Alternator noise on Xmit only

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)

On Jun 29, 12:02 pm, "Vaughn Simon"
<vaughnsimonHATESS...@att.FAKE.net> 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

On Jul 1, 6:34 pm, David Lesher <wb8...@panix.com> wrote:
> "Mike Noel" <miken...@comcast.net> writes:
> >So running a grounding wire from the jack to the radio tray is not enough?
>
> You want to isolate the jack from the panel ground.
>
> --
> A host is a host from coast to coast.................wb8...@nrk.com
> & no one will talk to a host that's close........[v].(301) 56-LINUX
> Unless the host (that isn't close).........................pob 1433
> is busy, hung or dead....................................20915-1433

He's right you know. Otherwise, small alternator induced voltage
drops in the airframe will show up in series
with the mic. You puny little wire won't change that picture.
Isolation washers are the answer!!

Should be on the phones too.

Bill Hale

"bill_hale@verigy.com" <33bonanza@**********> wrote

>He's right you know. Otherwise, small alternator induced voltage
>drops in the airframe will show up in series
>with the mic. You puny little wire won't change that picture.
>Isolation washers are the answer!!
>
>Should be on the phones too.

I agree with the above; however it depends on which bit of the
airframe the grounds of all the loads are returned to.

I fly a Socata TB20 and on that, everything is grounded to a common
point, close to the firewall. So there are no currents of any
significance flowing through the "passenger" portion of the airframe.