Noise in LEMO jacks.

[AK737pilot]

I'm an A&P and just did a complete instrument panel and electrical overhaul on my 182. In the process I upgraded to a PlanePower alternator and regulator. I also added LEMO jacks to the pilot and copilot panels. They're wired directly to the mic/phone jacks. When the engine is not running, I have no noise in the audio system. When the engine is running (alternator on or off), I get a zip zip zip with my wingtip pulse lights as well as a steady squeal when those lights are on steady in the Land mode or Taxi mode. With a headset plugged in to the mic/phone jacks (not using LEMO) I don't hear the noise, nor is it heard from the back seat jacks. Only when using the LEMO jacks.

Any ideas???

 

[Bell206]

Any ideas???

Are you supplying power through the LEMOs to your headsets?
How did you "directly" wire the LEMOs to the standard jacks?

 

[AK737pilot]

Yes, power from my avionics bus goes to the LEMO jacks, ground to my main grounding block. The other mic/phone leads are wired through the standard jacks as per the Bose wiring diagram.

 

[Bell206]

Yes, power from my avionics bus goes to the LEMO jacks, ground to my main grounding block.

Where did you terminate your wire shields?
How long is the jumper between the LEMOs and existing jacks?
Where does the audio source come from? Brand?
Could be a ground circuit difference between your power return, shielding return, and/or LEMO grd return.

 

[AK737pilot]

Standard jack shields are terminated at the audio panel backshell (GMA350). LEMO jumpers are about 8” with the shields terminated at their respective jack barrel.

The copilot standard jacks were silent. I didn’t get to check the pilot jacks- my son was flying the plane and I didn’t want to distract him too much.

If one of those jacks wasn’t insulated well from the panel, could that cause the pulse light interference, or would it be a steady squeal regardless of what lights are on?

I need to get up under the panel and look, but if the jumper is physically in contact with the landing light bundle, would that cause it? (Quasar CX wingtip lights).

 

[Kent Wien]

One hack you can do (I know it’s not a true fix, but it does wonders) is to turn the volume down on your headset and bring the volume up on the intercom and radio. Try it out. You’ll be surprised.

 

[Half Fast]

Yes, power from my avionics bus goes to the LEMO jacks, ground to my main grounding block.

Another thing to consider is the routing of the power and ground wires. If they're widely separated, they'll create a loop which can pick up noise. Ideally, your connections to the power bus and the grounding block should be close to each other, and the power and ground wires should be a twisted pair to minimize the loop area.

 

[Bell206]

LEMO jumpers are about 8” with the shields terminated at their respective jack barrel.

Look at this. I've always been told to ground all shields at the audio source. In the same context, unless specifically stated, to have all ground returns terminate at the audio source as well to include items like the LEMO 2 grd.

If one of those jacks wasn’t insulated well from the panel, could that cause the pulse light interference, or would it be a steady squeal regardless of what lights are on?

IMO, if you had a jack insulation problem it would be evident with the standard plugs.

I need to get up under the panel and look, but if the jumper is physically in contact with the landing light bundle, would that cause it? (Quasar CX wingtip lights).

The shorter the jumper the better. I believe less than 12" in length no shielding is needed. However, anything is possible. Since the problem appears LEMO related, may want to verify all your grounds are good and terminated properly. I'm not an expert on this stuff but learned from some top shelf sparkies. Perhaps PM @Jesse Saint for some input?

Standard jack shields are terminated at the audio panel backshell (GMA350).

I've either heard or read that some LEMO panel installs can be a bit finicky. Given you have a Garmin panel maybe compare that diagram routing vs the Bose diagram. I've only installed LEMO receptacles in helicopters with Garmin equipment and used their routing. So I don't have much to compare on yours.

 

[Lindberg]

Another thing to consider is the routing of the power and ground wires. If they're widely separated, they'll create a loop which can pick up noise. Ideally, your connections to the power bus and the grounding block should be close to each other, and the power and ground wires should be a twisted pair to minimize the loop area.

It's always better to have the source be as loud as necessary so that you're not amplifying noise downstream.

 

[AV8R_87]

Audio grounds might be creating a ground loop with the power grounds.

I'd consider routing the power grounds close to where the audio grounds/shields are terminated.

 

[Papa Pilot]

It might be your headsets.

I had this issue with headsets I retrofitted with ANR. With the retrofit, I used an off the shelf power regulator to bring the voltage down. Turned out it was a switching power regulator which acted as an antenna and picked up the strobe EMF emissions. I ended up getting a pair of used Bose X and didn't have any more issues.

 

[AK737pilot]

Thanks for taking the time to reply! I really appreciate the insight!
I've got some troubleshooting to do and will report back when I find something!