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Mag noise

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nuckolls.bob(at)aeroelect
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Posted: Thu Sep 13, 2012 9:47 am Post subject: Mag noise

Noise in radio. Newly overhauled Bendix mags, new ignition harness new P leads, Noise only appears with mag switch on Both, is not there when switch is on left or right individually. Rechecked all ground connections, shielding is grounded at both ends. Engine is grounded to engine mount, engine mount grounded to airframe. All connections at switch have been checked. Aircraft is a Glastar with composite fuselage. Someone has suggested using double shielded wire for P leads. We have tried everything to solve the problem and have run out of options. Would really appreciate your help. I have your loose leaf book but could not find anything that specifically addressed this problem. Thank you.



         Do this experiment. Disconnect both the p-lead

         center conductors -AND- ground wires at the switch

         and leave them hanging. Fire up the engine and

         see if the noise persists. Kill engine with

          mixture.



         If noise goes away, reconnect wires to the

         switch but REMOVE local ground for the shields.

         Wiring data in the book is figure Z-26.



 [img]cid:7.1.0.9.0.20120913123717.01e22bb8(at)aeroelectric.com.0[/img] 



 

         The shields should be grounded at the engine end only.

         I think this may fix your problem.



 

 

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Sep 14, 2012 9:33 am Post subject: Mag noise

>The shields should be grounded at the engine end only.

          >I think this may fix your problem.



Thank you for your quick response to my problem. I have been 

wrestling with this for two weeks. Every book I have on the subject 

says to ground both ends of the shielding. In any event I took your 

advise and the noise is gone. Medicine Lodge brings back memories to 

me, I went to school at Emporia and my roomate was from there. I 

trained on B-29s at Smoky Hill Air Base in WWII. Thank you again for 

your help, it is much appreciated. Ed Dunn



    I think this is perhaps the third time this

    particular set of conditions propagated ignition

    noise into the rest of the airplane. I'm not

    sure I understand the exact mode of propagation

    for noise but whether we understand it or not,

    it's a demonstrably good thing to do.

   Bob . . .

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