Shugart 800-8 media centering problem

[tony duell]

> Except that when the media is aligned to the spindle correctly, the
> drive performs flawlessly, including exchange with another drive.  If
> the spindle itself were wobbling so much as to cause the other symptoms,
> I don't see how it could ever work right.

> On these drives, the centering cone (i.e., that which centers the media
> on the spindle) is actually part of the spindle.  The part that comes
> down onto it is a set of nylon fingers, all one piece, with a spring
> surrounding them.  They call it the "Spindle Hub Clamp".  All this
> latter part does is clamp the media to the spindle.  The SA-800 doc
> makes it hard to see what is going on, but it is at

In every 8" (and 5.25") drive that I've seen the spindle (the bit driven, maybe
indirectly, by the motor) has a female cone on the end. There is then the clamping
cone, often plastic which fits into it through the hole in the disk.

That plastic part may be shown as one piece in the service manual as individual parts
are not available as spares, but most of the time it's actually 2 bits with a spring inside.


> Haven't taken the spindle apart.  (Page 6, parts 2 through 10).  Not
> sure I would want to - if it is that worn somewhere, requiring changing
> out its bearing or the like, then for me the case is probably hopeless.

If it's not the clamping cone then it pretty much has to be the spindle. A comment
about pot metal got me thinking. If the cone has swollen due to oxidation of one 
component of the pot metal, it may be that the plastic cone is compressed too far too
early and the media slips out of alignment.

I would inspect the metal part of the spindle very carefully. I would check for excessive runout 
of the conical section. I would look for damage (burrs, etc). Do you have mechanical measuring
tools (callipers, etc)? 

The ball races are most likely standard parts and can be obtained. Other parts may have to
be made.


> My gut feeling about this remains that something interferes with the
> media centering itself on the cone of the spindle before it gets clamped
> down.

My feeling is similar, that the clamping cone allows the media to slip about before it
is finally clamped so that sometimes it works, sometimes it doesn't

> What I am really looking for is someone who had a drive with similar
> behavior that can say "hey, I had that too, and it was ......".

Firstly I think it's a long shot that somebody else will have had this and secondly
even if they have had the same symptoms it might be due to a different cause. Better
to actually work out what is wrong with _your_ drive (sorry, but I have never gone along
with the idea of stock faults and cures [1]).

[1] May be OK if you have a lot of similar units to repair, say you are a repair shop and
you keep seeing the same model of TV come in. Stock faults may then let you get 90% off
the bench very quickly. But they are not much help with one-off machines like we tend to 
get on this list

-tony


JRJ