Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!van-bc!ubc-cs!alberta!calgary!ctycal!ingoldsb
From: ingoldsb@ctycal.UUCP (Terry Ingoldsby)
Newsgroups: sci.electronics
Subject: Slowing down a disk drive
Keywords: 350 rpm -> 300 rpm
Message-ID: <324@ctycal.UUCP>
Date: 1 Feb 90 19:32:14 GMT
Organization: The City of Calgary, Ab
Lines: 43

I have an interesting problem that someone out there who has an
IC master might be able to help me with.  I have an ancient
(venerable) computer that uses a 96 tpi double sided double
density floppy disk drive.  Due to the fact that one of my
(late) nephews knocked it on the floor and cracked the head,
it doesn't work anymore.  The drive was the old fashioned
low density type drive, not withstanding it having 80 tracks.
ie. it ran at 300 RPM and laid down twice as many tracks
as a conventional PC floppy drive.

Thinking that since a PC high density drive also has 80 tracks
per inch, and that the only difference was the rotational
speed, AND that the data format is generated by the disk
controller, I bought a high density 1.2 MByte disk drive.
Every drive I had ever seen before had a pot on it to adjust
the rotational speed.  Not this one.

The motor, a direct drive affair, is driven by a chip marked:
  B 839008
  M51785SP
I believe it has 32 pins.  Monitoring the pins with a scope
indicated an oscillator that seems to be controlled by a
small, green, square edged device on the printed circuit
board, about .7 cm wide, .3 cm thick and .5 cm high.  The
device appears to be a ceramic capacitor EXCEPT for the
fact that all other caps on the board are silk-screened as
C1, C4, etc. and this one is called Y01.  Removing the
unidentified component caused the motor to slow to a crawl,
and the oscillator to stop.  Putting any cap I had in my
junk box in place of Y01 caused the motor to whirl like
a dirvish.  Either this is supposed to take a very small
cap (a few pF) or else it is something other than 
capacitor.  The resistance of the original green device
appears to be infinite.  The device is much too small to
be a crystal (I've forgotten the oscillator frequency,
but I think it was about 800 KHz).

Any ideas?

-- 
  Terry Ingoldsby                ctycal!ingoldsb@calgary.UUCP
  Land Information Systems                 or
  The City of Calgary       ...{alberta,ubc-cs,utai}!calgary!ctycal!ingoldsb