Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!oliveb!intelca!mipos3!cadev4!dbraun
From: dbraun@cadev4.intel.com (Doug Braun ~)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Norton Speed Disk Bug and Fix?
Message-ID: <1673@mipos3.intel.com>
Date: 9 Feb 88 18:40:59 GMT
References: <327@ttrdf.UUCP> <2023@bsu-cs.UUCP> <990@neoucom.UUCP> <20291@amdcad.AMD.COM>
Sender: nobody@mipos3.intel.com
Reply-To: dbraun@cadev4.UUCP (Doug Braun ~)
Organization: Corporate CAD, INTeL Corporation, Santa Clara, CA
Lines: 50
Keywords: Speed Disk Bug

In article <20291@amdcad.AMD.COM> philip@amdcad.UUCP (Philip Freidin) writes:
>In article <990@neoucom.UUCP> wtm@neoucom.UUCP (Bill Mayhew) writes:
>>
>>I have read several times and have seen discussed on this network,
>>a phenomenon referred to as sector creep.  Apparently, sector creep
>>is a cumulative phenomenon that causes disk data errors after
>>extended time has passed.
>>
>There is no way
>that the error could accumulate, as the writes ALWAYS start in the
>splice region.
>
I looked at schematics for a (Shugart 10 meg) winchester, and I
noticed something interesting.  This drive, and most smaller, cheaper
winchesters, use diodes to switch the heads.  Each head is center-tapped,
and a positive voltage is applied to the center tap to forward-bias
diodes that run from the end-taps on the heads to the preamp inputs.
The circuit looks like this:

                                      ________
           --------------->|------+---|       |
          (                       |   |       |
      ||| (       other head ->|--'   |       |
      ||| (                           |       |
head  ||| (------- +V                 | preamp|------------
coil  ||| (                           |       |
      ||| (                           |       |
          (                           |       |
           --------------->|------+---|       |
                                  |   |       |
                  other head ->|--'   -------- 


The same switching voltage also provides the write current.

The point is that to forward bias the diodes, there must
be a dc current running through the heads at all times.
This would produce some magnetic field.  This field,
possibly in conjunction with the earth's (rotating at 
3600 rpm relative to the platters) could eventually
start erasing the disks.


Doug Braun				Intel Corp CAD
					408 496-5939

 / decwrl \
 | hplabs |
-| oliveb |- !intelca!mipos3!cadev4!dbraun
 | amd    |
 \ qantel /