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Tone throught the audio panel

Printed From: Avidyne
Category: Avidyne General
Forum Name: AMX240 Audio Panel
Forum Description: Topics on the Avidyne AMX240 Audio Panel
Printed Date: 27 May 2024 at 2:30am
Software Version: Web Wiz Forums 12.01 -

Topic: Tone throught the audio panel
Posted By: M20Kid
Subject: Tone throught the audio panel
Date Posted: 11 Aug 2016 at 2:41pm
I have a 240 audio panel that has been in the airplane for over 6 months.  On the flight home from Oshkosh last week I noticed a new high pitch tone coming through the intercom.  In trying to isolate the tone I tried adjusting the volume control of all the radios and the intercom.  No matter what I turned I couldn't seem to affect the volume of the tone.  The way I solved this issue was to select the Phone symbol and turn it Green - the tone was gone.  Deselect the Phone and the tone returns after a second and come on with increasing volume, just as it would if it was a music source that was overridden for com traffic.  Select the Phone back to Green and the tone was off again.

I was on a trip again yesterday and had the exact same issue.  There is no phone or other device connected to the bluetooth (as far as I'm aware).  Has anyone else had this issue?

The concern I have is that this is a new issue that just started last week.  I do have an IPad Air; I suppose it's possible it has started connecting through the bluetooth.  Anyway, are there any suggestions??

Posted By: dpcraig
Date Posted: 12 Aug 2016 at 8:04pm
On the subject of the 240, I read before I purchased the unit the unit will allow you to monitor a frequency and when the primary com receives a transmission it will mute the monitored frequency. My avionics shop has made contact with Tech support but not heard back from them yet. Does anyone have knowledge of that functionality?


Posted By: dmtidler
Date Posted: 13 Aug 2016 at 11:05am
I don't recall the MON (standby) audio ever muting when the active com receives a transmission and could not find any mention of automatic MON audio muting in the AMX240 Pilot Guide.

I use the MON feature of my AMX240 frequently and have the standby com volume set to a lower level than the active Com volume in the AUX - Audio Tab of my IFD540. This allows the active com reception to always be heard over the standby com reception when using the MON feature.

Posted By: edanford
Date Posted: 13 Aug 2016 at 12:06pm
I believe this was supposed to be a feature in the early development  of the AMX240 and was later dropped.


Posted By: AviJake
Date Posted: 14 Aug 2016 at 1:03pm
I'm not aware of hearing of a field report of a tone like M20Kid reports above.  If still present, the best recommendation is to contact Avidyne Tech Support and have them work the problem.

As for muting during MON operations, it is true that was the original intent but it we did not have success in getting that to work and was dropped well before the first product ever shipped.

Steve Jacobson

Posted By: AUXAIR
Date Posted: 14 Aug 2016 at 1:24pm
Does the tone vary with engine speed, or is it constant?

David E.
Cessna 182 RG II

Posted By: M20Kid
Date Posted: 17 Aug 2016 at 12:33pm
The tone is constant and it's something that is being generated somewhere in the radio system.  When I deselect the phone (back to white) after a couple of seconds the tone slowly comes back up, just like music would if muted for comm traffic.  The intercom thinks it is supposed to be playing something and this high pitch tone is the result.  Like I said, pressing the phone to green makes the tone stop.  Seems like a tech person would be able to isolate this pretty quickly.

Posted By: MysticCobra
Date Posted: 17 Aug 2016 at 5:59pm
I have something similar in my setup.  What I've noticed is that certain maneuvers can cause the tone to decrease or disappear.  I suspect there's a wire somewhere that has enough play that it can come close enough to a source of interference.

I had the opposite thought.  Figured I'd be writing a blank check if I asked the avionics shop to track it down.

Posted By: petersk
Date Posted: 19 Aug 2016 at 9:24pm
Your best test for this tone is to start by varying the engine speed. Does the tone change frequency? If so, try turning off the alternator side of the master switch. Odds are the tone will go away.

If that's all true, then you still have the question of why the alternator is generating that tone? It is most likely a grounding problem between some of your radios or connections. Perhaps a remote headphone jack has become grounded? Or you've hard mounted a music jack to a panel, without an isolation washer?

The hum in these cases comes from grounding loops, and when you turn on the telephone input it is muting whatever system has that grounding loop. The fact that the tone comes back in a volume ramp suggests the noise is in your music input.

One other cause can be when you use a power supply to feed your music source device... a charger on your cell phone while it's also connected to your music input, for instance. That creates a ground loop, which causes the hum.

There are various potential solutions for these things, but they basically boil down to having a single ground point and using isolation transformers when you just can't avoid that second ground, as in the power supply example.


Keith Peterson
Cardinal Flyers
Cardinal RG N177KP

Posted By: M20Kid
Date Posted: 25 Aug 2016 at 3:41pm
Keith, In my particular case I don't think it's an alternator noise because it only comes through on the one channel of the audio panel and it can be muted.  It seems that it must be some signal coming over the bluetooth phone link, but no bluetooth device is active at the time.

I'm hoping my avionics guy who installed all the new stuff can find the noise.

Posted By: DavidBunin
Date Posted: 26 Aug 2016 at 9:01am
My guess is that your music input jack is not isolated from the airframe.  I had the same problem until I put the music jacks on a plastic sub-plate.  The main clue for me is the muting behavior you describe.


Posted By: M20Kid
Date Posted: 09 Sep 2016 at 7:33pm
Thank you David!  I'll let me avionics installer know what you said and see what he can find.

Posted By: muddy00
Date Posted: 25 Jan 2019 at 2:58pm
You ever find the problem or solution to this ?

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