Take out the ELT and see what happens. Sometimes they will cause problems.
Mine did.

"Curator" N185KG

"Ray Andraka" wrote in message
...
I'm wondering if anyone here has seen anything similar to this or might
have some ideas on where to look:

A buddy of mine has a new noise problem with the radios in his Arrow, but
it is a little different than the run of the mill noise problem because it
is only in the comm radios (you only hear it during transmit and when
receiving a signal that is strong enough to break squelch, otherwise his
intercom is crystal clear, as are the nav radios, marker beacon, and ADF).
The odd thing is both radios make the same noise, which would seem to
indicate the noise is getting into the system somewhere other than in the
radios. He's had it to two avionics shops, both of which were happy to
take his money but didn't really do anything.

Here are the particulars:

Equipment: KMA-24 Audio panel w/marker beacon, KX155 comm 1, KX170 comm
2, PS Engineering 4 place intercom (PM-1000 I think)

Observations:
We get noise in the audio during transmit and when it is receiving a
signal that breaks squelch (not as strong as in transmit). There is no
noise otherwise.
The same noise is present during transmit and receive regardless of the
radio selected.
The same noise is present regardless of which PTT switch is pressed, and
is audible in both the pilot and copilot positions (didn't try the back
seats)
Noise is gone when not receiving (signal not breaking squelch) or
transmitting, as well as when the selected comm radio is turned off.
ATC reports noisy transmission when we hear the noise, so it apparently is
getting transmitted as well.
The noise sounds like random impulses, but doesn't seem to be directly
related to the engine speed.
When I heard it, the noise was masked by the background noise if I
defeated the squelch. I suspect it was still there, but I really couldn't
tell.
The noise only seems to occur at higher airspeeds. We have not observed
it on the ground or at pattern speeds. (I'm thinking it is more a
vibration issue than an airspeed issue).
Intra-cockpit communication via the intercom is crystal clear, ie. the
noise is only there when you key the mic or the selected radio breaks
squelch.
The noise is also not present if I turn off the comm radio and key the
microphone (I also don't hear the side-tone).
When I listened, the noise level was noticible, but not bad enough to make
the radio unintelligible. My buddy said it gets a lot worse at times, and
has been so bad that when keying the microphone it made him "want to
immediately unplug the mic", and that it made the radio useless.
There may be some correlation to recent rain. He thought the problem had
been solved with static wicks, but then it returned after a heavy rain.
Swapping the KMA24 with another does not change anything, so it is
probably not the audio panel itself.
The noise is not present in the nav audio on either radio. It is also not
present on any of the other audio sources other than comm1 and comm2

My hypothesis is that the noise is coming in through the comm radio audio
amplifiers since it is only present when the radio is producing audio
(either un-squelched receive or transmit side-tone). Since it is the same
for both radios, The source would seem to be from outside of the radios.
If the audio panel had some part that was unique to the comm radios but
common to both receive and transmit it could be there, but based on what
you told me previously it doesn't sound like that is the case. My thought
then was that it was a grounding issue. I didn't know if the radios get
grounded through the cases and tray bonding or not though.

Things I know of that have been tried so far:
My buddy hadn't noticed that the noise was also in the received signal, so
he was concentrating on the transmit. They had swapped antennas, tried
re-bonding the antennas, and added static wicks without result before
telling me about the problem. They also flew with the alternator
completely removed from the airplane, no change.
The back seat intercom jacks were disconnected at the intercom, no change
as a result.
The radios and audio panel were pulled out and the contacts cleaned. While
they were quite dirty on the kx170, the others were clean. No change as a
result.

We (my A&P and I) took out the trays, cleaned up the mating edges (they
didn't look bad) and bonded the audio panel tray to the airframe with some
braid. After putting it all back together, the noise is still there but a
little different: 1) it is much more apparent in the KX170 than it is in
the KX155 now, 2) it is much more apparent on transmit than it is on
receive (not sure if this is any different than it was). We also played
with engine rpm on the ground and got the noise to occur on the ground,
which we hadn't achieved before (hopefully will help with tracking it
down). It seems to be vibration related, and in particular seems to have
some mechanical resonances that depend on engine RPM on the ground in that
it will only start to happen at particular RPMs, but once it starts it
will persist over a wider range of RPM. Once you move out of that range,
it stops and won't start again until you get into a narrow range (one
resonance was at about 1100 RPM, +/- about 25 RPM, but once started would
persist over a range of +/- about 200 rpm).

The audio grounds at the KMA-24 are apparently tied to a bolt on the back
of the tray rather than to pin1 as called out in the KMA24 installation
manual, as there are 3 ring lugs attaching 8 or so wires to the tray. I
found this tonight while looking through a copy of the install manual I
found on the internet.