[vcf-midatlantic] Drive repairs, was Hard drive art

David Gesswein djg at pdp8online.com
Thu Apr 18 21:58:18 EDT 2019

On Wed, Apr 17, 2019 at 12:36:04PM -0400, Dave McGuire via vcf-midatlantic wrote:
> On 4/17/19 11:46 AM, Jason Perkins via vcf-midatlantic wrote:
> > Another issue we'll run into is servo tracks, and no way to re-create them
> > once they fade. With the software / fixtures only available at the factory
> > I don't know how we could recreate those tools.
>   First you'd need to first determine the exact format of the servo
> tracks.  Keep in mind that they may not be "digital" tracks.  If the
> format cannot be determined from patent info, etc, then the only choice
> might be to temporarily disassemble a functioning drive of the same
> model, identify the servo head (assuming a dedicated head for early
> drives), connect it to a signal processing chain and then digitize it.

On some of the drives I've poked at you can get at some of the servo
signals on the circuit card. May or may not be available as analog waveform
at convenient levels. The low level head signals should be available on
the head cable. None of the drive I have poked at have more than a head
amplifier in the HDA. Only a few drives have schematics and hit or miss on if
datasheets are available for the major chips. Some of the drives encode
data in the servo identifying track and areas outside data tracks.

I haven't looked much at embedded servo since its rare in the SA1000/ST506
type drives. I was surprised that the Quantum Q20X0 drives and the 
Syquest SQ306 use an early form of embedded servo. The SQ306 uses servo with
a stepper motor I assume in microstepping mode.

>   Then it should be possible to synthesize those waveforms and lay them
> back down on the platter.

Accurately positioning the heads seems like a challenge. I assume you could
do some sort of optical position measurement and make your own servo
loop to position them. That would require opening the drive.

>   Not easy, of course, but similar things have been done by smart and
> motivated people with, say, QIC tapes. (I'm looking at YOU, AJ!)

I think it may be possible but significant work for the first drive and
more for each additional make of drive so I don't expect it to become common.

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