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From: info-mac@uw-beaver
Newsgroups: fa.info-mac
Subject: Re:  Programmer's switch
Message-ID: <287@uw-beaver>
Date: Fri, 21-Dec-84 21:15:42 EST
Article-I.D.: uw-beave.287
Posted: Fri Dec 21 21:15:42 1984
Date-Received: Sun, 23-Dec-84 00:35:14 EST
Sender: daemon@uw-beaver
Organization: U of Washington Computer Science
Lines: 33

From: olson@harvard.ARPA (Eric Olson)

I read a good theory in INFO-MAC a while ago about why external drives
sometimes start trashing disks.  The programmer's switch/power off issue
may seem to help, but I think it helps by side-effect only.  To summarize:

The external drive sits in some random position outside the Mac.  When the
mac reboots (via power cycle or programmer's switch) it resets the video
controller (possibly) causing a massive collapse of flux in the flyback
transformer (or some other transformer).  If the read/write head in the
external drive is in the path of this field, its head could become
semi-permanently magnetized.  It will then trash disks inserted because
it writes all over them with a (semi-permanent) magnet.  To cure this
problem, the suggestion was made to take a disk and keep inserting it
in the drive and formatting it.  This will put the write-bias current
through the head a lot, and hopefully demagnetize the head.  Putting a
plane of conductive metal (like a cookie sheet) between the Mac and the
drive cured the problem [I wish I could take the time to attribute the
creator of this wisdom, but I can't look through the archives for that--
leave it at: this isn't original thought].

Considering that the power cycle is probably much more fluxxy than the
programmer switch reset, I wouldn't recommend using the power cycle to
get the machine to behave better.  It's just possible that power-cycle
works both ways:  sometimes magnetizing, sometimes demagnetizing.

Whatever.

-Eric

[For those of us without cookie sheets, would it be wise to always
position the external drive on the RIGHT side of the Mac, farthest
away from the fly back transformer(?)  --bc]