RE-POST: AeroElectric-List: Magneto Noise

Sam Hoskins

From Bob Nicholls email list.


Sent via wireless gizmo.

---------- Forwarded message ----------
From: "Jeff Luckey" <jluckey@...>
Date: Apr 25, 2014 3:08 PM
Subject: RE-POST: AeroElectric-List: Magneto Noise
To: "aeroelectric-list@..." <aeroelectric-list@...>

Per request from J. Davis...

----- Forwarded Message -----

From: "Robert L. Nuckolls, III" <nuckolls.bob@...>
To: aeroelectric-list@...
Sent: Tuesday, April 8, 2014 8:06 AM
Subject: Re: AeroElectric-List: Magneto Noise

--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob@...>

At 12:04 PM 4/6/2014, you wrote:

I flew w/ a friend recently and when trying to get awos for our
destination airport (which is a pretty weak signal) it was almost
impossible to hear due to magneto noise.  The radio is a Garmin 430W
in a Cherokee and we were around 10 miles from the airport.  When we
got within 3 or 4 miles the signal was much clearer.  All other
transmissions with ATC & towers were acceptable but I noticed at high
volume I could clearly hear magneto noise in the background.

I'm wondering if that's just the way it is or if there is a problem
with the magnetos or P lead wiring.

    Was this a new condition or has it been that
    way 'forever'. It's difficult for the neophyte
    observer to tell the difference between ignition
    noises getting into the system by way of plug wire
    radiation versus p-lead radiation.

    First, make sure that the perceived noise goes up
    and down with volume control settings on radio.
    This all but guarantees that it's coming in through
    the antenna.

    Then, disconnect both p-leads from the back of the
    magnetos and run the engine. See if the noise is
    still heard . . . turn to unused frequency . . .
    open squelch.

    If you can still hear it, then look for bad plug
    wire. If not, hook up one p-lead at a time and
    repeat experiment . . . see if noise is predicated
    on either or both of the p-leads being hooked up.

    Once you've identified the offending radiator,
    you need to figure out why. If this is a 'new'
    condition, then you're looking for something that
    has changed. Bad cap in mag (probably not both)
    broken ground lead on p-lead shield. If possible/
    practical wire p-lead wires per Figure Z-27

    Ground p-lead shields at magneto end only . . .
    use p-lead shields to ground mag switch. Remove
    and existing ground to the mag switch.

    If the noise is coming from both p-leads,
    there are 'approved' magneto noise filters

    that most mechanics will install with a simple
    log-book entry.

    Bob . . .

  Bob . . . 


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Bruce Crain

Bruce Crain