where exactly to put the noise filter

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

It may be a shortcoming with the CP136 design too. If the volume of the
non-selected radio is up on mine, you can hear it. Same with the VORs. Was true
both before and after I pulled one of the MX11s and a Nav11 and replaced them with a
KX155. Turning down the volume on the offending unit fixes it, so it is definitely
something with the audo circuit. Since any radio including the VORs, marker beacon
and ADF do it, I assume it is the CP136M that is providing the bleedthrough path.

Jim Weir--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

Since any radio including the VORs, marker beacon
and ADF do it, I assume it is the CP136M that is providing the
bleedthrough path.

Nuts. And to think I just worked three days to repair this audio panel!

Is there a solution that my avionics shop can do, or is it just something to
live with? It's not a real hardship -- heck, I can't even detect it with
my ANR turned off -- but it is one of those minor annoyances that grates on
me every time we fly.

Thanks Ray, Jim, Nathan, Mike and everyone...
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

On Sat, 21 Feb 2004 20:49:18 GMT, "Jay Honeck"
wrote:

Is there a solution that my avionics shop can do, or is it just something to
live with?

Put on your headphones at home and listen to the 1812 at high volume
about 20 times. Or spend a day at a pistol range.

Nature is always the best remedy.

Eh?
Don

Put on your headphones at home and listen to the 1812 at high volume
about 20 times. Or spend a day at a pistol range.

Heh.

Along those same lines, I've slowly fixed everything that was wrong with The
Mighty Grape (our fuel truck) when we bought it. For just $1800, you expect
a LOT of cosmetic problems, and, one by one, I've fixed them all.

The last thing wrong is that the "Check Engine" light is on, all the time.
There's nothing wrong with the engine, according to my mechanic, but the
Nissan dealer wants $70 to hook it up to his machine, and turn off the
damned light.

A piece of black electrician's tape did the job perfectly -- just like your
pistol range...

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

On Sat, 21 Feb 2004 21:57:45 GMT, "Jay Honeck"
wrote:

Put on your headphones at home and listen to the 1812 at high volume
about 20 times. Or spend a day at a pistol range.

Heh.

Along those same lines, I've slowly fixed everything that was wrong with The
Mighty Grape (our fuel truck) when we bought it. For just $1800, you expect
a LOT of cosmetic problems, and, one by one, I've fixed them all.

The last thing wrong is that the "Check Engine" light is on, all the time.
There's nothing wrong with the engine, according to my mechanic, but the
Nissan dealer wants $70 to hook it up to his machine, and turn off the
damned light.

A piece of black electrician's tape did the job perfectly -- just like your
pistol range...

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


sometimes you can get rid of those lights by disconnecting the battery
for a few min.

sometimes you can get rid of those lights by disconnecting the battery
for a few min.

And lose all my radio stations? :-)

I'll try it -- thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:
sometimes you can get rid of those lights by disconnecting the battery
for a few min.

And lose all my radio stations? :-)

Plug a 9V battery into the cigarette lighter plug.