Uncontrollable Microphone Squeal - How to Resolve

Fearless Tower

Fearless Tower

Touchdown! Greaser!
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Fearless Tower
Could use some ideas on where to look to troubleshoot a mic transmit problem.

Issue is related to an HGU-55 helmet. When using the helmet in the T-6 (Garmin SL 40 Comm and PS Engineering PM3000 intercom). The helmet was refurbed by Gibson and Barnes and has a civilian electret mic. G&B previously bench tested the helmet and found no issues. I have used the helmet in other aircraft with no issues.

In the T-6, the mic seems to work fine with the PTT intercom. But as soon as you press the PTT button for the transmit, you get a very loud squeal that makes it impossible to effectively use.

Other headset mics in the same aircraft seem to work fine, but this is the only helmet I have.

Any ideas where to troubleshoot?
 
Have you tried pushing the plug in farther, or barely pulling it out a little as you try to transmit?
 
The feedback can come from two sources ... RF and audio.

RF feedback comes from when there is enough of a mismatch at the antenna that RF comes screaming back down the outer braid of the coax and sprays all over the internals of the airplane. The innards of the electronics rectify the RF to audio, patch it down the mic line into the radio and round and around we go. Most CIVILIAN headsets solve this problem with a bit of bypassing, but military headsets use such a low impedance microphone that it isn't necessary (something on the order of 5 ohms). Try wrapping a piece of tinfoil around the antenna coax ... say a piece about 3" square or so. Slide that sucker back and forth along the coax (at least 48") and see if at some point the squeal goes away.

The other cause is audio feedback, but I doubt you have a speaker going in the aircraft, so we can sort of eliminate that.

Report back ... and don't forget what the "experts" in these groups always attribute it to ... "a loose wire, a broken ground, or something else"

Jim
 
I’d be willing to bet you’ve got a loose wire somewhere.
 
wanttaja said:
Some Icom radios have a similar problem. Folks have had some success wrapping the headset cable around ferrite cores:

http://www.aircraftspruce.com/catalog/pspages/icoma6a24.php?clickkey=2129817

I strongly suspect these WON'T do the trick for you (something else going on), but they're only $10 and simple to install.

Ron Wanttaja
Click to expand...

Here's that info, from ICOM:

http://icom.custhelp.com/ci/fattach/get/1265/0/filename/A24+Headset+Noise+Modification.pdf

Of course, this didn't help with my ICOM/DC headset/remote PTT combination.
 
Huh. Just had that exact same problem happen last week while travelling in that -10F degree nasty weather, with an otherwise known good Lightspeed headset. Worked with the intercom, squealed to all get-out with the PTT.

I am attributing it to a stiff and poorly sealing foam earseal, and am holding judgement until I fly again in warmer weather.

In order successfully to talk to the tower, I finally figured out I could cup a hand around the mike and my mouth to block the errant audio.
 
Might be that the Side tone is set to high. You'll only hear side tone when the Radio is TX'ing so it won't affect the intercom, just when you key up the radio. The side tone is coming out the ear speakers in the helmet and feeding back into the mic. Place your hand over the mic to shield it while testing, if it's still an issue it's RF related, if your hand stops it, it's audio related. Turn down the side tone or if able turn down the MIC gain. (might be a small pot on the mic itself.)
 
Don’t rule out the helmet. I had an hgu55 that was wired up by Gibson and Barnes. Everything they did was complete ****. Had lots of issues with their work. Never again will they get my money. Headsets Inc out of Texas completely required the helmet and installed their anr system for me. No issues after they worked on the helmet.

I had feed back issues with ptt and intermittent shorts in the speakers. It was just crap all the way around.
 
Pat Elliott said:
Might be that the Side tone is set to high. You'll only hear side tone when the Radio is TX'ing so it won't affect the intercom, just when you key up the radio. The side tone is coming out the ear speakers in the helmet and feeding back into the mic. Place your hand over the mic to shield it while testing, if it's still an issue it's RF related, if your hand stops it, it's audio related. Turn down the side tone or if able turn down the MIC gain. (might be a small pot on the mic itself.)
Click to expand...
This was the issue with my SL-30. If the SL-40 is like the SL-30, sidetone level is a menu driven procedure described in the manual under "Advanced Operation", "System Mode", "Comm Options".

"Sidetone Level
This function displays and adjusts the sidetone audio level that is heard when the transmitter is keyed. Turn the SMALL knob to change the value. The range is from 1 to 100. Setting the value to variable slaves the sidetone level to the volume control knob."

In my case, changing the setting from a fixed value to variable (slaved to the volume control) cured the feedback squeal. If that doesn't work, try changing it to a lower fixed value until the squeal stops.
 
I downloaded the SL-40 User Guide, the sidetone level range is 0 - 255. Setting the value to 0 (zero) slaves the sidetone level to the volume control knob.

SL40%20Sidetone%20Level_zpssdszt2jh.jpg
 
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Stan Cooper said:
I downloaded the SL-40 User Guide, the sidetone level range is 0 - 255. Setting the value to 0 (zero) slaves the sidetone level to the volume control knob.

SL40%20Sidetone%20Level_zpssdszt2jh.jpg
Click to expand...
I saw that to. I’ll play around with the radio settings when I fly it next and see what happens.
 
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