Servo tracks on SMD disk

Tony Duell ard at p850ug1.demon.co.uk
Sun Mar 28 13:23:50 CDT 2010


> 
> Hi,
> 
> I have some trouble with a 14" removable SMD disk drive (Ampex DFR 996): 
> When it comes to the outer tracks it starts making noises like a cat and 

Based on my experiences with Pentina, I don't believe a cat makes a 
unique sound...

Anyway, I assume you know it's not headcrashing (which can make a noise 
like a very angry cat). I think you'd know if that was the case...

> recalibrates. Endlessly. With some disks I have it manages to crouch up 
> to the end, track by track with many retries. The problem seems to exist 
> with most of the packs I have. I played around with the head cables 
> while the heads are further outside. Seems to be no contact/strain 
> problem here. The drive seems to be working fine for the rest of the 
> surface. But having the end unusable makes the disks unusable for RT11 
> with its replacement tables. And if the drive doesn't even manage to 
> arrive at the end, RT11 won't even write a new directory to the disk.
> The drive does not recalibrate on bad blocks. So the problem should be 
> in the servo system and nowhere else.
> 
> Does my problem sound familiar in any way?
> 
> Playing with the servo head amplifier gain adjustment did not help. In 
> fact I found out that there's a working range, and the original 
> adjustment was quite in the middle of that. So I kept it the way it was 
> adjusted. For real readjustment of the drive I'd need a disk exerciser 
> board, CE cartridge and some other special purpose bits. None of them 
> are available to me.
> 
> Another idea is that the servo data is decaying more quickly in the 
> center of the disk: The disk hub is held by a magnet in on the spindle. 
> The magnetic field and the mechanical shock while inserting the 
> cartridge could affect the disk. Or/and the problem could be caused by 
> thinner magnetic coating in the center (too close to the inner edge of 
> the coating?). Vague theses....
> 
> The disk drive also has a fixed disk with three disks and 5 data plus 1 
> servo surface. That works flawlessly.

This sounds somewhat similar in concept to the CDC 'Phoenix' drive. 
There's a separate servo surface for the remvoable pack (one servo, one 
data), and the 3 fixed disks (1 servo, 5 data surfaces).

What do you mean by 'outer' tracks? Normally, I would take that to mean 
ones closest to the edge of the disk, but it appears you mean ones neares 
the spindle.

Do you have schematics and a 'scope? The first point I would look at is 
th edrive to the positioner amplifier, just to make sure there's a 
component of the right frequency there to procude the noise.

Since it works correctly on the fixed disk servo surface,  Iwould guess a 
lot of the electronics must be working correctly. So the problem might 
well be in the removable paclk's servo head, or the preamplifer stage 
after it. Are there any other electronci changes made when going from the 
fixed ot the removable pack (change in gain?). What about any velocity 
feedback signal? That would have been an early guess of mine (lost of 
this can make the servo unstable), but why would it only affect the 
removeable pack?

Can you look at the output of the servo head preamplifer when using the 
fixed disk (which works correctly) and the remveable disk? Step one 
cylinder at a time going towards the 'bad' part and see if you can notice 
any changes in the signal.

I do have a CDC Phoenix service manual, if you think the drives are 
related I can take a look...

-tony



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