The problem comes from the
"click" switch at the end of travel of the pot. The entire string of
post lights & internal radio lights is switched directly with the little
"click" switch. On my airplane, this is 2.5A tungsten through a little
switch!

And not to forget the inrush current which is somewhere between 3 and 10
times the quiescent current with an incandescent bulb.



*I* think the OP has a bad alternator diode or 2. Turning on the panel
lights draws 2A which starts the whining. Pressing transmit draws 8A
which drowns out the transmitter with the whining and pulls the system
voltage down so that the lights dim. Cherokee 140's have a very marginal
40A alternator to begin with.

Hm. His complaint didn't seem to be "whine" but more of a static or
rat-a-tat-tat sort of noise. Let's wait for the OP to get back to us with
exactly what he is hearing.



If the OP checks by turning on pitot heat or the landing light I think
you can diagnose this right away.

THat's true. If it is alternator diodes. I was just thinking that the
dimming he was seeing was from not having the engine turned on and running
on battery alone. That was one of the next questions as soon as he finishes
answering what I've already asked him.


Jim