Radio feedback problem

[Spitzair]

Hello everyone,
I'm hoping someone can help me out with an issue we're having with one of our Astar helicopters... We have 2 comm radios in this machine among other things, and when transmitting on Comm 2 you get this horrible squealing feedback, sounds like when you hold a microphone by a speaker... Here's what we've tried so far:
-Switched out radios
-Switched out Audio controller
-Switched antennas
-Replaced the entire coax cable from radio to antenna
-Tried different headsets
-Put grounds from different locations on the antenna
We hooked up a dummy load instead of an antenna as well and when we do this we don't get the feedback. I've run out of ideas and was hoping someone here might be able to help is out! Oh and I forgot to add that Comm 1 works flawlessly.

[Dougie_diesel]

Is it a ground loop?

Instead of moving the antenna (and coax?) grounds, have you tried REmoving them? Temporarily of course.

[Hasherucf]

So what maintenance did you before this happened . Or has it always been this way.

[lowfat]

eurocopter usual have "simulcom" which reduces the sensitivity of the rx side of the none transmitting radio..
I would check the ground planes of the aerials.
if one is on the cabin roof as its composite there is a copper tape which forms the ground plain it can corrode/split.
is the feed back in comm 1 when you transmit? does the radio actually tx and whats the quality like?
is it a problem with your sidetone?

[aveng]

Check the bonding of the comm receiver tray and the braid on the antenna coax.

[Kiwiconehead]

Did you replace Com 1?

Just been thru this on a Bonanza with KX155s.

Com 2 feedback from Com 1 because no TX interlock/supression.

IF Audio from Com 1 feeding back into Com 2.