In article ,
wrote:
As far as the *location* being the copilot, you could check to make sure
that
when the plug is inserted into the jack that it doesn't touch anything when
the prongs
spread. IIRC, the SPA-400 has pilot/copilot isolation, so the signal will
have
different paths depending on which plug is connected... separate internal
circuits.

I'm not convinced that it's a ground loop because the intercom system
has a single-point ground. I have the system grounded where at least
SOME other avionics are grounded, but not sure that all avionics are
grounded at that point.

Nothing is touching any metal or anything else in the panel--I've
already been down that road.

I'm wondering whether there's just enough noise around the bus area (and
the b+ in particular) to create a problem. The lighter receptacle is
located above the copilot jacks--not sure whether this is relevant to
the problem or not.

Also, I've noticed a crackle interference in BOTH headsets when the
landing light is turned on, but I suspect that the landing light leads
are brushing against the muffler shroud. I need to check this out, but
it's not related to the high-pitched whine as far as I can tell.




JKG