Tone throught the audio panel

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??

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.

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.

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.

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.

 

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.

David

You ever find the problem or solution to this ?