Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!lll-winken!uunet!mcvax!ukc!acorn!moncam!emmo
From: emmo@moncam.co.uk (Dave Emmerson)
Newsgroups: comp.sys.atari.st
Subject: Disk Drives
Keywords: disk floppy drive
Message-ID: <249@marvin.moncam.co.uk>
Date: 30 Jul 89 14:08:09 GMT
Organization: Monotype ADG, Cambridge, UK
Lines: 63

Since this may be of general interest, I'll post it here, rather than
reply directly.

All the disk drives I have ever seen, hard or floppy, have a jumper
on their pcb near the data connector, which allows you to set its
device number. Some only allow you to set it to 0/1 (a/b), most have
4 possible settings. No rewiring is needed, just find the appropriate
link and move it to its new setting. If you are at all unsure about
this, sketch all the link positions before you move any, and use a 
disk you won't mind reformatting to test it with. The links are often
marked DX and usually numbered either 0 to 3 or 1 to 4. DONT touch
any marked MO, TB1, TB2, DCG, MN, PAR.

Problems..
The following is a brief note of 2 of the causes of my own disk drive
problems over the years. Both can cause intermittent or seemingly
permanent problems on any computer.
a) using the wrong type of floppy disk. Seems too obvious? I bought a
   box of re-marked Sony DSDD floppies from a local (normally reputable)
   supplier. They worked fine for weeks, then I started 'losing'
   files. It was some months before I found out that they were only
   single density. All that work salvaging files.... Aaaagh!
   (I didn't suspect them as I could see the removed Sony legend under
   the new print. It was only when I looked much more closely that I
   saw they were SD.)
   Using HD disks on a DD only drive isn't a good idea either.

b) Power supply overloaded/out of adjustment. This one can be much more
   sneaky. The +- 5% tolerance quoted in many drive manufacturers' specs
   is often optimistic. I have had drives fail with their 12 volt rail
   over the 11.5v this allows. Due to bitter experience I *never* use
   a drive with its 12v supply under 12v, I'd rather have it nearer its
   upper limit. Linear power supplies often exhibit a tendency to droop
   a little in the wee hours when mains supply voltages frequently drop
   by 10% from their norm in my area at least. (The supply company are
   probably breaching a statutory regulation).
   To check this, you'll need a 20v meter or an accurately calibrated
   'scope. The ground (0v) pins are the middle two on the drive, with
   +5v one side, +12v the other. If they read OK when you first check,
   keep everything handy till things start going wrong.
   Curing this problem is a little more tricky. Many switch mode supplies
   have no adjustment, some others have only one adjustment, - for
   the +5v rail. The other outputs are designed to be 'in spec' when the
   5v is correctly set up and supplying its correct load. If you are 
   overloading it slightly it won't trip the protection circuit, but the
   other output(s) is (are) 'starved' for current so their voltages drop.
   It should read between 5.0 and 5.4v. at the drive. Normally the 12v
   rail will drop long before the 5v does.
   Supply voltages DO drift a little as components age, and a 6 monthly
   check/readjustment by a competent engineer is common practice in
   industry. BEWARE though, some types of supply can hold a potentially
   LETHAL charge even when switched off. By all means check it yourself,
   but unless you know what you are doing, get a qualified technician
   to service it for you!
   Do make sure that the power supply gets plenty of fresh air. Burying
   it under piles of printout etc. is asking for trouble. So is letting
   it fill up with dust and fluff.
   For that matter, your ST needs some ventilation too, don't use it on
   a carpeted floor or on top of your bed or you'll block the vent slots.

   If all this helps anyone at all it'll be worth its bandwidth.

   Dave E.