On Jun 3, 2:54 am, mikem wrote:
On Jun 1, 7:35 pm, mikem wrote:

On Jun 1, 5:03 pm, Steve - KDMW wrote:

Another tidbit of info. The intercom comes back to normal if the
Garmin radio is on and not selected. It doesn't load down the intercom
simply by virtue of being on, it has to be on and selected.

...

Another thought: If the summing resistor normally in series with COM2
input inside the Audio Panel has been removed or shorted out by some
strapping jumper, then you would get this behavior. I'm not familiar
with your audio panel, but each input should have a series resistor
between the input pin and the summing junction (op amp input). If for
whatever reason, this resistor is missing, the SL40's low output
impedance would upset the balance at the summing junction. Opening the
COM2 switch would make the problem go away. If the old COM that was
removed had some resistance in series with its output, then it would
NOT have upset the balance...

I got ahold of a KA-134 schematic and discovered that there is no
summing amplifier in a KA-134. The radio inputs go through a purely
mechanical switch matrix for the Speaker switch and the Both switch,
but are fed directly to the Headphones output of the audio panel. It
is purely a mechanical swtiching device for the audio portion. So the
SPA-400 is seeing the radio output directly, and not isolated/
conditioned by the audio panel whatsoever. I don't see how I can
attach the schematic file here (a .jpg file), or I'd attach it so you
can see.