10 Oct
2005
10 Oct
'05
9:10 a.m.
Tony Duell wrote:
A couple of days ago I got a comment that I was starting at too basic a
level :-). Oh well...
The track 0 sensor detects when the head assemnby is at cylinder 0 -- the
outermost cylinder. Typically it consists of a slotted optoswitch mounted
on the drive chassis and an opaque 'flag' on the head carriage which
interrupts the light beam in the slotted optoswitch when the head is at
cylinder 0.
A slotted optoswtich is typically a black plastic device with a slot in
it. I would think you could find a picture of one on the web somewhere --
try the RS components (not Radio Shack!) or Farnell catalogues.
If this drive is the one I think it is, it was called the 'Slimline
Diskette Srive' by IBM. I have a schematic (although not a mechanical
layout) in front of me.
The track 0 sensor seems to have 4 wires which go to pins A3, A4, B3, B4
of connector J3. Also on this connector are the in-use LED, the spindle
motor and its tachogenerator, and the head stepper motor. You should be
able to find it by tracing wires.
As for the circuitry : A slotted optoswitch consists of an infra-red LED
shining at a phototransistor across the slot. Put something (that 'flag'
I mentioned) in the slot, and the light can't reach the phototransistor.
In this case, the LED is connected between A3 (connected to +5V via a 150
Ohm resistor R61) and B4 (ground). The phototransistor between B3
(ground) and A4 (connected to +5V via a 47k resisotr R43). The last is
the output from the sensor, of course. It also goes to TP8 and to the
input of U16d ('14). The output of that is inverted by U14c ('00, inputs
connected together), then NANDed with a couple of signals from the
stepper motor driver circuit by U7c ('10). THat's inverted again by U16a
('14), then fet to one input of U11b ('38) which drives the track00 pin
on the interface connector (the other input to this gate comes from the
drive-selected signal).
What I would do next is power up the drive as usual, and with the head
stepped away from track 0, look at the output of U14c -- that is pin 8 of
U14 -- with a logic probe. It should be low, and should go high if you
put something opeaque (to IR) in the slot of the sensor.
-tony
This sounds (!) like it's not detecting track
0 correctly, it's just
banging the head into the end stop.
My first thought is to clean the track 0 sensor (has it got dust in the
gap), then sit down with the drive and test gear and debug the circuitry.
I'm starting from ground zero. Where *is* the track 0 sensor? (a hard thing
to describe without pictures, I know... sorry)