Wayne wrote:

Thank you! I haven't dug the unit out of the panel yet so I hadn't seen
the labes. Still not sure where the negitive of the headsets and mic's go
but I imagine that will be obvious when I tear into it. Now I know that they
are labeled and that all the heatsets are wired the same and that the pilot
and co-pilot each have their own PTT and mic input. Thank you very much,
that explains things nicely. Not sure what to think about the diode thing
though, audio is AC current and and so are a mic's output so a diode would
make it pulsating DC rather than stopping backflow.


There are two aspects of this switching: Mic and PTT. The PTT "or gate"
is usually implemented with two diodes, wired so that if either Pilot
or CoPilot Key switch is depressed, then the PTT line of the Com
transmitter is pulled to ground.

During PTT, whichever (Pilot or CoPilot) Key switch is depressed causes
the Mic line from the respective side to be connected directly to the
Com transmitter's input. In this way, only audio from the selected
Mic modulates the transmitter, Moreover, during transmit, the Mic bias
is taken from the Com radio (not from inside the intercom like it is
when the PTT is not keyed). Most older intercoms used a reed relay
to acomplish the internal mic line switching (Three states: Intercom-
niether relay on, Pilot Ptt- one relay on, CoPilot Ptt- other relay no)

All audio grounds (including shields) are connected to pin A
(Intercom Power ground). To keep alternator whine and strobe squeal
out of the audio, (especially) the rear seat jack (Mic and Hdphone)
barrels must not be "grounded" to the airframe. Instead, use
plastic sholder washers to keep the jack barrels from touching, and
connect the barrels to either a "ground wire" inside the shield, or
use the shield itself to connect the remote jack barrels to Pin A.
Trust me on this: I know of which I speak....


MikeM (PhD EE)
Fixed dozens of aircraft audio systems...

Skylane '1MM
Pacer '00z