Shugart 800-8 media centering problem
cube1 at charter.net
Fri Aug 14 11:27:40 CDT 2015
I have a Shugart 800-8 with a media centering problem. I know it is
definitely that and not something else (say, electronics, TRK00
position, etc.), because I can see the "wobble" in the signal coming
from the heads on a 'scope. Depending on how I load the floppy, the
attenuation can be from nothing (in which case the floppy reads/writes
fine, including exchange with other drives - the Altos has a nice floppy
diagnostic), to as much as 80% (in which case I cannot boot from it, it
won't read anything at all). So, it isn't the TRK00 position or
anything like that.
With a 'scope, I can see that the wobble is a bit over 80ms in period -
1/2 the rotation time, confirming that it is wobble.
It isn't slipping or anything like that, and I have tried different
media as well, to make sure that the issue isn't a worn hole in the
I can usually get it to center properly if the drive is on its side, and
when I close the door I close it slowly, and "massage" the door (a bit
more closed, a bit more open a couple of times) just as the clamp starts
to engage the spindle - say, two out of three insertions work fine.
This also confirms that the spindle itself is not wobbling, aside from
In the normal position of this system, the drive is upside down, with
the motor-driven spindle on top. In that position, I am lucky to get it
to work one out of five times. The issue is not the head load tension
or pad, either -- even when off center, the peak amplitude is just fine,
and when properly centered, the amplitude is the same as the other drive
on this system on tracks 0, 38 and 76.
I have exchanged the spindle clamp with another drive, and also the
eject mechanism (the spring was a bit stretched when I first got the
drive, but I repaired that) as well, to no avail.
The handle was a little out of adjustment - coming down a bit too close
to the spindle (but not interfering with it), but adjusting that did not
seem to help either.
When I look at the cartridge guide without the spindle clamp in place,
it is now centered above the spindle, and at the same distance from the
spindle as my drive that does not have these issues.
Anybody have any experience like this? Thoughts? There really isn't
much to adjust on these things.
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