Avionicswest.com, under "Articles," has a couple on noise problems,
plus other cool stuff.
Where they talk about a capacitor on the alternator, that's a
relatively small-value one to suppresses RFI that bugs ADF as they
imply, and it may do no more than make the hum more musically
pleasing.
They do talk about ground looping and such, but they can't mention
chokes (FAA-illegal here w/o mfr imprimatur). Places like Radio Shack
sell a big one for automobile use (plus a mysterious Ground Loop
Isolator). But I bet there's a choke inside the Rotax alt reg, and
hence the big 25,000uf filter cap (could be bad). Else it wouldn't do
much, as a storage battery is the world's largest, readily available
zillion-farad electrolytic capacitor!
But rather than brute force method at that end, one can try a little
one in series on the 14V line that feeds the intercom. For an audio
panel problem, I used one of unknown value, a 1" O.D. toroid core with
about 30 turns of wire on it, but had to try an assortment from the
electronic surplus place. It looked too few turns to work than one
that did look like enough, but took most of it out. Guess it was the
right mH's at offending freq to allow it's filter cap, at its value,
to earn its keep.
This presumes the actual source isn't the comm radio. A good design
will have a choke, which won't fail should it have one, but its filter
cap can. Or try an external choke there.
Best,
Fred F.
Alan D Stewart wrote:
>
> Hi Guys,
>
> After four years of satisfactory service in my Europa, the Flightcom 403
> intercom installation has begun to
> exhibit a highly irritating background hum with headsets attached and
> engine running.
>
> The hum frequency follows the engine RPM closely. Pulling every circuit
> breaker in the panel, bar the intercom
> itself does not alleviate the problem.
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