Super Sleuths: Solve This Transmit Squeal Problem

UPDATED. SEE POST #7.

On occasion, the unfortunate receiver and I hear a very loud, ear-piercing squeal, I'd guess about 1,500hz. Voice modulation is fine, but drowned out by the squeal. What's the cause?

Sometimes hours pass between occurrences, not related to temperature, whether or not the engine's operating, and when the phenomenon starts, the squeal is faint, builds, and a couple of mike keys later is in full swing, then after some period of time, maybe minutes or hours or days, disappears - until the next occurrence.

It's not the SL-30 radio (subbing a -40 didn't help).
Shutting off all electronics other than the radio doesn't help.

Here are some thoughts for rumination:

A weak ground at the antenna, mike jack, coax connector, shielding... but manifestation is much more like a semi failure progression, not a mechanical chatter.

Even though the PS3000 intercom is off, the pilot's mic still passes through, so maybe it remains the antagonist though powered down?

Hmm. I haven't tried another headset (a Lightspeed Zulu 2). That's the first alteration I'll make since the squeal is much like feedback from placing a mic too close to the speaker.

Any "Ah, ha!" moments out there?

John Siebold

Try a dumb, non-noise cancelling headset...

I had a fault with my Lightspeed Zulu 2, that when the FO pressed his PTT I would get the feedback squeal in my headset.
The fix was to move the two sliders for volume on the Lightspeed Zulu 2 down to about half way instead of MAX.
My intercom Vol and headset Vol are both around half way.

I agree with Gil, try a different microphone which of course means different headset.

I started phase 1 with the telex echelon that I had been using for years but it did the terrible squeal. I could never tune it out via mic gains and other adjustments.

I got the impression it had something to do with the difference between carbon and electret mics.

Changed headsets. Gone. Never looked back.

This happened to me with two different Lightspeed Zulu headsets. Both times I sent the headset back to Lightspeed. They did some kind of internal adjustment and have worked trouble free ever since.

John,
I don't know if this helps but I am having a similar problem. On first flight this week I didn't have sidetone. What a pain. I read the Garmin instructions and set the sidetone higher. Perfect on ground channel 121.--. However when I went to tower 135.25 I got a unbelievable feedback (squeal). Adjusted the sidetone partway back down and now it is gone. I would like to find a better solution as the sidetone is still a little low. I am also using the Zulu's.

Recent Info

I called Lightspeed. Customer Service without hesitation suggested moving DIP switches 3 and 4 to their opposite setting (if on, turn off; vice-versa). She suggested this usually solves a grounding issue.

Obviously, others encountered and reported this phenomenon enough to make Lightspeed jump to conclusions. It may take many flight hours before I'm confident the problem is corrected - or not - by this fix.

So now the concern. If this is the fix, why would a headset that worked well for hundreds of hours need reconfiguration?

John Siebold