29 Nov
1999
29 Nov
'99
4:02 p.m.
please see comments embedded below.
Dick
-----Original Message-----
From: Tony Duell <ard(a)p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp(a)u.washington.edu>
Date: Monday, November 29, 1999 7:45 PM
Subject: Re: Needed: 1 IBM 8" alignment disk.
These are not the usual things for which you'd want an alignment
diskette.
> It isn't alignment as I can manually move the
head very slightly back and
> forth which does affect the data coming into the controller assembly but
> does not help it find the actual sector... Therefore, hardware or an
anlog
> adjustment
>
> I would like an alignment disk so I can use "their" tuning procedure to
make
> this drive work quickly. It has a number of
adjustments including :
Clock,
window, gap,
etc...
Most alignment disks do not include such tests (I am not saying that the
IBM one you asked for doesn't). They're just raw drive alignement disks
with catseye patterns for aligning the radial position, an index timing
burst, etc.
If you think there's an electronic problem then you don't really need an
aligment disk. Start with the spindle speed, which I assume is right
since it almost certainly uses a mains-powered induction motor. Still, it
can't hurt to check that the index pulses occur at the right frequency.
They'll be a master clock, probably also used for writing. Check this
with a 'scope or frequency counter. If it's incorrect, find out why.
Generally, there isn't a master clock. Among the drives I've been working
on over the past months, none had onboard oscillators with the exception of
the microprocessor-controlled Mitsubishi. That's why there are one-shots
and the like. The writing is accomplished by using both outputs from a
flipflop which is toggled by every positive edge on the data stream which is
generated on the controller.
Then take a formatted disk (format one yourself on your old CP/M machine
if you have to ;-)). Read a sector continously. Display the off-disk data
on one trace of a 'scope and the read clock on another. Does the read
clock seem to be the right frequency? Is it locked wrt the data? If not,
look at that read PLL (or whatever it uses).
Again, most drives don't have PLL's on them, but rather leave that to the
controller. It does appear that this drive and controller may not be
entirely separate. The adjustments for "window" and "gap" do sound
like
they are just trims on a one-shot, though. I've seen adjustments like that
on old hard disks. As you may recall, almost all the early FDD's had FM
clock/data separation on board. This drive is probably no exception.
Index timing is either very important (if, for example, the controller
expects to see an address mark as the first thing after an index pulse)
or not at all important at this stage.
At this point you should at least be recovering data bits from the disk.
So if it still can't find a sector, find out why not. Can it not find an
address mark at all?. Or can it not find the header you're asking for.
This is likely to be a digital problem. As I don't have schematics for
this unit I can't comment any further.
-tony