OK, let me reiterate what you have told me. Please take your time to
respond so that we don't go chasing down blind alleys. THere are some
things you have said that are contradictory.

1. THe nav lights (you are talking about the ones on the wingtips -- red,
green, white in the back) are working just fine. What would this have to do
with the problem?

2. The internal instrument lights are working just fine. I'm presuming
what you are telling me is that if you connect a wire directly from the +12
supply to the instrument lights then they illuminate at the proper level.
You DID take the dimmer out of the circuit for this test, didn't you?

3. WHen you turn up the dimmer potentiometer so that the "instrument
lights" (do you mean the radio backlights or do you have some sort of
lighting system on the gyros?) then you get a certain amount of noise,
mainly in the headsets but some in the speaker as well.

4. You also said somewhere along the line that you cannot transmit when
this noise occurs, but that you CAN transmit using a handheld mic. However,
when you key up the transmitter, the radio lights dim.

5. You have a fixation on a "bad" transistor or a "bad" potentiometer.
Please let's not guess at solutions until we can prove something.

6. You said that you can't hear radio transmissions, yet in a subsequent
post you said that you CAN hear radio transmissions. Which is it?

7. I guess I'm a little unclear about the difference between "nav" and
"instrument" lighting. Can you elaborate?

8. I don't suppose there is a chance in hell that you have a schematic of
the dimmer?

9. What sort of test equipment can we presume as we toddle down the fixit
trail? A handheld AM broadcast band receiver is a hell of a good buzz
detector.


Jim


"Jonathan Goodish" wrote in message
...
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

It sounds like there are two likely possibilities: bad potentiometer or bad
power transistor. When I crank the pot up so that the internal radio lights
illuminate, I get an escalating buzz that quickly squelches the panel mount
intercom. Although I can hear the buzz in the headset (but not as easily in
the aircraft speaker), I can also hear radio broadcasts, but I can't
transmit and am unable the communicate via the intercom. When I turn the
lights back down, the problem goes away.

However, when I key the hand mic with the radio lights illuminated, the
radio lights dim significantly. I thought this was odd considering I don't
ever remember that happening in the past, and it certainly didn't happen
previously when the mic was keyed via the intercom.

A similar circuit that controls the panel lights produces no such effects.



Any ideas? Does this sound like a bad potentiometer, transistor, or

something else?