Chasing a ground loop

[smithflys23]

I have been plagued with strobe noise ever since my -8 was wired by one of the "pro's". I have Aeroled multi voltage nav/strobes. I recently rewired my strobes hoping to eliminate the pulsing I hear with the strobes on. I ran separate ground wires for each strobe and grounded them all at the same point, the main ground bus. I grounded the shields independently on the light frame, and at a single point on the main ground bus. This was per Aeroleds suggestions.
I still get the pulsing noise in my headset. Very frustrating after all the work to rewire them.

Some other clues I've picked up along the way. The noise is not present when running only on the battery, and while on the ground. Ill have to try this airborne by shutting off the alternator, but I don't think this will make a difference. The noise only comes through the intercom. The pulsing does seem to change frequency slightly based on engine RPM. I also get a slight hum when my smoke pump is turned on.

Can anyone offer suggestions on where to look next? Does this sound like an intercom problem? It appears at first glance that the intercom is grounded correctly. Your help is appreciated!

[N804RV]

These gripes are difficult to troubleshoot without really digging into your electrical and avionics systems.

One easy thing to rule out is your electrical system. You say it's most noticeable when you're flying. Have you ever done a bonding test on your electrical system's ground leg? If the ground leg from the negative post of your battery isn't doing its job, that can set you up for problems. You may also notice other problems like your starter doesn't turn over as fast as your friends airplane with the same set up.

Likewise, if you use the "forrest of tabs" type grounding system aft of the firewall, you need to make sure your grounding strip is likewise bonded. Under test conditions, you need to have less than 2.5 mili-ohms. And, a regular ohm meter won't do it, it has to be a bonding meter.

Another thing, one I'm sure youre already aware of, is that all your audio "common" wires have to be electrically "common" with the audio ground of your audio panel/intercom. And, they're not supposed to be grounded on the same "forrest of tabs" grounding strip your power circuits are grounded to.

This is all I can think of without knowing more about your airplane. I hope it helps.

[smithflys23]

Originally Posted by N804RV

These gripes are difficult to troubleshoot without really digging into your electrical and avionics systems.

One easy thing to rule out is your electrical system. You say it's most noticeable when you're flying. Have you ever done a bonding test on your electrical system's ground leg? If the ground leg from the negative post of your battery isn't doing its job, that can set you up for problems. You may also notice other problems like your starter doesn't turn over as fast as your friends airplane with the same set up.

Likewise, if you use the "forrest of tabs" type grounding system aft of the firewall, you need to make sure your grounding strip is likewise bonded. Under test conditions, you need to have less than 2.5 mili-ohms. And, a regular ohm meter won't do it, it has to be a bonding meter.

Another thing, one I'm sure youre already aware of, is that all your audio "common" wires have to be electrically "common" with the audio ground of your audio panel/intercom. And, they're not supposed to be grounded on the same "forrest of tabs" grounding strip your power circuits are grounded to.

This is all I can think of without knowing more about your airplane. I hope it helps.

Thanks for the reply, Ken.
I’m new to tracking down electrical gremlins, so The things you mention are things I know little about. I can certainly do some research in an effort to educate myself.

I have not done a bonding test. I’ll have to research this.

The battery appears to be grounded to the battery box frame.

I do have a forest of tabs behind the firewall.

I’ll also have to research the audio comm wires and electrically common.

One thing I forgot to mention, is that I have a VPX. I don’t know if that in any way changes things.

[BobTurner]

Originally Posted by smithflys23

I ran separate ground wires for each strobe and grounded them all at the same point, the main ground bus.

The noise is not present when running only on the battery, and while on the ground.

Are the separate ground wires mentioned above running along side the power wires, both inside a shielded cable? Despite the manufacturer's instructions I would only ground the shield at the firewall, not the other end.

Are you saying there is no noise when on the ground, with the alternator and strobes running?

[smithflys23]

Originally Posted by BobTurner

Are the separate ground wires mentioned above running along side the power wires, both inside a shielded cable? Despite the manufacturer's instructions I would only ground the shield at the firewall, not the other end.

Are you saying there is no noise when on the ground, with the alternator and strobes running?

Thanks for the reply, Bob.

The strobes are grounded with a separate unshielded wire running to each light. It runs along side the shielded 3 conductor wire that powers the lights.

FWIW, the grounding scheme came direct from their head electrical engineer. He said this way the shield would act as an actual shield. I simply followed his advice.

On the ground with the engine off, and running on the battery only, there is no noise when the strobes flash.

I’m going to see If the noise is present with the engine running and the alternator off.

[rgodsy]

Make sure all of your audio line shields are grounded on one side only. This includes your headset. Prevents noise that gets in an avionics unit from using the audio ground/shield as a path/loop.

[smithflys23]

Originally Posted by rgodsy

Make sure all of your audio line shields are grounded on one side only. This includes your headset. Prevents noise that gets in an avionics unit from using the audio ground/shield as a path/loop.

Thanks. I’ll take a look at that.

[cbennet1]

Make sure your intercom jacks are physically isolated from the panel or wherever they are mounted assuming it is conductive. Insulated shoulder washer and flat washers on all jacks.

Lots of other great suggestions already posted. Good luck!

[Mich48041]

The strobe negative and positive power wires should be a twisted pair.
Try running the negative strobe wire all of the way to the alternator mounting bracket.
If nothing else solves the problem, try increasing the alternator "B" lead wire size.

[gasman]

Originally Posted by smithflys23

Thanks for the reply, Ken.
I’m new to tracking down electrical gremlins, so The things you mention are things I know little about. I can certainly do some research in an effort to educate myself.

I have not done a bonding test. I’ll have to research this.

The battery appears to be grounded to the battery box frame.

I do have a forest of tabs behind the firewall.

I’ll also have to research the audio comm wires and electrically common.

One thing I forgot to mention, is that I have a VPX. I don’t know if that in any way changes things.

I would not consider this a good ground location. The full load of the starter must pass through the battery box bolts.

The forest of tabs should be bolted to the firewall with a passthrough brass bolt. The battery ground should connect to the brass bolt on the engine side. And a ground strap should go from the same bolt, to the engine case.