Alternator noise on Xmit only

[Vaughn Simon]

(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.

[Tauno Voipio]

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)

[[email protected]]

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

[MikeMl]

Tauno Voipio wrote:

> 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.
>

Bingo,

Proper installation of aircraft mic and headphone jacks requires the use
of insulating bushing where the threaded sleeve on the jack would
otherwise come in electrical contact with the airframe. The wiring
behind the panel from each jack requires that there be three conductors
(one each for tip=ptt, ring=micHi, sleeve=micLo & Ptt return) surrounded
by a shield. Good practice connects the shield to airfame ground at the
radio end.

None of the mic or headphone jacks should be touching airframe ground.
None should utilize the airframe as the return conductor for any audio
signal, mic or headphone!

The sleeve connection from all jacks should go to a "single-point audio
grounding point" at the rear of the audio panel tray.

The factory installed "original mic jack" usually did not follow the
proceedure cited above. It is likely that the alternator whine is
getting into the transmitted audio due to a ground drop between where
the radio is grounded and where the mic jack touches the airframe.

[David Lesher]

MikeMl > writes:

>> Short from microphone connector ground to chassis.

>Bingo,

>Proper installation of aircraft mic and headphone jacks requires the
>use of insulating bushing where the threaded sleeve on the jack would
>otherwise come in electrical contact with the airframe.

And those those who've not seen them, you need shoulder washers.



...[======| |======]
PANEL| |****PANEL
.......[==| |==]
...[======| |======]



The outer [here lower] washer is two-step; the inner part centers the
jack in the panel hole. Sometimes the inner [here upper] washer is flat,
sometimes it's the same as the lower. Both are usually made of phenolic
or such; but could be any insulator.

And note that if any mike jack wiggles loose/gets off-center, you can get
a short. It's an easy thing to inspect.

And if the jack is mounted on a plastic panel; you can skip the whole
issue. I don't know how many are.


--
A host is a host from coast to
& 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

[Mike Noel]

So running a grounding wire from the jack to the radio tray is not enough?

I installed a jack to run the audio from my 496 to the audio panel in the
radio stack and at first just used the instrument panel as ground. I did
get a significant amount of noise. Then I added a ground wire from the plug
to the radio tray and got rid of the extra sound effects.

--
Best Regards,
Mike

http://photoshow.comcast.net/mikenoel


"David Lesher" > wrote in message
...
> MikeMl > writes:
>
>>> Short from microphone connector ground to chassis.
>
>>Bingo,
>
>>Proper installation of aircraft mic and headphone jacks requires the
>>use of insulating bushing where the threaded sleeve on the jack would
>>otherwise come in electrical contact with the airframe.
>
> And those those who've not seen them, you need shoulder washers.
>
>
>
> ..[======| |======]
> PANEL| |****PANEL
> ......[==| |==]
> ..[======| |======]
>
>
>
> The outer [here lower] washer is two-step; the inner part centers the
> jack in the panel hole. Sometimes the inner [here upper] washer is flat,
> sometimes it's the same as the lower. Both are usually made of phenolic
> or such; but could be any insulator.
>
> And note that if any mike jack wiggles loose/gets off-center, you can get
> a short. It's an easy thing to inspect.
>
> And if the jack is mounted on a plastic panel; you can skip the whole
> issue. I don't know how many are.
>
>
> --
> A host is a host from coast to
> & 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

[David Lesher]

"Mike Noel" > 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
& 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

[[email protected]]

On Jul 1, 6:34*pm, David Lesher > wrote:
> "Mike Noel" > 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
> & 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