Trouble with Terra 230 Audio panel

My partner and I own a 1967 Cherokee 140 with a Terra 230 Audio panel.

Currently we use COM2 as our main COM. Whenever we attempt to listen
only on COM1, COM2 drops to almost inaudible, with the plane running
it is inaudible. If you put COM1 on the speaker it is better, you can
still hear COM2 in the headphones and COM1 on the speaker.

This came up while flying to SnF this year with another plane. We had
ATC in COM2 and quickly learned that if we tried to listen to our 2nd
plane on COM1 through the headphones we lost our ability to hear ATC
in COM2.

Transmitting is not a problem. We can transmit on both COMs.

COM1 is a Narco Mark 12 (in the shop getting the upgrade)
COM2 is a Bendix King KX125

Any ideas why this is happening?

Also can someone point me in the right direction to find the pin out
diagram for a Terra 230. It seems they've gone out of business and I
can't find a resources on the web.

Thanks for any help you may provide.

Mark