cctech Digest, Vol 12, Issue 33, Re: Drive Alignment

From: Barry Watzman <Watzman_at_neo.rr.com>
Date: Tue Aug 24 12:45:56 2004

RE: Drive Alignment

8" drives are usually pretty stable once aligned, but perhaps not over a
period of decades.

There are two ways to check/set alignment.

The "classical" (mid-70's) way is to use an analog alignment diskette and a
dual-channel oscilloscope. You connect the scope to two channels of the
read head amplifier and seek to the alignment track of the disk (my
recollection is track 38 decimal, the center of the 77-track diskette). You
should see a kind of "filled in dual sine wave". If it's symmetrical or
close, the alignment is correct. If one lobe (every other one) is of
significantly greater amplitude than the lobes in between, you are out of
alignment and you adjust the position of the head to get it right (this part
varies from drive to drive). Then you have to check some other things, like
the track 0 sensor and track 0 phase (on some drives). You need the analogy
alignment disk, a scope and an "exerciser", or a computer with software that
could let you manually seek and load the head as desired.

The "modern" (about 1981) way was with a digital alignment diskette. This
was a normal data disk, almost, but various tracks on the diskette were
either correctly aligned, or intentionally mis-aligned by varying known
amounts. By seeing which tracks could be read, and which could not, you
could determine the alignment status of the drive. This was useful for
checking alignment, but not nearly as good for setting it. SOME of these
disks ALSO had the analog patter on track 38. You needed the digital
alignment disk and software that knew what to do with it.

The problem today is that the alignment disks (of either type) are not
readily available, and most PC hobbyists don't have a scope. Documentation,
knowledge and experience are also issues.
Received on Tue Aug 24 2004 - 12:45:56 BST