Too much sidetone / mic gain in GNS 430

[Ultranomad]

I'm finally finding time to do some work on the radios in my AA-5B. The problem persisting ever since the purchase (as witnessed by Genghis the Engineer) is a loud noise in the headsets caused by the microphones picking up too much cockpit noise. Previous owner flew with David Clark ANR headsets, while I have two simple PNRs - a Harry Mendelssohn's HM40 and an old DC with an articulated mic boom, so I thought I would first tweak the audio gain and see if I can optimise it. As I connected the GPU and powered up the radios, I realised the magnitude of the problem: even the softest whisper was heard in the headset at least as loudly as a normal conversation. No wonder even the noise-cancelling microphones would pick up a lot of engine noise in flight.

The radio stack has two very different animals in it, a KX155 and a GNS430. As the latter one can be easily adjusted directly from the front panel, I switched it into configuration mode and reduced sidetone from 30 to 01 and mic gain from 45 to 01. To my surprise, it didn't seem to become much better - my soft whisper is still amplified to nearly full volume. KX155 has about the same gain level. Unfortunately, I can't contact the previous owner to ask if he had the radios specifically tuned to his ANR headsets, but the current situation seems clearly abnormal to me.

The audio panel is a KMA24, which has nothing but the switches in the microphone circuit, so it's not to blame. Has anyone encountered anything like this? And why does GNS430 fail to respond to my adjustments?

[wigglyamp]

Is the noise problem on the intercom or only when transmitting?
Playing with the mic gain on the GNS430 as described is going to reduce modulation depth so transmit is going to be very poor - carrier power won't be affected but there will be very little signal. It's quite likely that you have a blind intercom amplifier fitted (ACS-3 is common on many AA5s) and it's the gain of this that will be affecting intercom levels and consequently the background noise.

[Ultranomad]

Wigglyamp, thanks!
I understand the consequences of reduced mic gain, it was done merely for diagnostic reasons. The problem is on the intercom - it's the noise you hear all the time, proportional to the loudness of the outside noise, nothing fatal but heavy on the ears. Up until now, I would suppress it by turning the mic switch knob on the audio panel to an unused position when not maintaining active radio communication.
The documentation I have does not mention an intercom amplifier, but given a few other discrepancies I discovered, I won't be surprised if there is one lurking in there. I couldn't find any information on ACS-3 - what does it look like and where can I get a manual for it?