[Boatanchors] Need magnetic shield for shaded pole motor

[J. Forster]

Can you replace the fan with a muffin style unit, possibly a brushless DC
version?

-John

==============


> Good afternoon Al, Charlie, Bob ... and thanks for all the replies.
>
> Here is my problem:  I am trying to mitigate a minor HUM problem with a
> transceiver that uses a shaded pole blower.  Worked on the Hum problem for
> far too many hours and found it to be the blower motor  ----- Who would
> guess????
>
> The shaded pole motor has an incredible amount of 60 cycle magnetic
> leakage extending out to about 3 inches.  Problem is the 60 cycle magnetic
> field easily passes through the aluminum chassis to influence the audio
> preamp circuits that are just below the blower.  To make it worse the
> audio preamp tubes are directly adjacent to the motor.  It is definitely
> the Gaussian not electrostatic effect because leaving the 115v hot line to
> the blower live is not a problem.  Was a design error that was never
> important enough to divert resources apparently.
>
> I have tried the mu metal shield for a 3” scope tube, strips of
> transformer laminate, and that incredibly expensive mu metal that we use
> to shield RF stages.   The most effective material is the transformer E
> sections but would like to try a solid sheet that I could solder into a
> congruent loop completely around the motor to short circuit the
> inconsistent magnetic fields.
>
> Mu metal/soft iron is only effective to short circuit two poles of a
> magnet.  If you expose a single pole to mu metal it will easily pass
> through and be felt on the other side.  Believe that to be the problem
> with shaded pole motors --- inconsistent field patterns.
>
> This is actually only a problem for a very fussy listener with a perfectly
> noise free antenna.  It was identified by Bill at Electrovoice 47 years
> ago when testing the radio for the Hallicrafters gang.  He detected it
> when listening to very weak CW signals.
>
> Thanks for the suggestions ---- Kindest regards Jim K9AXN
>
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> ---------------  ORIGINAL MESSAGE -------------------------------
>
> Message: 7
> Date: Thu, 02 Jan 2014 14:33:36 -0500
> From: Al Klase <ark at ar88.net>
> To: boatanchors at mailman.qth.net
> Subject: Re: [Boatanchors] Need magnetic shield for shaded pole motor
> Message-ID: <52C5BF10.1000009 at ar88.net>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> Jim,
>
> Non-ferrous metals, copper etc., will do little to attenuate a magnetic
> field. There is an alloy, know an mu-metal, that was designed just for
> such applications, but it's expensive and hard to come by. Google same.
> Start here:
> http://www.pmlindia.com/hi-perm/Magnetic-Shield.htm?gclid=CJ3ghvWc4LsCFTNp7AodLUsAmA
>
> However, mu-metal only has a slight edge over soft iron or even steel.
> The coffee can may be a little too thin. The wreckage of a LARGE
> transformer may be the place to start.
>
> Al
>
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