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

[Dave McGuire]

On 4/18/19 9:58 PM, David Gesswein via vcf-midatlantic wrote:
>>   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.

  Agreed on all points, and yes the only thing really ever put in the
bubble is the head amplifiers.

> 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.

  Ok that's neat, I hadn't heard of a servo-positioned stepper head
positioner.

>>   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.

  But if we're rewriting the servo tracks anyway, it won't matter,
unless we're trying to recreate them to align to existing data.
Otherwise they can be anywhere, and the other tracks will follow suit
when the drive is low-level formatted.

  I guess the question is whether we'd do this to recover the
functionality of a drive or recover the data from a drive.

             -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA