Rheostat Audio Interference

This evening, I noticed a problem with the nav/instrument lighting and
panel intercom in my Cherokee that I haven't noticed before. The nav
lights and internal instrument lights (I have KX170Bs, KMA20 audio
panel, etc.) work just fine. The problem is that when I turn the
rheostat up so that the internal instrument lights come on, there is an
escalation in electrical noise in the intercom to the point that the
intercom cuts out (i.e. I can't communicate with any other person in the
airplane, can't hear radio transmissions, etc.) When I turn the
rheostat down to the point where the instrument lights are out, but the
navs are still on, there is no problem. I have no problem with the
panel light rheostat.

This problem wasn't always present, and I was caught off guard by it
tonight. I did swap the original non-shielded intercom wiring with
shielding wiring a couple months ago, and brought everything back to a
single-point ground, but I can't image that this has anything to do with
the rheostat issue.

Does anyone have any ideas?



Thanks,
JKG