Hum in headset after wrapping panel

[jefish]

I had the whole instrument panel pretty much done, everything worked. Then I wrapped it.

I pulled everything out of the front that could be (HDX, OLED COM and Intercom) and pulled everything else from the back (prop controller, breakers) so each item remained pretty much in place, electrically connected.

After installing the vinyl and reconnecting all devices, I now have a very distinct hum in the audio. Gets MUCH worse when hitting TX. I also noticed on first boot that the radio was stuck in RX mode, but a second boot seemed to fix that. Odd. Changing the Dynon Dim Offset to -3 or +7 removes the hum.

Anyway, I've done the "touch a wire between the HDX case and the intercom case, hum will disappear" test. It didn't.

I traced all ground wires back to the central grounding block about 12" away: the HDX and EMS were connected to one pin. The OLED COM and Intercom panels were connected to another pin. Disconnecting all 4 of these, all devices remained powered on, so all devices appear to grounded to, getting ground path, from being mounted in the instrument panel? Connecting all of these ground wires only to each other, no change.

Doing a continuity test between the HDX case, the OLED COM panel case and connector shell, the Intercom panel case and connector shell, and the airframe all give a positive beep.

I don't know if this is the desired result, or if one of those test points should be isolated from the airframe.

Also noteworthy: when powering off the Intercom by pressing it's volume knob, then powering it back on, the OLED COM panel reboots. I hooked up the old LCD COM panel and it does NOT do this. I can't help that this is a symptom trying to tell me something, but maybe that's just how the new OLED COM panels are?

Tips please? What's supposed to be airframe grounded vs grounded to each other vs isolated? Should all Dynon devices remain powered on when their ground wires are disconnected?

I've been building for 700 hours and I've never been this frustrated.

 

[PaulSS]

I’m no expert but I would certainly try isolating your equipment from the panel, as it sounds like you’re getting a ground loop by having multiple grounds to the same bits of kit. Just use the ground wires that you’ve installed and not the panel as well. All my stuff is mounted in a carbon panel, so it’s definitely only grounded through the wiring. It all works properly.

 

[jefish]

I’m no expert but I would certainly try isolating your equipment from the panel, as it sounds like you’re getting a ground loop by having multiple grounds to the same bits of kit. Just use the ground wires that you’ve installed and not the panel as well. All my stuff is mounted in a carbon panel, so it’s definitely only grounded through the wiring. It all works properly.

The weird part about that is, the panel was bare aluminum before, and it was working perfectly for almost 2 years. Add some insulating vinyl, and suddenly I have more grounding?

But I think I'll test the theory by removing the Dynon gear from the panel and doing a "bench test" with everything sitting on the seat and floor.

 

[Rhino]

Have you reviewed this?

Ground Loops and Noise in Aircraft Systems

Quite a few forum threads and articles have at various times addressed ground loops, and their potential to cause noise or other interference in your avionics system. Simply put, a ground loop occurs when different paths to ground are used. The difference in resistance, or impedance, on those...

forum.flydynon.com

 

[PaulSS]

Is the vinyl wrap preventing the panel grounding to the airframe? Perhaps the bare aluminium was grounded to the airframe but, now it's not, you're getting some strange loops. I would stick with only having the instruments grounded through their wires and not through the panel as well.

 

[MikeD]

Were the headset jacks removed? Do you have shoulder and fiber washers installed on the headset jacks?

 

[jefish]

Were the headset jacks removed? Do you have shoulder and fiber washers installed on the headset jacks?

The jacks were not removed, they're mounted under the seats with fiber washers as before. The audio wiring harness was not moved or changed, the intercom was merely unplugged from it.

 

[jefish]

Is the vinyl wrap preventing the panel grounding to the airframe? Perhaps the bare aluminium was grounded to the airframe but, now it's not, you're getting some strange loops. I would stick with only having the instruments grounded through their wires and not through the panel as well.

The wrap is only on the face of the panel, the edges where the panel mounts to the airframe is still bare.

I might try taping up all the Dynon gear and reinstalling it.

 

[owenssonex]

I had something similar happen with no real solution, but to say you are not alone. My hardware version 1 HDX1100 died so I installed a hardware version 4 HDX1100 in its place while I shipped the original back to dynon for repair. The hum in my left ear was so bad I disconnected Pin 13- audio out- left. Decreasing the screen dim away from 0 caused a minor hum in my right ear. I also tried the comm to HDX ground, no change. My original display was repaired, Pin 13 reinstalled, original display has no humming. The original display works great including the harness based 3rd USB port (really grinds my gears that Dynon removed that support). Sorry I don't have a solution for you.

 

[RVDan]

I often witness testing of avionics in the lab environment for EMI and audio interference. Generally it is essential that the equipment cases are grounded to the airframe, typically with a DC resistance of 2.5 milliohms or less. I am guessing that when you wrapped the panel you lost the common case grounds somewhere for the panel mounted equipment. You need a milliohm meter to measure milliohms, not a typical VOM.

 

[jefish]

I often witness testing of avionics in the lab environment for EMI and audio interference. Generally it is essential that the equipment cases are grounded to the airframe, typically with a DC resistance of 2.5 milliohms or less. I am guessing that when you wrapped the panel you lost the common case grounds somewhere for the panel mounted equipment. You need a milliohm meter to measure milliohms, not a typical VOM.

Same thought. I've gone over and grounded every case directly to the airframe for just this reason, but no change. It's all just so odd.

I've put out a call for a local Dynon/audio guru to come to the house and "just do it." No bites yet.

 

[Rhino]

Grounding from a physical box to the airframe is system voltage grounding, not audio grounding. There should be a single common grounding point for each power and audio. If you are grounding everything to the airframe individually instead of to a common ground point, you're just asking for ground loops and noise in your system. That is discussed in the link in post #4.