Chopping Static in my Radios

[Rmag]

I've been trying to hunt down the source of a chopping static in my radios that's been a tough one to nail down.

The attached audio file is what the static sounds like while I was flying at a 2400 rpm setting.  The chopping static changes frequency with a rpm change.

My radios are Garmin GMA 35c operated through a Garmin GTN 750 and a Garmin GNC 255 is my NAV/COMM 2.

Both radios exhibit the same static.  The static comes through any (noise cancelling or normal) headset as well as the cabin speaker.  The static will also come through a handheld COMM within 20 feet of the aircraft so its throwing off RF into the air.

Troubleshooting we did:

Tested Headphone jacks for proper grounding.

Pulled every breaker (including alternator) in the aircraft except the radio to eliminate any other electrical systems..

Turned off the Master and Alternator (complete shutdown of electric systems) and the static is still in the handheld.

Both L and R mags make no difference, the sound is there for both.  Key off will kill the static before the engine quits.

We tested all the magneto/ignition wiring, it is all properly grounded.

We put filters on the P Leads from the magneto.

We ran the plane with the P Leads removed to take the ignition wiring completely out of play.

We changed the spark plugs and tested the spark plug wiring.

Tested transmitting, the radios do not transmit the static.

 It is not in the intercom   

We removed the coax wires in the back of the radios.

We removed each radio one at a time from the panel.

The only thing I haven't done yet and that is where we are heading is to take the magnetos apart to check/replace the condensers.  But a bad condenser you can usually find either L or R mag switch, unless they both are bad and they are both newly overhauled mags.

Anybody see anything we might have missed trying to hunt this thing I go through the expense of breaking into the magnetos?

The plane is squawk free other than this radio chop, its really bugging me.

 

static.m4a

[Marauder]

You have an engine analyzer? And if you do is it a JPI?

Sent from my iPad using Tapatalk Pro

[Marauder]

This fixed it.




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[Rmag]

It's not an open squelch it's a chopping noise. When not receiving on the radio the squelch kills the static.   I have a JPI 930. We disconnected the sensor cables to the JPI and we have also pulled the JPI"s circuit breaker. No change. 

[tony]

you have a tough one.  Is the engine grounded properly to the frame?

[takair]

Key off kills the static, but switching to individual mags does not?  What if you remove both P leads and run an extension into the cockpit so you can ground them outside of ignition switch? (Use care) . Do you have shower of sparks?  Is it possible there is some short in the switch causing this through some other path. Long shot, but you've done most everything else.

[jasona900]

Do you have a USB adapter in your 12v cigarette lighter port?  I found I would sometimes get static so bad I couldn't hear over the radio when I would charge my iPad.  I unplugged the iPad, and the static would immediately go away.

[Yetti]

I vote USB charger.

Or capacitor has gone bad in the noise filter.  There may be a big ole noise filter on the copilot side firewall on the engine side.

[Rmag]

12 minutes ago, jasona900 said:

Do you have a USB adapter in your 12v cigarette lighter port?  I found I would sometimes get static so bad I couldn't hear over the radio when I would charge my iPad.  I unplugged the iPad, and the static would immediately go away.

Nothing plugged in. 

[thinwing]

I think it's magneto /ignition lead noise....when did this noise start?when we're mags overhauled?The fact that you pick this up outside the aircraft with a handheld is very telling...Assuming it is detectable with both mags shorted ..that points to an ignition lead (large area chafed baring shield to let rf escape)regardless that is classic high voltage arcing producing rf...mags or leads 

[Yetti]

The magneto P leads are shielded.  You can resolder them or cut back the insulation and run a ground from the outershield to one of the ignition wire caps.

[carusoam]

The two things that stand out in the description to me...

1) The noise is broadcast outside of the plane... typical of high voltage RF noise

2) It stops immediately with the switching off the magnetos to off. (Both mags grounded)

Then I typed some more.....

3) Capacitors are often used to curtail electronic noises.  There is a capacitor in each mag.  It's primary function is building and storing electric energy to generate the spark.  It may have a second function of noise reduction.

4) check the presentation on how a mag works. And see if  swapping in new capacitors makes sense.

5) Back in the day noise filters were mounted in cars to keep alternator whine from getting to the radio.  The filter was a giant capacitor mounted on the input power source to the radio...

6) alternator whine is typically rpm sensitive, increases in pitch with increased rpm... 

7) there are six(?) legs in an alternator.  Losing a single leg isn't very noticeable in terms of generating electricity... but it can generate a lot of RF noise.  Hmmmmm?

 

Now to put on My Piloto Thinking cap... (he knows this area really well)

1) mount a large capacitor on the pilot relief tube.... . (Not helpful!)

 

More PP pseudo knowledge, not anything else.

Best regards,

-a-

Edited May 18, 2017 by carusoam

[thinwing]

2 hours ago, carusoam said:

The two things that stand out in the description to me...

1) The noise is broadcast outside of the plane... typical of high voltage RF noise

2) It stops immediately with the switching off the magnetos to off. (Both mags grounded)

Then I typed some more.....

3) Capacitors are often used to curtail electronic noises.  There is a capacitor in each mag.  It's primary function is building and storing electric energy to generate the spark.  It may have a second function of noise reduction.

4) check the presentation on how a mag works. And see if  swapping in new capacitors makes sense.

5) Back in the day noise filters were mounted in cars to keep alternator whine from getting to the radio.  The filter was a giant capacitor mounted on the input power source to the radio...

6) alternator whine is typically rpm sensitive, increases in pitch with increased rpm... 

7) there are six(?) legs in an alternator.  Losing a single leg isn't very noticeable in terms of generating electricity... but it can generate a lot of RF noise.  Hmmmmm?

 

Now to put on My Piloto Thinking cap... (he knows this area really well)

1) mount a large capacitor on the pilot relief tube.... . (Not helpful!)

 

More PP pseudo knowledge, not anything else.

Best regards,

-a-

And the next time you really gotta go!.....ZAP....there goes MR HAPPY!Kidding aside..as a hobby I restore old 2 stroke outboards with simple Magneto ignition...they produce the same noise (but louder) interference..( google early spark gap transmitters for expo)

[Rmag]

I initially though P leads as well, but we disconnected the P leads from the magnetos to completely remove the ignition wiring from the equation. Still had the noise.

 

[takair]

6 minutes ago, Rmag said:

I initially though P leads as well, but we disconnected the P leads from the magnetos to completely remove the ignition wiring from the equation. Still had the noise.

 

 Do you have impulse coupling or shower of sparks? Shower of sparks provides another circuit to the mag that may be causing noise.

[Andy95W]

1 hour ago, takair said:

  Shower of sparks provides another circuit to the mag that may be causing noise.

With a starter switch that is functioning normally, this is extremely unlikely.  And if this were the case, the noise would be a continuous crackling.

[Piloto]

Check the engine ground strap connection at the back of the engine. Without this strap the engine acts as antenna for the magnetos.

José

[tony]

thats what I think the issue is too Jose

[bradp]

Plus one for something not grounded. 

[thinwing]

Than the starter would not function 

[thinwing]

Of course a quick is to attach temp alligator jumpers from both mag attach bolts to airframe ground...but I doubt it...I haven't heard if this started after mag overhaul...regardless if the mags checkout than its on to ignition leads...one or more shields are ungrounded or leads that have been ziptied back on themselves and center conductor has migrated to arc on shield

[Yetti]

15 hours ago, Rmag said:

I initially though P leads as well, but we disconnected the P leads from the magnetos to completely remove the ignition wiring from the equation. Still had the noise.

 

The shielding is completed by the running the ground via the shield to the magnetos.  Disconnecting them would break the shields.  The condition of the wire will tell the story here.

[EricJ]

It seems pretty clear that it's ignition noise, but it can come from any of the plugs or leads, not just the magnetos.   A loose lead or a damaged lead shield, or even a faulty plug could do this. 

 

[thinwing]

Look for a >5000 ohm plug

[Yetti]

And evidence of arching....