Reading discs written by miscalibrated drives

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Posts: 5
Joined: Wed Feb 16, 2011 12:07 pm

Reading discs written by miscalibrated drives

Post by RichAplin »

It's probably possible to microstep the heads on a disk drive to read stuff that was written on drives with bad alignment.
This would be an advanced hack, and is just a suggestion..
Basically if you're reading a 40 track disk on an 80-track drive (5.25") you've already got half-track stepping, which I've seen is used already for those disks, but my thought is that you could fairly easily-ish modify a 3.5" drive to microstep; probably at 4x or 8x normal track resolution.
Microstepping is basically just PWM modulating the existing stepper coils so the rotor stays at midpoints between normal stops. Sounds hacky but actually works really well.

Three ways to do it;
a) replace stepper in drive with higher resolution one. Tricky.
b) try to microstep the existing drive by high speed wiggling of the existing step lines
c) disconnect existing stepper motor from drive PCB + connect it to your own driver, then microstep it in software

A is tedious and no fun, b may work somewhat but is likely to be foiled by large variations between drives and EMI suppression filters, and c is quite likely to work with a small amount of wiring and a suitable driver chip (e.g. ULN2003 which is about $1)
You basically need two more output pins (so you have all 4 phases, currently you have "step" and "direction") and a driver chip.
To get the PWM juuuust right would probably be good idea to use a second Kryoflux board just as a microstep controller.

With this setup, any normal head misalignment issues should be readable, although I'm not an expert on what goes wrong with floppy drives.

Anyway just a thought. Someone would have to be really interested in a "forensic drive" mod like this to do it, but it can probably be done.

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Joined: Wed Jul 20, 2011 9:12 am

Re: Reading discs written by miscalibrated drives

Post by turnkit »

Brilliant! Love to buy a unit with a special drive if it would solve read issues!

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Re: Reading discs written by miscalibrated drives

Post by mr.vince »

I'd assume cost for such pre-modded device would be about EUR 300,- to 500,- for parts and labour... Too costly for private users I'd say...

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