Jonathan Goodish wrote:
: That's very helpful, but unless I'm missing something, I'm not sure that
: it explains why I'm only having the whine problem on the right side of
: the cabin (in both front and rear jacks). All jacks are isolated from
: the airframe, and I'm running a common ground back to the intercom
: ground. The braided shield is floating at the jack ends and grounded at
: the intercom ground point. The whine on one side of the cabin would
: make more sense to me if the jacks were grounded locally to the
: airframe, but not sure how noise would enter (or exit) the system on one
: side of the cabin with the intercom system on a single-point ground.

The jacks may be a red herring. If they truly are isolated from the structure
and grounded only to the intercom, you're right... they *cannot* be causing a ground
loop problem. The shield could still be allowing some noise coupling, but I doubt it.

As far as the right side/left side stuff, remember that there's a *BIG*
current flowing out of the alternator (well, *into* the alternator's ground). That
current is just meandering through the structure somehow. It could certainly be more
preferential to one side or the other when you're talking a few miliohms total through
the structure.

: Outside of the stuff I've posted to the newsgroup, one other oddity:
: when I activate the marker beacon output on the audio panel, the whine
: is more noticeable.

Does the *audio panel* run to the same single-point ground as the intercom?
If they're spatially separated (especially if one is on port the other's on
starboard), it could do what you are describing. Of course *any* chunk of avionics
grounded elsewhere could be inducing the ground loop as well.

: Obviously, it sounds like there's a ground problem somewhere, but I
: don't know where to start. I'm baffled as to why I'm only hearing the
: problem on one side of the intercom system and not the other, with none
: of the jacks or PTT switches grounded directly to the airframe.

Do you have the schematics available for the intercom? I managed to find a
set for both of the intercoms I've had in my plane. They generally have different
circuitry for the pilot vs. copilot. That might allow for different coupling of the
noise.

The : noise must be alternator-induced, because it is only present when the
: alternator field is activated. If something was looping through the
: audio panel or somewhere else, why would it only affect the jacks on one
: side of the aircraft?

Again, different inputs. It *does* seem a bit odd, but I haven a hard time
believing that it's the shield. Again, you're *SURE* that the jacks are floating,
right? ... and they don't accidentally contact any chunk of airframe when a jack is
plugged in? For the longest time we didn't have side panels in our plane and I just
had the rear seat intercom jacks zip-tied to the side. Once in awhile they'd move and
rattle against the airframe and make a serious crackle racket in the intercom system.
Just a thought.

-Cory

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************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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