[sci.electronics] Slowing down a disk drive

ingoldsb@ctycal.UUCP (Terry Ingoldsby) (02/02/90)

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

whit@milton.acs.washington.edu (John Whitmore) (02/03/90)

(Terry Ingoldsby writes:
>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.
	Well, you have definitely found the frequency-setting
component.  It is a crystal resonator made of a polycrystalline
material (ceramic), and "poled" into the correct behavior by
electrical biasing while it was being boiled in oil...
Such ceramic resonators, and the similar ceramic filters, are
cheaper than quartz crystals, and more than accurate enough
for most applications.  The "Y01" is a crystal designation,
and I guess the ceramic components are close enough to qualify.
	Usually a replacement for such a component would
be a "pi" network; input node to capacitor to ground and to
an inductor, other end of inductor to capacitor to ground, and
to the output.  Replacing the inductor with a quartz crystal, this
becomes a parallel resonant crystal circuit.  If you want to
change the value, find capacitors and an inductor such that
1/SQRT(L * C) = 2*pi*<frequency>
where 1/C= 1/C1 + 1/C2 for the two capacitors
Depending on the nature of the resonator, it may have external
capacitors already connected in the appropriate spot.
	Good luck.

I am known for my brilliance,                 John Whitmore
 by those who do not know me well.

blarson@dianne.usc.edu (bob larson) (02/03/90)

In article <324@ctycal.UUCP> ingoldsb@ctycal.UUCP (Terry Ingoldsby) writes:
>I have an ancient
>(venerable) computer that uses a 96 tpi double sided double
>density floppy disk drive.

>Thinking that since a PC high density drive also has 80 tracks
>per inch, 

80 tracks at 96tpi you mean

>and that the only difference was the rotational
>speed,

Nope.  They use different magnetic media (not interchageable),
different clock speed, and probably different read/write heads.

> AND that the data format is generated by the disk
>controller,

partially true

> I bought a high density 1.2 MByte disk drive.

Mistake.  As far as I can tell, even the ones that claim to be able to
handle both formats don't.  Do yourself a big favor and buy a good
drive, like a TEAC 55f.  (I've never hand any problems with them, like
I had with several other brands.) Don't get stuck with a 55g or any
other HD drive.  The 720k 3.5" drive I have works with no problem as
a second drive.

-- 
Bob Larson	blarson@dianne.usc.edu		usc!dianne!blarson
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