Impedance Mismatch?

[Rockyjs]

I finished flying Phase I and last week took my first passenger (son) for a ride. Interphone is loud and clear on battery and ships power, but as I taxi out the interphone volume is very weak. After the flight we test the interphone volume on the battery and it is fine. I do another flight and the same thing happens. I troubleshot and found the volume loss on the intercom happens when the SL-30 is on and selected on the audio panel. If I switch the audio panel transmit to the 300XL #2 the interphone is fine. If I leave the switch in number 1 and turn off the SL-30 the interphone volume is also fine. It appears that there is some mismatch between the interphone and the #1 position on my audio panel, or internally in the SL-30. The internal interphone function on the SL-30 is disabled. I'm going to pin swap the two radios tomorrow and see if the problem follows or remains with the #1 position. Any other ideas? Anyone else with a similar problem?
Equipment list:
Sigtronics SPA 400 Interphone
King KMA 20 Audio Panel
Garmin SL-30 (Comm 1)
Garmin GNC 300Xl (Comm 2)

 

[vlittle]

Hi Rocky.

The problem that you are having is that the output impedance of the SL-30/40 is very low, and cannot be selected directly to your Sigtronics intercom.

The fix is to wire in a resistor between the SL output and the audio panel. Any value between 150 and 600 ohms seems to work. Start with 220 ohms and see how that is.

You can see from this appnote what I'm getting at. They don't show an audio panel involved, but its the same issue.

That's the fix, guaranteed!

Vern

 

[Rockyjs]

Thanks

Thanks Vern,
You just saved me a lot of work. I figured it was a mismatch, but the KMA 20 is pretty old, but everything works as advertised. The other option was to just fly solo. Well, not really.

Thanks again, I'll place a resistor in tomorrow and let you know, but I'm sure that that will work.

 

[Rockyjs]

Worked Like a Charm

Vern,
That worked perfect. I put a 220 ohm resistor in the headphone feed from the SL-30 going to the KMA 20. Now I have no noticeable change in the intercom level when selecting the SL-30.

I owe you a few cold beverages of your choice when we meet.

Thanks
Rocky

 

[6 Gun]

hey vern

got one for you
i have a kma 20 with 2 mx170b's
when i turn the volume up on both radio's i can not transmitt on either radio without interference so i turn the volume down on the one im not transmitting on and no interference any idears
thanks in advance bob

 

[vlittle]

got one for you
i have a kma 20 with 2 mx170b's
when i turn the volume up on both radio's i can not transmitt on either radio without interference so i turn the volume down on the one im not transmitting on and no interference any idears
thanks in advance bob

Dang, I new this would happen. One lucky guess and now I'm Kreskin! Seriously, it sounds like an installation problem.

Either your wiring is suspect, your antenna coax is bad or your antennas are too close to each other. This is the area of mysticism and whiffle dust-- you'll need an avionics guru on the ground to work it out.

Sorry, no magic answers.

Vern

 

[vlittle]

Vern,
That worked perfect. I put a 220 ohm resistor in the headphone feed from the SL-30 going to the KMA 20. Now I have no noticeable change in the intercom level when selecting the SL-30.

I owe you a few cold beverages of your choice when we meet.

Thanks
Rocky

I'll take you up on that one day! This is an obscure problem that I've dealt with before. In fact, some of the Vx Aviation products incorporate this fix right into the design of the output structures.

Good luck,
Vern