Bendix King KMA20 - Stuck mike open.

I had a "mike stuck open" problem on my 1977 Piper Archer.
The PTT switch tests out fine and seems that the KMA20 audio panel is the
culprit.
I worked all the front panel switches many times and the problem has gone
away for the moment.
The mike unstuck as I was working the Marker Hi Lo switch [Extreme right]
I can't locate a either a user manual or maintenance manual.
Can any one help with suggestions on obtaining a circuit diagram so that I
can trouble shoot the electronics.
Also any tips on using and fault finding the KMA20 would be appreciated.

Thanks guys.

Roy Page
Beloit, OH 44609