On Jun 3, 6:28 pm, Steve - KDMW wrote:
....
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.

Ok, then the fact that the Narco's headphone audio output had a higher
output impedance than the SL40 must be the key. Just go ahead and put
47 to 220 Ohms in series between the SL40 and pin D, and call it
good... You will have to run the volume on the SL40 slightly higher,
but that probably wont matter...