Rheostat Audio Interference

In article ,
"RST Engineering" wrote:
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?

Nothing, other than the fact that they are activated by the same switch.
in the Cherokee.



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?

No. With the dimmer in, the instrument lights (radio backlights)
illuminate and dim just fine.




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.

I have two circuits, panel lights and the radio backlights. The circuit
in question is the radio backlights.

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.



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.

That is correct. The hand mic causes the radio backlights to go from
full bright to dim when it is keyed.



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.

I don't necessarily have a fixation on anything, otherwise I wouldn't be
asking for opinions. It's pretty obvious that something is wrong, but I
don't know what that "something" is at the moment.



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?

I can hear transmissions at all times, even when the buzzing is present
through the intercom when the radio backlights are at full brightness.



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

Yes, I do.



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.

I have an AM radio and a digital VOM and that's about it at the moment.




JKG