Anyone has a "Fone" CB on their CB panel

[Tommy]

I have managed to hunt down the source of static whine on my KX175BE. It goes away when I pulled the "fone cb" but nothing was turned off when I pulled it. So what is it for?

 

[Jerry 5TJ]

It could be a remnant of an old headphone/speaker amplifier system. 

If you still have the KMA20 audio panel that CB may be connected to an intercom, or what was an intercom. 

Attached is a partial schematic for the KX175B from the installation manual.

There is often a lot of orphaned wiring in our older planes.

 

KX175 Schematic Snip.pdf

[FlyDave]

Tommy,

 

I had one of these in my previous J (N1160N). Believe it or not It was for an "Air Fone" (satellite phone) - in a Mooney 201!

The CB was connected to a 12 - 24 volt converter in the avionics bay. This converter was still in the plane but the "Air Fone" equipment had been removed.

On my way back from Oshkosh in 2012 I was in the clouds at 8,000' and hit a bump. I heard a chirp in my headset and smelled a puff of smoke - yikes! I waited a few seconds, then a few more and a few more. Everything seemed OK so I cautiously continued to my next stop. On descent to my fuel stop I hit another bump and got another chirp and puff of smoke. I told the controller I wasn't declaring an emergency but I needed lower NOW - which he gave me and put me in VMC 2 miles from my destination.

I began troubleshooting as soon as I landed. What I found was the "Fone" CB popped and if I turned the avionics master on with "Fone" CB in it would pop. So I figured out what it went to and disconnected the cable from the converter in the avionics bay and left the CB pulled for the remainder of the flight home. I didn't have any more problems after that.

Let us know if you have the same thing in your avionics bay and if disconnecting it fixes your static issue.

 

Dave