Re: anyone know where I can find a service manual for Disk II

["Michael J. Mahon" <mjmahon@aol.com>]

Silicon Sam wrote:
> On Apr 10, 5:22 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
>
> > >  Not sure, but since the sine wave was a slow one, I would say it
> > > only had to wobble once per revolution.  The object was to write a
> > > signal from 1 track to the other, and measure the track in between.
> > > Obviously the disk can sit still and the head move back and forth
> > > between tracks while writing.  But it would have to be a mechanism far
> > > more precise than the cam that Shugart and Alps used.
> >
> > >  Yes, there are several signals on the disk, this is just how they
> > > made the cats eye recording.
> >
> > I'm having a hard time seeing how a wobbled recording makes a
> > cat's eye diagram.
> >
> > It seems like the display would be amplitude modulated at the wobble
> > frequency when off-center in either direction and amplitude modulated
> > at twice the wobble frequency when on-center, assuming that the
> > amplitude of the wobble was a bit more than a track.
> >
> > And I agree that this would require a special drive to record--
> > either one with a voicecoil actuator or (kind of fun) one with
> > an eccentric spindle.  ;-)
> >
> > The dual-frequency recording I described could actually be made
> > in a non-phase-coherent way, with a single headed drive.  One
> > frequency would be recorded with the track positioned +0.75
> > track, and the other at -0.75 track.  The only problem would
> > be ensuring that the edge of the erase head on the second track
> > written didn't put the "equal amplitude" point off the center
> > of the nominal track.
> >
> > -michael
> >
> > NadaPong: Network game demo for Apple II computers!
> > Home page:  http://members.aol.com/MJMahon/
> >
> > "The wastebasket is our most important design
> > tool--and it's seriously underused."
>
>
>   A cats eye is a signal that is very strong, then drops off to
> nothing then back strong again.  Like a sine wave on AC voltage.  What
> you see when you look at track 16 is a strong amplitude signal when
> the signal written is spot on track 16, and it drops off the as the
> signal get weak as it moves over to track 15 or towards track 17.  It
> returns again as the disk spins and the amplitude of the signal gets
> stronger, then weaker, then non-existent when it is spot on track 15
> or 17.  Repeat as long as the disk spins.  Adjust the head until the
> highest peak of the signal is the strongest.  Page 3.16 of the disk2-
> tech manual mentioned above.
>
>   About the best way I can describe it, but I can try again...

Please do...

If there were a signal written with a "head wobble", that would not
produce the result you describe.

I presume you are describing a high frequency "carrier" that is
amplitude modulated by a sine wave at the wobble frequency (or
twice the wobble frequency).  (Such a "carrier" could be a 250kHz
waveform resulting from writing all 1's to the track while wobbling.)

If the test drive is properly aligned, sitting on track 16, with
a "wobbled" signal moving under the head, you would see an amplitude
modulated signal at twice the frequency of the wobble.  The test
signal's amplitude would diminish twice per wobble cycle--once when
the wobble was off-track to the inside and once when it was off-track
to the outside.  Of course, that means it would also peak twice per
wobble (when the wobbled track was directly beneath the head).

In other words, the modulation sine wave would be at twice the
frequency of the wobble (two per revolution if the wobble is once
per revolution).

Centering the head would correspond to adjusting radial position
until both modulation peaks (or minima) are *equal*, not maximal.

The peak amplitude would remain the same if the head were mispositioned,
since one part of the wobbled track would always pass right under the
head, producing a maximum amplitude signal.  If the head were off-center
on the wobbled track, however, one of the "minima" would shrink and the
other would grow until the modulation sine wave was at the same
frequency as the wobble.  This corresponds to the head being positioned
so that it only senses one "edge" of the wobbled track.

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it's seriously underused."