The airplane I fly (Mooney M20C) has a two place intercom. I have a
Pilot PA-400 3BL portable 4 place intercom, to allow 4 headsets to be
connected. I bought it used and don't have a manual. So, first, does
anyone know where I can find a copy of the manual, either for purchase
or online?

It worked beautifully for awhile but a problem has developed. It was
intermittent but always fails now. It used to be that you could plug it
in again (and again) and it would work. When I plug the units
microphone plug into the pilot side (it's OK on the co-pilot side) it
keys the microphone as if the PTT had been pushed.

The Intercom has a pigtail with standard headset and microphone plugs on
it. These plug into the usual jacks on the panel, then you plug the
headsets into the box, which has labeled jacks (pilot, co-pilot,
passengers). There is nothing on the box to activate the push to talk,
which is still handled by the PTT switch on the control yoke. However,
as mentioned, when I plug the unit in (no headsets, either on or off, no
changes of switches changes anything) the T comes on in the radio
indicating the mike has been keyed.

Another thing I don't understand... On the intercom microphone plug (on
the pigtail - plugged into the panel) there are two jacks on the back
for pilot and copilot. So, you've got a plug with two jacks on the
back, one jack labeled pilot and the other co-pilot. These make no
sense to me since a typical headset only has about 6 inches of available
cord between the microphone plug and the headset plug and there's no way
to make the microphone plug reach that far away from the headset plug.
The intercom box has a 6 foot cord on it (or so). I'm inclined to
remove this and connect a microphone jack directly, but don't want to do
so without knowing the function of the two plugs.

Thank you for any help. My understanding, from a google search, is that
to key the mike (PTT) the tip of the microphone plug is brought to
ground. This leaves the middle portion or band to carry the microphone
signal and the long portion is ground. Is this correct? What does the
circuit look like, particularly with a pilot and co-pilot, each with
their own PTT.

Thank you for any clarification. GGK