Malfunctioning 5.25" drive: please assist

[crownmi@zot.io.org (Myles Fudge)]

Your assistance would be appreciated with troubleshooting a 
malfunctioning Apple II disk drive that an acquaintance has
asked me to repair.  Thank you in advance for your time.
 
This drive is a third party disk II drive, connected to my IIe clone while
I'm testing it.  I also have an original Disk ][ drive that I'm using to
compare notes with.  The Disk ][ drive is in near perfect condition.
 
The 5.25" clone drive receives power and spins, however it does not
read the disk.  My Apple reports an I/O ERROR when I attempt to
CATALOG a freshly formatted 5.25" disk with the bad drive.  The disk, 
however, works just fine in my Disk ][.  I have found that once I've 
attempted to catalog a good disk in the malfunctioning drive the disk no 
longer works in the Disk ][ drive.  It seems that the malfunctioning 
drive is damaging the disk.
 
I am using Locksmith to check the disk's speed and Locksmith indicates
that the drive's speed will not come within tolerance.  That is, the
indicated speed is either too slow or too fast, and never within range
of acceptable drive speed.  I've turned the potentiometer inside the
drive and it sounds like the drive spins from slow to fast smoothly -
with no jump from slow to fast.  I cannot recall the program that
indicates drive speed in numbers, so I'm going with Locksmith's HIRES
speed graph.
 
I've physically inspected each component inside the drive
and they don't look or smell burned out.  I haven't gone through the 
trouble of testing any or each of the components out-of-circuit.  No
inordinate amount of heat is coming from any component.
 
All wires and connectors appear to be in place, the diskette I'm using is
not write-protected, and Locksmith ceases to spin the drive when I've 
opened the drive door during the speed test.
 
There is a pressure switch on the underside of the drive that is
depressed by a plastic arm when the stepper motor moves the read/write 
head to the track 0 position.  At this point in time, the drive is
in the track 0 position with the switch depressed.
 
Where do I go from here?  Thanks again.

Myles Fudge  (crownmi@io.org)
http://www.io.org/~crownmi/pleasure.html

P.S.  Whoops -- my apologies for not limiting the length of my text to 70 
      characters, or so, in advance.
-- 
Myles Fudge  (crownmi@io.org)
http://www.io.org/~crownmi/pleasure.html