Audio panel troubleshooting - feedback squeal

[Jonathan Hutchinson]

We've just acquired a new aircraft (UH1H). It has an A711 audio panel. (there's a second A711 in the rear that we don't intend to use). With only the master on (no radios powered), and just my headset plugged in - increasing the ICS volume to midrange will cause some hiss and then runaway feedback - deafening squeal. Covering my headset Mic doesn't help. I swapped the two panels (actually just removed the rear panel and installed it in the front). This had no effect. I tried several different headsets brands.

Any thoughts on how to troubleshoot this further?

 

[cgrab]

It appears you are just overloading the amplifier. Another headset may increase the impedance and reduce feedback. Plug in a headset for the other seat and set the volume to audible levels and then see if you get feedback.

 

[Bell206]

I tried several different headsets brands.

Are these fixed wing type headsets? A lot of helicopters are set up to use dynamic mics especially the older ones which can cause feedback issues if you use other types of mics.

 

[Jonathan Hutchinson]

It appears you are just overloading the amplifier. Another headset may increase the impedance and reduce feedback. Plug in a headset for the other seat and set the volume to audible levels and then see if you get feedback.

Thanks. I should have also said that we first noticed the problem during the ferry flight home - with multiple headsets plugged in. With the ICS volume set at a comfortable level, just a slight increase to the volume will cause the squeal. It's so painful when it happens that I'm tempted to glue it in place until we sort it out.

 

[Jonathan Hutchinson]

Are these fixed wing type headsets? A lot of helicopters are set up to use dynamic mics especially the older ones which can cause feedback issues if you use other types of mics.

hmm, well it's a '67 Huey, but the audio system is all fairly modern. Let me retest, but I think It happens with Lightspeed Zulu, Bose A20, DC, and modern Gentex helmets with and without ANR.

 

[Bell206]

well it's a '67 Huey, but the audio system is all fairly modern.

It's all in the microphone. There are modern dynamic mic systems as well. What did the previous owner use? Whether ANR or not is not a factor in most cases. System impedance can also be an issue but this sounds more like a mic problem. Good luck.

 

[MajorTurbulence]

Check that the plugs on the back of the audio panel box have not backed out. They need to be tightened to the box to allow full seating. I had what seemed to be the same issue, which frustrated me and my avionics guy despite switching audio panel units. Finally, the tech looked at the back of the box and found those loose connectors. I was however able to hear things well only from the planes internal speaker system during the problem.

 

[Jonathan Hutchinson]

It's all in the microphone. There are modern dynamic mic systems as well. What did the previous owner use? Whether ANR or not is not a factor in most cases. System impedance can also be an issue but this sounds more like a mic problem. Good luck.

Well, I plugged in a set of David Clarkes, and disconnected the Mic. So the Mic input is now open circuit and the problem persists. Today I'll try eliminating the ICS tie line which is a long run to the aft audio panel.

 

[Jonathan Hutchinson]

Check that the plugs on the back of the audio panel box have not backed out. They need to be tightened to the box to allow full seating. I had what seemed to be the same issue, which frustrated me and my avionics guy despite switching audio panel units. Finally, the tech looked at the back of the box and found those loose connectors. I was however able to hear things well only from the planes internal speaker system during the problem.

Thanks for the thought, but this audio panel has the spring-loaded locking connectors, so I'm quite sure they are seated.

 

[Bell206]

So the Mic input is now open circuit and the problem persists.

Was this a problem before you bought the aircraft or did the previous owner remove any avionics before your ferry flight? Do all the headset jacks have the same squeal or just the PIC jack? When you break squelch/key mic does the squeal transmit to the receive side? When you power up the radios does the squeal change or xmit out to the rec side?

 

[Jonathan Hutchinson]

Was this a problem before you bought the aircraft or did the previous owner remove any avionics before your ferry flight? Do all the headset jacks have the same squeal or just the PIC jack? When you break squelch/key mic does the squeal transmit to the receive side? When you power up the radios does the squeal change or xmit out to the rec side?

I'll record it for you today, with those scenarios. Thanks!

 

[Bell206]

Thanks!

May want to determine the answers to my first question on previous problem/avionics removal first. I'm no sparkie but have fought similar issues on "new" to you helicopters over the years and if you bought it with such a problem my checks probably wont do you much good.