cctech Digest, Vol 92, Issue 24 - test-drb@ccmp.vtda.org

16 Apr 2011

On 2011-04-16 19:00, cctech-request at classiccmp.org wrote:
...
  Are you saying that this uses a rack-and-pinion type
of hard positioner?
 If so, it's the first one I've heard of in a floppy drive, but anyway... It
is indeed a rack-and-pinion positioner.
...
  You would want som way to eliminate the backlash
between the rack and the
 drive gear. One common way to do thsi is to make the gear in 2 'slices'.
 One is fixed to the spidnle, the other is free to move by a small angle,
 but there;s a bias spring forcing it in a particualr direciton relative
 to the spindle. The idea is that said spring is pre-tensioned when the
 mechnaism is assembled so that the teeth of the 2 gears are forces to
 commpletel fill the gaps between the teeth on the rack. I know what you mean. There
is nothing like that here, everything is 
probably too small. The pinion is about 2 mm diameter and the teeth on 
the rack are something like 3 per mm...
...
  What I would do next is with the drive removed, try
carefully moving the
 head back and forth and/or rotatign the stepper motor spindle and see
 what moves. If something is stripped then moving the head will not
 rotrate the motore and vice versa. Of course you may find that something
 is jammed solid, in which cae that could be the problem. I removed the motor, it
turns out the rack and the pinion is in good 
shape. What I was seeing was just a deposit of grease and/or gunk. The 
head assembly was locked solid. It runs on a steel rod approx. 2 mm 
diameter which goes through what looks like bushes, and it was 
completely jammed from lack of lubrication, dirt etc. The bad news is 
that now everything is out of alignment since the head assembly came 
loose when I was trying to move it (it was locked really, really solid), 
and I don't have an alignment disk. Still, it was a learning experience...
I have ordered a new drive from a local shop, they don't keep them in 
stock. They are only 6 Euros so replacing it will be a lot less trouble 
than making it work again. I would have liked to fix it, but that is 
simply not practical, sadly.
Interestingly, Atari drives are sold for about $50 on eBay, with 
shipping to Sweden another $50. I suppose they are "collectable". That 
works out at about 10x the price of a new one. The difference is that 
the original drive is 720K and has a special bezel and eject knob, a new 
one is 1.44M and the bezel and knob can be transferred from the old one. 
The machine itself works with a 1.44M drive.
...
    have a
metalworking workshop with a lathe available to me:-)  ).  Also remember that if you
dismantle the head poositioner, you will need to
 set the radial alignment, which needs an alignment disk. Something else
 you may well not have. Indeed I don't... Next time, if there is one, I shall
try squirting some 
white spirit or something on to the guide rod with a syringe, carefully 
work the head assembly loose and then lubricate the guide rod sparingly.
...
  While it certainly isn't worth getting a lathe and
all the add-ons just
 to repair one floppy drive, it is somethign that you may want to consider
 at some point. Having the ability to make mechancial parts really
 increases the things you can repair, and it also removes worry from doing
 some jobs ('If I mangle this part getting it off, I can always turn
 another one.'). Yes, a lathe is a very useful machine. Unfortunately I
can't really see 
where to put one in my flat, except possibly a watchmaker's lathe. Of 
course one of those would have been just the job here if I needed new parts.

/Jonas