Terrible whistle using Garmin GNC255A

PH-SCP · #1 11-13-2020, 04:33 AM

Hi Guys,

When 8.33 Khz spacing became mandatory in Europe, I purchased a Garmin GNC255A VHF radio. Also bought a GMA240 audiopanel and kept the SL30 as a second radio.

When using the GNC255A I get a terrible whistle when pushing the PTT button. When I start to talk the whistle reduces but does not go away completely.

I tried every combination on the audio panel but that does not help.
Selecting COM2 (SL30) gives a crisp and clear sound without any whistle.

With the experiment, I think I isolated the problem to the GNC255A but I am no avionics expert. Thought I would give this forum a try for advice.

It sounds as if there is some sort of open loop whereby I re-transmit my own transmission. I pulled out the co-pilots headset too without success.

Is there some sort of internal setting I can tweek to make the whistle go away ?

Thanks in advance !

Walt · #2 11-13-2020, 06:54 AM

Try turning down the MIC gain in the 255.

scard · #3 11-13-2020, 06:57 AM

Antenna line issues?

Southern Pete · #4 11-13-2020, 07:28 AM

Look in the manual under side-tone - its a software function in the set-up menu. Turning it down should help.

PH-SCP · #5 11-13-2020, 07:49 AM

Thanks for the quick replies.
Antenna is not the problem. I switched antenna's between COM1 and 2 and the issue remained the same. I will try "Mike Gain" and "Sidetone" and keep you informed how this works.

PH-SCP · #6 11-14-2020, 10:52 AM

Turned the Microphone Gain dowm from 12 to 6 and that took the whistle away. Thanks a lot for your input, happy days ! You guys rock 👍👍