[Boatanchors] Need magnetic shield for shaded pole motor

[rbethman]

Jim,

Which specific transceiver is this?

Perhaps we can look at alternatives. In some instances I've gone to 
using computer fans.

Bob - N0DGN

On 1/2/2014 3:43 PM, james.liles at comcast.net wrote:
> 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
>