Re: Apple II drives and automatic monitor degaussing

["Michael J. Mahon" <mjmahon@aol.com>]

jsnospam@cimmeri.com wrote:
>  Hi folks,
>
>       Anyone have any thoughts on how close Apple drives (such as any
> 5.25" or 3.5") can be to monitors that automatically degauss upon
> power up?    Or if the degauss effect might even be beneficial to the
> drive heads as well?   There's plenty of thoughts via Google on
> degaussing near magnetic media, but I did not find mention of effects
> on hardware.

Demagnetizing disk heads is not necessary, so there is no real benefit.
Further, unless your heads are within inches of the outside edge of the
CRT, the field would be too weak to do the job.

In any case, there is no danger from the degaussing coil operation.

>    I'm thinking of rearranging my Mac desk to have the VGA monitor
> sitting on top of the IIgs + System Saver.   This raises the monitor
> about 7" off the desk.   Next to the IIgs is the floppy stack of 2 x
> 5.25" drives and 2 x 3.5" drives.. which puts the stack right up next
> to the monitor.   I suppose I could split the stack with 2 drives on
> one side and 2 on the other to get them under the monitor height.
>
>     Thoughts or guesses -- intelligent or otherwise -- appreciated.

The real issue is interference with disk reading from the stray fields
from the monitor deflection yoke and sweep circuits.

Whether or not this is a problem for a particular setup is a function
of the location and intensity of the fields generated by the particular
monitor, the susceptibility of the particular disk drive, and the exact
position of the two relative to each other.

Try the setup by reading disks--if it works reliably, great!

If it works unreliably or not at all, then...more distance is, of
course, better.  But the distance is not measured between the outside
of the cases, it's measured from the primary source of interference
(usually the yoke or horizontal output transformer) and the head of
the disk drive.  You may find that one side of the monitor or disk
drive is much more of a problem than the other.

Intervening magnetic shielding is also a major factor.

High frequency magnetic fields (1kHz up) can be effectively shielded
by an eddy current shield of any sufficiently conductive material
between the source and the sensor.  Some people even report that
aluminum foil has been effective, though a somewhat thicker sheet
of aluminum or copper would be preferred.

Low frequency magnetic fields will generally be more efficiently
shielded by a high-permeability magnetic material.  Soft iron is
somewhat effective, but there are specialized magnetic shielding
alloys (like Netic and Conetic) that are hundreds of times more
effective, though more expensive.

-michael

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