Re: how to adjust a faulty 5.25" drive

[mjmahon@aol.com (Michael J. Mahon)]

Ernest asked:

>I have a couple of Micro-SCI floppy drives that are having a hard time
>reading disks. They both power on and seem to work OK but one drive won't
>read disks at all, and the other seems to be on edge of not being able to
>read. Basically, I think they just need to be adjusted a little but I've
>never done that and I'm not sure of how it's done.
>
>These drives have two adjusting pots. One pot is on the card that is on top
>of the drive, and the other pot is on the back of the drive. The top pot is
>small and fat an the back pot is long and skinny.

The skinny trimpot in the back is for adjusting drive speed.

There are several disk utilities that have a speed adjusting program,
though I'm drawing a blank at the moment...did Locksmith have one?

The top pot is an adjustment for the read amplifier, and should
probably not be tweaked without an oscilloscope and a drive
service manual.

>Can anyone give me some tips on how to re-align these drives? Are there any
>utilities that make the process easier?

In my experience with several dozen 5.25" drives, I have found that
disk rotation speed is frequently inaccurate.  Belt-driven drives have
a tendency to develop a lot of "wow" in the speed, as a result of belt
stiffening and shape memory.  This may even out as the drive is used
more over a period of weeks, or it may simply require a new belt.  ;-)

Apple's RWTS is relatively tolerant of speed variations, but if a drive
gets more than +/-10% out, things may not work.  Assuming that the
drive (and the diskette) are not sticking and the belt is OK, the speed
adjusting trimpot will bring the speed right back where it should be--
just a percent or two slow is best.

The other major adjustment issue is head alignment.  A drive which
has been treated roughly may need to be aligned.

There are special alignment disks meant to be used with an
oscilloscope to center the head on the standard track position.
These disks are recorded specially, and cannot be duplicated by
any ordinary disk drive.

In the absence of the standard factory alignment setup, I have found
that a "cut and try" method works pretty well, given a disk that is known
to be properly recorded (i.e.: not recorded on the faulty drive!).  It would
be very helpful if RWTS were patched to produce a "tick" or "beep"
whenever a read error was detected, so that the head alignment could
be adjusted in real time to the approximate center of the track.

The head adjustment depends on the type of head actuator, but for
the original cam actuator, alignment is adjusted by loosening the two
screws that fasten the head stepper motor to the frame.  The motor
body can then be rotated slightly clockwise and counterclockwise in
the slotted screw holes to achieve proper alignment.  When the correct
angular position is found, the two screws should be carefully tightened.

If the head actuator is a split-band type, there will usually be an
adjustment which produces an analogous slight in-and-out movement
of the head.  It is usually sealed with glyptol paint at the factory.

I have always wished for a real-time indication of disk I/O errors,
short of the gross one of head recalibration when all normal retries
are exhausted.  Although it might make people a little less trusting
of the infallibility of their drives, it would be a valuable warning when
margins are growing thin.  (I had a patch for this several years ago,
but can't locate it at the moment.)

-michael

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