Intercom Problem with New Radio

[Steve - KDMW]

I had my old Narco COM 120 replaced with a new Garmin SL40 and now
when the new radio is turned on my intercom volume drops by at least
half. Anyone seen this before? It has the avionics shop baffled. I
have an SPA 4000 intercom and a King KA-134 audio panel. I looked on
the Bendix website and I see a notice in their manual's list for a
"blocking diode for audio muting" installation but I can't get the
manual without a dealer login. I wonder if that has any relevance.
Anyone come across this before?

Steve
KDMW
PA28-151

[mikem]

On May 27, 5:32 am, Steve - KDMW > wrote:
> I had my old Narco COM 120 replaced with a new Garmin SL40 and now
> when the new radio is turned on my intercom volume drops by at least
> half.

Steve, coupla questions?

Is the "HEADPHONE" OUTPUT of the SL40 wired into a COM1 or 2 INPUT on
the Audio Panel?

Is the OUTPUT of the audio panel wired to the SPA400 INPUT?

Is the SPA OUTPUT tied to the aircraft headphones?

Have you defeated the built-in intercom inside the SL40?

Did everything work right before the SL40 install? (All the installer
should have had to do was wire the SL40 headphone out to the whereever
the previous COM was wired)

It sounds like they may have tied two OUTPUTs together, which with
modern avionics is a no-no. Even though the headphone audio is
nominally at 500 Ohms impedance, the OUTPUT impedance of the headphone
amplifier inside the SL40, and the one inside the audio panel are
really a voltage source with VERY LOW output impedance, and two of
these cannot be tied together ( and there is actually no reason to tie
them together) without using series resistors, otherwise they will
fight each other, sort of like your symptoms.

[mikem]

On May 27, 9:28 pm, mikem > wrote:
I forgot to mention, I have a KX155, SL40, KMA24, SPA400, and other
stuff, and it all works perfectly. I have the SL40 interfaced to a
Garmin 496, and can load the SL40 standby frequency from the 496...

[Steve - KDMW]

On May 27, 11:28 pm, mikem > wrote:
> On May 27, 5:32 am, Steve - KDMW > wrote:
>
> > I had my old Narco COM 120 replaced with a new Garmin SL40 and now
> > when the new radio is turned on my intercom volume drops by at least
> > half.
>
> Steve, coupla questions?
>
> Is the "HEADPHONE" OUTPUT of the SL40 wired into a COM1 or 2 INPUT on
> the Audio Panel?
>
> Is the OUTPUT of the audio panel wired to the SPA400 INPUT?
>
> Is the SPA OUTPUT tied to the aircraft headphones?
>
> Have you defeated the built-in intercom inside the SL40?
>
> Did everything work right before the SL40 install? (All the installer
> should have had to do was wire the SL40 headphone out to the whereever
> the previous COM was wired)
>
> It sounds like they may have tied two OUTPUTs together, which with
> modern avionics is a no-no. Even though the headphone audio is
> nominally at 500 Ohms impedance, the OUTPUT impedance of the headphone
> amplifier inside the SL40, and the one inside the audio panel are
> really a voltage source with VERY LOW output impedance, and two of
> these cannot be tied together ( and there is actually no reason to tie
> them together) without using series resistors, otherwise they will
> fight each other, sort of like your symptoms.

Thanks for your suggestions. The COM 120 was wired to the COM 2 input
of the audio panel (KMA-134), and the audio panel output wired to the
SPA-4000. I believe the shop just removed the COM 120 and wired the
SL40 in it's place, so it should still be the same. I haven't noticed
anything different or unusual, i.e. if I select COM 2 I only hear COM
2.

I didn't check the intercom mode of the SL40. I'll do that.

[RST Engineering]

>
> Thanks for your suggestions. The COM 120 was wired to the COM 2 input
> of the audio panel (KMA-134)

You really need to be quite specific when you make statements like this.
What lead/terminal/connection of the COM-120 was wired to which lead/... of
the audio panel? Audio panels will generally have two inputs, speaker and
phones. Each is terminated differently and are quite different
electrically. Was this the only connection between the COM 120 and the
audio panel or were the mic audio and key lines also connected? Did they do
an exact wire-for-wire swap or did they do some other mickey motion with the
new radio installation?


, and the audio panel output wired to the
> SPA-4000.

Again, specifics please. Was the audio panel output wired to an input on
the intercom or were the two outputs paralleled?



I believe the shop just removed the COM 120 and wired the
> SL40 in it's place, so it should still be the same. I haven't noticed
> anything different or unusual, i.e. if I select COM 2 I only hear COM
> 2.

So how does "the shop" explain it? You paid good money for an installation
and you didn't get it. Can they explain it away to you?

>
> I didn't check the intercom mode of the SL40. I'll do that.

That's a possiblity if the SL40's intercom is inverted in phase to the
intercom's output.

You say the intercom volume drops in half when you turn on the SL40. Is it
just turning on the power or do you have to have both transmit and receive
selected to the SL40 before the volume drop occurs?

Jim

[Steve - KDMW]

On May 28, 1:36 pm, "RST Engineering" > wrote:
> > Thanks for your suggestions. The COM 120 was wired to the COM 2 input
> > of the audio panel (KMA-134)
>
> You really need to be quite specific when you make statements like this.
> What lead/terminal/connection of the COM-120 was wired to which lead/... of
> the audio panel? Audio panels will generally have two inputs, speaker and
> phones. Each is terminated differently and are quite different
> electrically. Was this the only connection between the COM 120 and the
> audio panel or were the mic audio and key lines also connected? Did they do
> an exact wire-for-wire swap or did they do some other mickey motion with the
> new radio installation?
>
> , and the audio panel output wired to the
>
> > SPA-4000.
>
> Again, specifics please. Was the audio panel output wired to an input on
> the intercom or were the two outputs paralleled?
>
> I believe the shop just removed the COM 120 and wired the
>
> > SL40 in it's place, so it should still be the same. I haven't noticed
> > anything different or unusual, i.e. if I select COM 2 I only hear COM
> > 2.
>
> So how does "the shop" explain it? You paid good money for an installation
> and you didn't get it. Can they explain it away to you?
>
>
>
> > I didn't check the intercom mode of the SL40. I'll do that.
>
> That's a possiblity if the SL40's intercom is inverted in phase to the
> intercom's output.
>
> You say the intercom volume drops in half when you turn on the SL40. Is it
> just turning on the power or do you have to have both transmit and receive
> selected to the SL40 before the volume drop occurs?
>
> Jim

Jim,
I haven't delved that deeply into the wiring (yet) because the shop
has told me that they will get me back on their schedule to bring it
in and they will make it right. They have a pretty good reputation, so
I'm not too worried yet (Penn Avionics at Brandywine airport near
Philly). If I hit a wall, I will pull out my multimeter and dive in
and make it right myself if I have to (and then never use that shop
again).

To answer your questions that I can answer right now...

They indicated that they did an exact wire-for-wire swap with the COM
120. The intercom volume drops as soon as you turn the SL40 on. You
don't have to be transmitting or receiving for the problem to occur.

Steve

[mikem]

On May 29, 6:30 am, Steve - KDMW > wrote:
> ...
> They indicated that they did an exact wire-for-wire swap with the COM
> 120. The intercom volume drops as soon as you turn the SL40 on. You
> don't have to be transmitting or receiving for the problem to occur.
>
> Steve

When the SL40 is powered up, the feedback around its audio output
amplifier becomes active, creating a very low- output impedance. If
this is paralled with something it shouldn't be, then this is what
caused the audio level to drop.

This leads me to believe that the headphone COM120 output was NOT
wired to the isolated summing input on the audio switching panel as it
should have been. AFIK, Narco products have an internal resistor in
series with the headphone output, which might prevent the volume drop
even as wired.

A half-assed fix would be to insert a series 100 Ohm resistor in the
wire coming from the SL40 headphone output. A better fix would be to
trace out where that wire actually goes...

[Steve - KDMW]

On May 30, 10:42 am, mikem > wrote:
> On May 29, 6:30 am, Steve - KDMW > wrote:
>
> > ...
> > They indicated that they did an exact wire-for-wire swap with the COM
> > 120. The intercom volume drops as soon as you turn the SL40 on. You
> > don't have to be transmitting or receiving for the problem to occur.
>
> > Steve
>
> When the SL40 is powered up, the feedback around its audio output
> amplifier becomes active, creating a very low- output impedance. If
> this is paralled with something it shouldn't be, then this is what
> caused the audio level to drop.
>
> This leads me to believe that the headphone COM120 output was NOT
> wired to the isolated summing input on the audio switching panel as it
> should have been. AFIK, Narco products have an internal resistor in
> series with the headphone output, which might prevent the volume drop
> even as wired.
>
> A half-assed fix would be to insert a series 100 Ohm resistor in the
> wire coming from the SL40 headphone output. A better fix would be to
> trace out where that wire actually goes...

Excellent suggestion. I'll ohm it out. According to my diagrams, pin
14 of the SL40 (headphone) should read to pin D of the KA-134 (Com 2
Audio In).

[RST Engineering]

Agreed. Good place to start.

Jim

"mikem" > wrote in message
oups.com...
> On May 29, 6:30 am, Steve - KDMW > wrote:

> This leads me to believe that the headphone COM120 output was NOT
> wired to the isolated summing input on the audio switching panel as it
> should have been.

[Steve - KDMW]

On May 30, 11:54 am, "RST Engineering" >
wrote:
> Agreed. Good place to start.
>
> Jim
>
> "mikem" > wrote in message
>
> oups.com...
>
>
>
> > On May 29, 6:30 am, Steve - KDMW > wrote:
> > This leads me to believe that the headphone COM120 output was NOT
> > wired to the isolated summing input on the audio switching panel as it
> > should have been.- Hide quoted text -
>
> - Show quoted text -

OK. Well, I ohmed out the wiring and found nothing unusual. The SL40's
headphone output was wired to the KA-134's Com 2 Audio In (and nowhere
else). I checked to see that it was not paralleled (shorted) to any
other inputs. All of the grounds read properly, i.e. to ground, and
nothing shorted to ground that shouldn't be.

The one thing I have not checked yet is to make sure that the radio
goes only to the audio panel, and not paralleled to the intercom. Only
the audio panel output should go to the intercom. I don't believe
there is a problem there or it would seem that I would always hear the
Com 2 radio regardless of audio panel selection. But I'll check it out
anyway.

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.

Still waiting for the shop to get back to me with their suggestion.
Maybe the 100 ohm resistor on the radio's headphone output is the way
to go. Also, I'll be going to the AOPA fly-in tomorrow and I'll pick
the brains of the avionics shop's booths and Garmin's reps.

[mikem]

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

[Steve - KDMW]

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.

[mikem]

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...

[Steve - KDMW]

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.

[Steve - KDMW]

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.

[mikem]

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...

[Steve - KDMW]

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).