Dying PCjr drive; options?

[Tony Duell]

> 
> Tony Duell wrote:
> > 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)

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