Servo tracks on SMD disk

toby at toby at
Tue Mar 30 01:33:15 CDT 2010

------Original Message------
From: Tony Duell
Sender: cctalk-bounces at
To: cctalk at
ReplyTo: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Servo tracks on SMD disk
Sent: 29 Mar 2010 19:14

> Hi Tony,
> > Based on my experiences with Pentina, I don't believe a cat makes a 
> > unique sound...
> You're speaking of the individuality of a DOG! A cat makes miau! :-)

I am sure Pentina is a cat. And he certainly males many different noises...

> > 
> > 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...
> No crash. But I have to admit that I got the drive crashed a few years 
> ago. After cleaning and using another pack it worked. I had 3 packs for it.

I assuem it has worked since the crash. Could the heads have been damaged 
by the crash?

> At some point in history I got a bunch of packs. They were used with a 
> somewhat dubious Nova clone system (DDC or DCC). They never really 
> worked fine.
> A few months ago now the drive completely broke down: After having 
> successfully used the machine for several hours and pausing for another 
> few weeks, I ran into disaster: A mechanical buffer at the back of the 
> head slide had changed to a sticky liquid and blocked the whole 
> mechanism. That resulted in the heads staying on disk while the drive 
> did an urgency spindown.

Hang on.. Are you saing the heads landed on the disk? And that the drive 
hasn't worked properly since that? I really wonder if the heads have been 

> I disassembled the whole thing and cleaned away the mess as good as 
> possible. I did not touch the head alignment. The heads stayed bolted to 
> their slide and were taken aside as a whole block. I also took care for 
> the heads not getting in contact.
> This week I reassembled the drive and tried it out. The cartridge that 
> was in the drive during the disaster was far gone and runs with 
> "pre-crash" noise.
> I realized that I had only the dubious packs. Some of them as I found 
> out yesterday can be formatted by the Emulex SC02 controller's low level 
> format routine, some stop shortly before the end (as I explained 
> before). When I then try to use them, I can run "INIT/BADBLOCKS" under 
> RT11 which says no bad blocks. Then when copying data in, I get write 
> errors and bad blocks. Number increases. They stay and are in the system 
> area most of the time. So I cannot use the packs :-(
> The whole procedure can be repeated using the low-level formatting 
> routine and then RT11 INIT.

OK... The formatting routine must move the heads across the disk surface, 
and it will read (but not write) the servo surface. Can you 'scope the 
output of the srrvo preamplifier while it's doing this? See if the signal 
changes in amplitude or whatever.

> That leads to the idea that there's something wrong about the data as well.
> > 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).
> Exactly.

But there are differences. I don't think there's any oil-filled damper on 
the Phoenix.

> > 
> > 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.
> Sorry, meant "inner", of course! I was tiredly writing in foreign English...

OK. I was just making sureI knew where the problem area was.

> > Do you have schematics and a 'scope?
> Yes. But... Sorry for saying that: That machine has a very very low 
> priority in my project queue as it's something from the 80s. I currently 

OK... Alas I suspect this is not going to be a quick fix.

> have enough older stuff to repair (RK05s, TU56s, PDP8/I, PDP8/Ls, 
> Honeywell DDP-516 and H316 reconfiguration and testing, etc.). I just 
> wanted to get it either working and back in the rack or out of the 
> window (i.e. give it to someone else, NOT trashing it!).
> > What about any velocity 
> > feedback signal?
> Speed might be worth a thought:
> - If running too slow, servo and d

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