where exactly to put the noise filter

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