Re: Alternator noise on Xmit only

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 bu****ng 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 ****eld. Good practice connects the ****eld 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.