Vaughn Simon wrote:
(I have only "seen" this plane on the telephone, but the folks I am trying to
help were pretty clear about the symptoms)

The basic problem is alternator noise on the transmitted carrier, receive is OK,
and alternator noise is not apparent anywhere else in the plane. The pitch of
the noise changes with engine RPM and disappears when you switch off the
alternator.

-No complaint of alternator noise in the intercom.

-The symptom is the same on both radios.

-If you turn off the intercom and key the radio, it still transmits the
alternator noise.

-They have tried different headsets

So far, I have told them to carefully check any ground straps under the cowl and
to check that the radio stack has a good ground.

Any other advice?

(Jim's take will be especially appreciated)


An educated guess:

Short from microphone connector ground to chassis.
The microphone should get its ground connection
from the radio (or audio panel) only, to prevent
the stray ground currents on chassis from creeping
onto the audio line.

--

Tauno Voipio (CPL(A), Avionics Engineer)