/audio panel problem.

Okay, so what would the "fix" be?

Thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

A. We know that this cannot be the problem (IF coupling) because Jay tells us
that if he turns the volume down to near zero on the AWOS radio that the
bleedover goes away. The volume control is WAY downstream of the last IF stage,
and has no control over the amplitude of the IF signal.

B. In forty years in this business, I've never had an aircraft radio (even when
two identical radios are in a stack on top of one another) bleed IF from one
into the other. Never. Not even through a ****poor receive antenna coupler
with lousy isolation.

C. The odds are that the headphone outputs of the two radios (which is what we
all use to drive both phones and speaker of an audio panel) are closecoupled so
that there is capacitive coupling from one headphone lead to the other. One
thing that Narco may or may not have done is to terminate the headphone leads
inside the audio panel with a resistive load (150-600 ohm resistor). If they
did NOT do that on this particular model, the capacitive crosscoupling between
the headphone inputs will be magnified.

It remains for somebody in this ng with access to the chassis schematic of Jay's
audio panel to see if Narco did in fact terminate the headphone inputs. If they
did, then a simple wrap of one headphone lead or the other with tinfoil will
tell us if it is wire to wire coupling, and then the fix is a simple replacement
of one of the headphone wires with shielded cable.

Jim



"Jay Honeck"
shared these priceless pearls of wisdom:

-
- If you can hear the AWOS, I suspect the problem is due to IF coupling
- between the radios. The reasoning being that if COM2 is still
- receving the AWOS and producing audio - but the bleedthrough is gone,
- than it is not a wiring/audio panel problem.
-
-Okay, so what would the "fix" be?

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Jay Honeck wrote:
: Jay - what make/model are the COM radios in your plane?

: Com 1 is a brand new Narco 810-R, digital. (It's the slide-in replacement
: for the Com 120)

: Com 2 is an old-but-perfect Narco 120.

Jay, just for you I tried out a little test this morning on my flight to
Martha's Vineyard. I borrored a friend's Arrow, which has
Narco CP-136 audio panel
Narko Mk12D nav/com + IDxxx (Integrated DME)
Garmin 300XL gps/com

As I arrived in the pattern, I had the ATIS on com2 and tower on com1.
I had the "COM1" button pushed in. I pushed in the "BOTH" button and
verified that I could hear the atis. I shut off the "BOTH" and I
could not hear the atis on top of the tower freq.

When I left I switched radios (ATIS on com1, tower on COM2), with the
same results: the radio switched out on the audio panel can't be heard.

I'm not sure what this means to you - you DID want a new audio panel,
didn't you?
--
Aaron Coolidge (N9376J)

I'm not sure what this means to you - you DID want a new audio panel,
didn't you?

No! It works and looks perfect now, after much work.

Interesting that you're not hearing the cross-talk. I suppose this must
mean Jim's (and/or Mike's) theory is correct? Something is wired
incorrectly?

Thanks for taking the time to check this out.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

I didn't say it was WIRED incorrectly. It was either DESIGNED incorrectly or
INSTALLED incorrectly or FAILED INTERNALLY or NOT JUMPERED CORRECTLY.

Jim


"Jay Honeck"
shared these priceless pearls of wisdom:


-Interesting that you're not hearing the cross-talk. I suppose this must
-mean Jim's (and/or Mike's) theory is correct? Something is wired
-incorrectly?



Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com