Intercom Problem with New Radio

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.

That sounds like the SL40's intercom function is competing with the
SPA400

On Jun 1, 9:35 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.

That sounds like the SL40's intercom function is competing with the
SPA400

I forgot to mention... I also checked to see if the Sl40's pin 12 was
shorted to ground, which would enable the SL40's Intercom function and
it wasn't. But just for the heck of it, I changed the Sl40's intercom
volume from 000 (slaved to the volume knob) to 001 (lowest volume
possible) to see if it would have any effect and it didn't.

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...

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...

The KA-134 has an open chassis. I could take a look. I'll trace the
solder track from pin D. If I don't find anything, I'll try a 100-ohm
resistor on the Com 2 input.

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.

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...

On Jun 3, 10:48 pm, mikem wrote:
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...

Success... I did a cut and jumper mod and added a 220 ohm resistor to
the Com 2 audio input of the KA-134 and now everything works great.
Intercom value is equal (loud and clear) with either Com1 or Com 2
selected.

Thanks for all of your input (no pun intended).