On Jun 29, 12:02 pm, "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)

Thanks
Vaughn

--
Will poofread for food.

We had a 172 that did exactly that, and I minimized it by
isolating the intercom from ground. It was a portable intercom,
screwed to a bracket that was in turn attached to the aluminum
airframe, and so provided a ground-loop path that picked up the
alternator's output pulsations travelling through the frame. Isolating
it removes that path. And you won't notice the noise in the intercom
itself because when you aren't transmitting, you aren't using the
pathway between the intercom and radio.
Older airframes seem to be worse for this sort of noise. The
riveted joints get corroded and so don't carry ground currents as well
as they did, and those currents will try to find easier paths across
those joints, paths like the one into the intercom case and through
its lines to the radio, though its circuitry to ground. An EE can
explain it better than I.

Dan