In article .com,
"nrp" wrote:

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

Lift that single ground ground and verify with a continuity test that
with power off, the intercom system is now floating. Make sure there
there isn't some other sneak path to ground that you don't know about.

I am quite confident that there is no sneak path to ground from the
intercom system, because I had everything disconnected when I rewired
(including the wiring harness). All jacks are isolated with fiber
shoulder washers. If there is a sneak path, I'm not sure where it would
be unless the SPA-400 is grounding through the mounting screws or on/off
switch barrel, but in that case I would expect the whine to be
system-wide.



Another ground loop vs B+ capacitive coupling check - does the whine
become worse as the alternator load is increased, say by turning on
pitot heat and the landing lights? If so, it is more likely a ground
problem.

Well, the whine goes away completely when the alternator side of the
master switch is turned off. I would say that it gets slightly worse as
more load is placed on the bus (landing light and pitot heat, for
example.) It is virtually non-existent in the pilot side, and is very
noticeable on the copilot side.

Before I dug into the intercom wiring last year, I did clean up the
ground at the battery box and strobe power supply (strobe noise problem
mostly cured with new power supply.) However, I haven't cleaned
airframe to engine ground or any grounds under the panel (other than
intercom system.)



Thanks,
JKG