Re: Diagnosing and fixing a faulty Disk II drive

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

EricN wrote:
> I have a Disk II drive that is giving me some problems.  When my Apple
> is first powered up and I run some standard disk alignment tests, I
> will get read/write errors in the drive.  However, after the drive has
> warmed up, the tests will pass 100%.
>
> Drive speed is right at 300 rpm and I've cleaned the read/write head.
> My thought is that the stepper motor might be a little sticky until it
> warms up and maybe needs some lubrication but this is total
> speculation on my part.  Does anyone have any suggestions?  If it's a
> sticky motor, what kind of lubricant is safe?

Stepper motors are positive indexing, so they will not have small
position errors.  If the lubricant has set up or there is another
major source of friction, then it will simply miss steps entirely.

If this is the case, it will be pretty visible if you look at the
seek cam while verifying or formatting a disk, since those operations
seek one track at a time (two pulses per track) across the full width
of the disk.  Any non-uniformity in movement will be evident.

From what you've said, I assume that the disk speed is correct both
when the drive is "cold" and when it has been in use.  If this is the
case, it rules out the cause I would suspect:  that the rubber belt
is stiff, and causes periodic speed variations until it has been run
for a while, causing it to forget some of its reating shape.

This leaves the analog card as a possible culprit.  If the temperature
is changing much (surprising if it is), then the read amplifier IC
could be marginal, and the temperature rise is improving its margins.

-michael

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