Weird feedback through the intercom

[aggie06]

I’m out of ideas. I have an intermittent feedback noise coming through my headset - sort of a warble. It has happened a few times and I’ve tried a bunch of things to isolate it to no avail. There are two separate KMA-24H intercoms in the airplane. The one time I had a passenger along when it happened, she got it in her headset, too, which is on the second audio panel. I have tried to turn off everything electrical I could (radios, lights, etc) and still gotten the noise. Last time I even tried turning off the alternator field. Noise kept going. All I can do is turn the overhead speaker on and turn up radios all the way while I turn the volume on the headset down. No noise in the overhead speaker and the noise seems to get much lower in the headset. This may happen a couple flights in a row and then not again for a year, which makes troubleshooting hard.

I have tried 3 different headsets (passive and active noise cancelling) with the same results. That makes me think that it may be related to interference somehow acting between the headset plug and the intercom, but I am out of ideas and am not even sure how to test that hypothesis. I’d like to figure out what is going on instead of just throwing a new audio panel at the problem and hoping.

Anyone have any ideas?

 

[SoCal 182 Driver]

Have you changed anything recently? New radio? Next, I would first try removing one audio panel and test, and then the other. If the problem doesn't appear when using only one of the panels, then there's probably some sort of cross-talk. If it appears with one panel, but not the other when you swap them, then the one where it appears is likely the culprit.

I'm an amateur, at best. Hopefully one of the pros will chime in with other (and better) ideas.

 

[denverpilot]

Next time it starts, unplug the mic only of each headset attached to the system. See if it stops.

Also be aware that ANR headsets with air leaks and worn out ear seals will usually “warble” as the ANR tries to fight the noise leaking into the ear cups. Is everyone aboard hearing the noise or only you?

Some really old ANRs will do it when battery voltage drops also. Keep fresh batteries handy and see if a swap stops it, if it’s only one person.

Why two intercoms? I can think of a couple of ways they could interact with each other squelch-wise, but... not sure I’d list that as a high possibility if this setup once worked and there have been no changes, including headsets.

 

[aggie06]

Nothing has been changed on the airplane, but I will try removing audio panels one at a time. The reason it has two to begin with is a for the FBI - when they used the plane, they had a guy in the right seat who could control all his own coms, including whatever the guy in the back seat plugged into his canon plug back there.

This wasn't an ANR air leak warble - it was a lower frequency sound and appeared with passive headset as well. Everyone is hearing it and the passenger deals fine by turning volume all the way down and keeping the ANR going.

I'm using it today, so I'll take an allen wrench and see if pulling the radios one by one in flight stops it. The fact it is intermittent and only in flight makes it extremely hard to diagnose, but that might be the way to do it. I hadn't thought about interference happening with the radios off.

 

[denverpilot]

Nothing has been changed on the airplane, but I will try removing audio panels one at a time. The reason it has two to begin with is a for the FBI - when they used the plane, they had a guy in the right seat who could control all his own coms, including whatever the guy in the back seat plugged into his canon plug back there.

This wasn't an ANR air leak warble - it was a lower frequency sound and appeared with passive headset as well. Everyone is hearing it and the passenger deals fine by turning volume all the way down and keeping the ANR going.

I'm using it today, so I'll take an allen wrench and see if pulling the radios one by one in flight stops it. The fact it is intermittent and only in flight makes it extremely hard to diagnose, but that might be the way to do it. I hadn't thought about interference happening with the radios off.

Just to be clear I wasn’t suggesting to pull the radios or the intercom but yeah, that’s a troubleshooting technique. I was suggesting to pull the mic cables out one at a time to see which input might be causing it.

Understand the FBI thing. The CAP and ex CAP 182s have an interesting STCed split audio panel setup including back seat PTT for the right side.

Interesting problem. Would be fun to troubleshoot in person. I like the weird ones.