Multiple Topics...

From: Tony Duell <ard_at_odin.phy.bris.ac.uk>
Date: Tue Aug 19 08:22:37 1997

> <If somebody could find a UK source of 8" alignment disks then I'd buy one
> <_now!_
>
> Dec diagnostic disks that havent been rewritten are generally very close to
> nominal alignment.

But as you said below, you need offset tracks to do an alignment properly.
Trying to find the points where the amplitude falls off and then setting
the head midway between them has never worked for me.

>
> <I was planning on removing the pin altogether and letting the head
> <carriage move freely over the leadscrew. Then fit a micrometer head and a
> <spring to pull the head assy agains the end of the micrometer. I should be
> <able to get about 1" of calibrated head movement.
>
> While you can do this there is one problem. Alignment disks are recorded
> with a narrower than normal trackwdith and also stagger tracks (recorded
> with offset either side of true) for alignment use.

Yes, I'd realised that. I was planning on making a circuit that triggered
off the index pulse and recorded 'bursts' of (say) 250kHz pulses on the
disk. A bit of logic would let me record a track offset towards the edge
of the disk, twiddle the micrometer to move the head to the same offset
towards the spindle and then record bursts between the ones I'd just put
down. Now align the target drive so that both types of burst are replayed
at the same amplitude.

>
> Another tool you need is a source of 125/250/500khz pulses that conform to
> FM timing (single density) for writing patterns. This is something you can
> build out a handful of counters and a 4mhz clock osc.

That's somewhat trivial to build IMHO...
 
> Over the years I've found that if the drive needs alignment it's wise to
> look for other problems like spindle and motor bearings that are tired

The bearings are AFAIK standard parts, and can be easily replaced.

> or head to actuator wear that will make for sloppy operation. Head
> alignment can signal other problems. I retired a SA800 for spindle
> bearings because the thing would not reliably read (had a new in the box
> spare). Generally I've found that it's best to put aside those drives in
> favor of a better one stealing the logic from it as needed. The only time I

The older SA800's used entirely standard logic, so it's not worth taking
parts from an old one. SA850's used custom chips in the read/write
circuit, and later SA800's (according to my service manual) used one big
custom chip :-(. I guess then you have to get spares from old drives.

> would fix it is when there is really no other choice. Unless doing a museum

Getting _new_ 8" drives is next-to-impossible, and if you use a
second-hand one, you've got no proof it's better than the one you're
replacing unless you test it with the alignment disk (after checking it
handles a scratch disk correctly, of course...).

> style restore I've found some drives are better dumped (sa400s in general)
> in favor of other better drives of the era.

The less said about that SA400 the better....

>
>
> Allison

-tony
Received on Tue Aug 19 1997 - 08:22:37 BST