We have 3 VHF radios in a Bell 206L-1. The #1 is with a Garmin 420, the #2 is a Collins and the #3 is a Technisonic. If I am monitoring all 3 radios and transmitt on the #3 there is very loud noise level on the side tone. If I have the #1 and #2 on the same frequency only the #1 is a problem. I appears that the #3 when transmitting is breaking the squelch on the #1 and is transmitting only noise on the #1. If I deselect the #1 and do not monitor it the side tone goes crystal clear.

We have relocated antennas, installed a grounding plane with the one Technisonic that was mounted on fiberglass. We even replaced the Technisonic with another unit. A noise filter was installed and none of this resolved the problem.

I was wondering if anyone out there has had similar probems or has any fresh ideas.