Bose A20 squeal on transmit

David Kagey

David Kagey

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Had an issue for the first time today. I have a Bose A20 with LEMO plugs. Tried to transmit and got a loud squeal. Could receive fine. Switched radios from 750 to backup KX155 and did the same.
Switched to the copilot side and got the same thing.
Then tried with an old non ANR headset in the original dual plugs. Problem went away and transmit is fine. Seems to rule out radios, audio panel and PTT. Agree?
Recent maint was the replacement of the P lead wire from the mag to the ignition switch. Looked under the panel and didn’t see anything loose. Headsets are getting power okay. Power lights on.
Anyone run into this before or have any suggestions? Read an old post about the problem if both comms are on and will check that tomorrow. Think only one but will be sure
Thanks
 
David Kagey said:
have any suggestions?
Click to expand...
Does the squeal happen with intercom only? Does the squeal transmit when you hear it or is it only in the A20? Also make sure all connections on the A20 are clean and secure.
 
Bell206 said:
Does the squeal happen with intercom only? Does the squeal transmit when you hear it or is it only in the A20? Also make sure all connections on the A20 are clean and secure.
Click to expand...
Was only me so didn’t check the intercom. Both radios receive fine and when using the A20’s with lemo on either side I get the squeal. It is very loud and high pitched in my ear and I asked for a radio check and the squeal transmits too. Makes me unreadable. Take the Bose out and plug in my old headsets with the dual plug and all is good. Works fine on both sides. To me this removes the PTT, audio panel and radios as the culprits. Suspicious of the work done on the P leads….new wires. Did they do something to the lemo plug. Used good shielded wire on the new P leads. Other one is the A20. Buddy has an RV with lemo and am going to plug them in to his plane to rule out a mic issue with the headset. Can you think of anything I missed? Thanks. Frustrating.
 
Bell206 said:
Does the squeal happen with intercom only? Does the squeal transmit when you hear it or is it only in the A20? Also make sure all connections on the A20 are clean and secure.
Click to expand...
Just thought of something I’ll check tomorrow. I should have turned in on speaker to see if it squealed then or just thru the A20. Thanks. Glad you made me think of that.
 
David Kagey said:
and the squeal transmits too.
Click to expand...
Could be your A20 mic has developed a fault. THis is just an internet guess but since a different headset does not duplicate.... Perhaps plug your A20 into another aircraft and see if problem follows?
 
Try that same headset plugged into a different jack. If the squeal follows, you've likely got a ground loop in your audio wires. Perhaps the shielding has been grounded at both ends (bad) or (worse yet) the wires aren't shielded at all. Be sure the jacks are insulated from ground (if they are supposed to be per the intercom/audio panel install manual). Audio is finicky. Could even be a squelch issue. Lastly, is it possible your transmission got stepped on or you keyed up and stepped on someone else? That's a noisy affair
 
Bell206 said:
Could be your A20 mic has developed a fault. THis is just an internet guess but since a different headset does not duplicate.... Perhaps plug your A20 into another aircraft and see if problem follows?
Click to expand...
Thanks. That’s the plan. Hope to do it in the next couple of days and will advise
 
Doug Reid said:
Be sure your ear seals fit snugly.
Click to expand...
I think they were and will check tomorrow, but out of curiosity…how come? Something to do with the noise attenuation? I put new seals on both sides within the past few months
 
Der Fliegermeister said:
Try that same headset plugged into a different jack. If the squeal follows, you've likely got a ground loop in your audio wires. Perhaps the shielding has been grounded at both ends (bad) or (worse yet) the wires aren't shielded at all. Be sure the jacks are insulated from ground (if they are supposed to be per the intercom/audio panel install manual). Audio is finicky. Could even be a squelch issue. Lastly, is it possible your transmission got stepped on or you keyed up and stepped on someone else? That's a noisy affair
Click to expand...
Thanks. Yes, it does follow to the copilot side to that lemo jack. As I shared, if I plug an old headset into the old dual plugs, all is good. The only recent "change" is that I had the P lead from the mag to the ignition switch changed. it was working fine before that. I suspected the Bose mic, but thought too coincidental that it happened right after the P lead replacement. I'll pass on your comment about the ground. The A&P/IA is going to look at it again today.
 
Der Fliegermeister said:
Try that same headset plugged into a different jack. If the squeal follows, you've likely got a ground loop in your audio wires. Perhaps the shielding has been grounded at both ends (bad) or (worse yet) the wires aren't shielded at all. Be sure the jacks are insulated from ground (if they are supposed to be per the intercom/audio panel install manual). Audio is finicky. Could even be a squelch issue. Lastly, is it possible your transmission got stepped on or you keyed up and stepped on someone else? That's a noisy affair
Click to expand...
I'll check the squelch, but since it worked fine when I plugged an old headset into the dual plugs, I ruled that out. Also, no on stepped on. Small airport and had to wait abit to even get a radio check. My guess is that when replacing the P lead on the ignition switch, they did something to maybe the ground. I wasn't sure about that in that the headset is getting power okay and it works fine on receive. Had no issue listening to traffic when I was doing my runup and then the loud squeal when I went to transmit departure.
Just looking for ideas. The only "change" was the P lead at the switch. Thanks again
 
Too high of mic gain in com settings can cause a squeal on transmit, but that doesn't sound like the symptoms described here
 
FORANE said:
Too high of mic gain in com settings can cause a squeal on transmit, but that doesn't sound like the symptoms described here
Click to expand...
I once had a similar problem; squeal when an ANR headset was used, no squeal with an non-ANR set. Fixed it with a reduction of sidetone volume (Icom handheld), which, I think, is essentially a reduction in mic gain. The ANR headset hasn't had an issue with any other radio.

It was either some sort of electronic feedback or some kind of acoustical coupling.

Ron Wanttaja
 
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