In article .com,
"mikem" wrote:
I concur. I dont think it is a ground loop issue. I'm thinking a wiring
error, or short which occurs only when headphone plugs are pushed in,
or an internal fault inside the SPA400. Too bad you dont live close, I
would lend you mine to see if it made a difference.

Well, I rewired the intercom with shielded cable and was pretty darn
careful to follow the wiring schematic, and the system does function, so
I'm fairly confident that it is wired correctly. However, I do
understand that just about anything is possible.

Believe it or not, I actually sent the SPA-400 back to Sigtronics a few
months ago for them to bench test, and it came back fine. That's not to
say that they didn't miss something, as I'm not sure what testing
procedure is used.



The SPA400 drives the pilot headphone jack differently than the other
three jacks. Is the problem only on the copilot's jack, or also on the
back-seat jacks?

The problem exists ONLY in the right side of the cabin (copilot and
right rear passenger.) The pilot side is fine (both front and rear.)
You can hear a very, very faint whine on the pilot's side if there is
radio silence, but it's almost unnoticeable even with the headset volume
set at max. Despite the whine on the copilot side of the cabin, voice
quality is still very good throughout the system.



I'm puzzled by the "crackle". The alternator whine caused by a ground
loop is usually musical, pitch varies with engine speed. Crackles are
usually vibration induced intermittent contacts such as an improperly
seated audio panel, or nav-com. These usually behave with the engine
off, but can be induced by bashing the panel with your hand. Crackle
could also mean loose electrical connections to the bus, unshielded
P-leads, noisy electric-powered gyro instruments, bad fuel senders,

I think I have two incidents of "crackle." One is only present when the
landing light is turned on, and I suspect that one of the landing light
leads in the cowl is occasionally contacting the muffler shroud.

The other one, however, sounds like a "chop chop chop" which increases
in speed with engine RPM and is often (but not always) present when the
throttle is advanced when positioning onto the runway. It is only heard
when the copilot PTT is keyed. It is never heard when the pilot's PTT
is keyed.

As for the regulator, I'm not sure when solid-state regulators became
standard equipment on airplanes, but as far as I know, mine hasn't been
changed since the airplane was manufactured in 1977.



JKG