tadsargent

tadsargent

Well Known Member
I have a loud scratchy sound when I PTT.

Here is what I have
Garmin 240 audio
Garmin 430
Garmin sl-40
Quiet Tech halos
Dual lightspeed Ign.

When I select both radios and PTT the 430 I hear it in my headset but the transmit is clear, when I deselect the SL40 it goes away and the same when using the SL40 only. I sent the 240 back to Garmin where they bench tested it and could not reproduce the issue. The problem only shows when both radios are selected and the PTT is used, otherwise they each work perfectly.

I rerouted the Coax and upgraded them to RG400. No change
I called Garmin and they suggested looping the two radios together to kill the transmit on the non Tx radio.

Any suggestions? Ill try anything
 
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Try a different headset?

Have you tried a different headset? There are others with a similar issue using the halo set... apparently they can install a bridge rectifier in the headset electronics that may solve it... best to start testing with other headsets first to isolate the problem.
 
Radio

Thanks, I tried that, and my co pilot hears the same thing in his Bose sitting next to me.

Please keep them coming
 
If I understand the problem correctly, it sounds like the transmitter of one Com radio is overloading the receiver of the other Com radio. Both Coms "should" have a transmit interlock function that reduces the sensitivity of the receiver when the other radio is transmitting. It's purpose is to prevent what you describe. However, it has to be wired in for it to work. Basically, it's a wire from the PTT lead of Com1 hooked to the transmit interlock pin of Com2, and vice versa. If you need more info, I can get the pinouts for you.
 
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radio noise

Mike, thank you for your reply. I talked to Garmin regarding the interlock feature. The 430 has this but I did not see the pins for the SL-40 can you provide it.
Thanks
 
You are correct, the Apollo series doesn't have it and I forgot about that. Even my old KX170B had it, not sure why they omitted that function.

You can at least wire up the 430 and fix half the problem. Another factor is that the com antennas are probably pretty close together (on the belly, side by side?). If you could increase the separation (one on top, one on bottom) that would probably help some but may not be worth the effort.
 
Dont leave me hanging

I wanted to share a follow up to my noise issue as a GOTCHA

I found the source of my problem to be the B&C crimps I made.
While at the avionics shop having my IFR Cert my Tech measured the output of my Transponder W/I 10 feet and was only able to read 76watts. He bench tested the unit (Garmin) and it passed. He then looked at the pig tail that was premade for me. I had to add a female to female connector to mate it to the antenna. He decided to remove the connector and replace the male end with a female end to minimize any loss. A retest proved successful with a reading of 206 watts. That started me thinking. The tech said sometimes the center connector gets pushed back over time due to removal and reinsertion. So.... I surveyed the connections I made and replaced them while lengthening them to protrude slightly from the flush position I had and also taking care not to crimp the buldge.

Problems solved and they work just as I paid for. Its just how I learn
 
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