What you describe is classical ground-loop troubles. Your first attempt to
fix the problem should have done it... insulating the jacks from the panel (mics in
particular). If that truly didn't fix it, you may need to look at other ground
locations. You did say you replaced/insulated *all* jacks, right?

You can try relocating the ground on the Sigtronics to another location (like
the same location as where the audio panel connects would be good). With a large
number of older avionics with a single-ended design, the noise could be coming from
other places.

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.

Basically, troubleshooting ground loops sucks.

-Cory

Jonathan Goodish wrote:
: I have a Sigtronics SPA-400 intercom in my Cherokee. Everything works
: fine, except that the right seat position has an annoying amount of
: electrical noise in the headset. It is a high-pitch whine (alternator
: noise), and the pitch seems to follow RPM. This is not noticable in the
: pilot's headset, even with the headset volume at full. It is much less
: noticable in the right seat headset with the headset volume turned all
: the way down, but obviously this makes things harder to hear for the
: right seat occupant (often another pilot).

: In an attempt to rectify the problem, I replaced the intercom wiring
: with shielded Tefzel cable, replaced all of the jacks, and isolated all
: of the jacks from the panel with fiber shoulder washers and brought
: everything back to a single point ground. That helped, but only slightly.

: I'm trying to figure out what's causing it or, at least, how to stop it
: on that particular jack. Obviously, the intercom wiring runs adjacent to
: the electrical buses on that side of the panel, so I'm not sure what
: role the buses may play, especially since I'm using shielded cable.

: Does anyone have any ideas?


: Thanks,
: JKG

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