6 Feb
2009
6 Feb
'09
7:31 a.m.
----------------------------------------
Date: Fri, 6 Feb 2009 11:38:21 +0100
From: cc at informatik.uni-stuttgart.de
To: cctalk at classiccmp.org
Subject: RE: speaking of 3.5
On Thu, 5 Feb 2009, Fred Cisin wrote:
Hi
First we are assumming that the drive already has a switch
for 720K that works.
On the ones I'm looking at, one could glue some equally
spaced tabs on the flywheel and add a opto pickup.
The 3.5 disk always mounts the same relative to the motor
and uses a sensor on the motor to create the index.
The Mitsumi drive I have in my hand right now has
just such an exposed flywheel/motor.
I looked at the Teacs I have and don't believe one
could do this because the motor is mounted differently.
Dwight
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Some of the older drives had a flywheel. A drill,
in indexing jig, a
light and photocell, any you could modify a 3.5" drive to provide 10 or 16
pulses from physically fixed positions, without being dependent on
repeatability of rotational speed.
I don't think that this would work, simply because the motor driver IC
has an internal PLL that controls the motor speed according to the output
of the flywheel sensor, and that PLL is crystal locked (a small resonator
of e.g. 981kHz, 1MHz or 493kHz). In order to change the speed you need to
change that ceramic resonator.
I know that because I've been trying to slightly reduce the speed of a
3.5" drive in order to write long tracks on an Amiga. That is BTW how I
found out that the driver IC I mentioned in an earlier post can do 600 and
720 RPM.
Christian