In article ,
"RST Engineering" wrote:
Nothing, other than the fact that they are activated by the same switch.
in the Cherokee.

The same switch as what? Are you saying that by turning the radio lights
pot from off to just barely on (switch snap) also turns on the nav lights?

Yes.


OK, let's agree on some terminology so we don't keep running down the same
rabbit path. Does the SAME dimmer run the instrument (post) lights as the
radio lights? If not, let's have an INSTRUMENT LIGHT dimmer and a RADIO
LIGHT dimmer.

No. We are talking about the radio light dimmer circuit.



You are correct, when I turn the pot so that the radio backlights
illuminate, I get noise that escalates in volume as I turn the lights
up. The lights appear to illuminate normally. When I get more than a
little current in circuit, the intercom squelches the mics on all
positions.

Squelching the microphones turn them off. Are you saying that the radio
dimmer turns ALL the microphones off when the lights start to get bright?

Yes.


OK, one last question. How long has this exact radio stack (radio,
intercom, headset, etc.) been in the airplane, and have you ever witnessed
it working correctly? Has anything ELSE electrical in the airplane changed
between the time it worked correctly and the present time?

It's all been in there for years. We used to fly quite a bit at night,
but haven't so much in recent years. We did the other night, and that's
when I noticed this problem. I don't remember it having been there
before. BUT ...

I didn't have much time tonight, but I did collect some more data. With
the master on, the avionics master off, and the radio light dimmer on,
the electrical noise is still in the headset even though the intercom is
not powered since the avionics master is off. The noise escalates as
the Century 1 autopilot gyro spins up... that's where the whine is
coming from. When I pull the connector out of the Century 1 to power it
down, the whine is instantly gone, but I can still hear scratches
through the headset when I move the radio light dimmer.

There is a terminal block under the panel to which it appears some of
the avionics is grounded, including my intercom. When I pulled the
intercom ground off of this terminal block and grounded it to the
airframe, even near to the block, the problem disappeared and I regained
normal intercom operation with the radio lights on full bright and no
noise in the intercom. I now wonder if the terminal block is grounded
to the airframe, or what is going on there. Would it be typical for a
terminal block to be anything OTHER than a ground? I didn't have
anything with me to measure voltage on this block or check for
continuity with ground.



JKG