18 Dec
2007
18 Dec
'07
1:03 p.m.
I've just had one of the disk drives in my TRS-80 Model III start to go
south on me, and before I crack the case and get down to serious diagnostic
work, I thought I'd check with the list to see if the symptoms ring a bell
with anybody.
Firstly a couple of general comments :
1) A lot of TRS-80 M3/M4 disk problems are due to the tapewire between
the CPU board and the disk controller board.. This is _very_ unlikely to
be your problem (it will affect both drives, it often makes the machine
not recognise it has a disk controller at all), but it's something to be
aware of.
2) I don;t know if you've been inside your M3 before, but if you haven't,
you should know that the monitor seciton -- CRT and video board -- is
fixed to the cocer, the rest of the machine is fixed to the baseplate.
Take out the screws on the bottom and the one on the back, put the
machine in the normal operating position and lift the cover straight up
(I find a hand flat on each side is the beast way to start gripping it),
turn in 1/4 of a turn counterclockwise, and put it down to the left of
the base. If you want to separate the 2 parts, there's an edge connector
on the video board that carries all the wires from the base section, and
a ground wire with a faston connector. Be very careful when removing or
fitting the op case not to knock the neck of the CRT. It's all too easy
to break this and then you're looking (of course) for a replacement.
I believe that the drive in question is a stock
factory install; it's the
secondary drive in a fairly low serial-numbered Model III (in the 50,000s).
The first sign of trouble was that the drive began returning spurious "Disk
is Write-Protected" errors, which would go away upon reinserting the disk.
Now it's begun episodes of returning TRS-DOS Error 08 ("Drive Not Ready")
messages for all operations. The activity light functions normally, and the
drive hasn't made any alarming sounds. It does not spin up the disk when
it's in this state. After a while, it'll begin working again on its own.
I realize there are any number of replacement drive solutions out there, but
this is a system where I'd really like to keep the original equipment up and
running until it just can't be patched together any longer. (I can't really
explain why I'm so fond of this thing; I just am.)
If it is an original drive, it's a single-sided 40 cylinder Tandon TM100
thing (maybe 'Texas Peripehrals', but as far as I can see, that and the
Tandon are identical). There should eb documentaion on the web -- the M3
technical manuial has scvhematics and alignment data in it.
A disk drive consists of a number of subsystems :
1) The spindle motor, mechanical drive to the disk spindle, and speed
control circuit (in some Tandon drives -- in fact most early ones -- this
speed control circuit is a separate PCB mounted o nthe rear of the drive
chassis). Therte's an input signal from the controller to start the
spindle motor
2) The head postiioner stepper motor, head positioner mechanism, and the
stepper motor driver circuit. This is controller by 2 signals from the
controller, one moves the head by one cylinder each time it's pulsed, the
other one determines which way the head moves. The _controller_ keeps a
record of which cylinder the head is on, and works out how many pulses to
send to move the head to th wanted cylinder.
3) The track 0 sensor which tells the controller when the head is at the
outpuermost cylinder, and also (normally) prevents the head from being
stepped out further. It consists of a microswitch or optical sensor
operated by the head carriage, the output of this is often gated with the
stepper motor drive signals to reduce the mechancial accuracy required in
the sensor.. Most Tandons use the microswitch!
4) THe index sensor, which is a light barrier shone through the hole in
the disk to detect when that hole comes round. It provides an index pulse
signal to the cotnroller, and may also be used to determine when the
drive is ready (if the index pulses are occuring at the right rate, the
drtive is up to speed and has a disk in it)
5) the write protect which detects the presence of the notch in the edge
of the disk jacket. It may be an optical sensor or a microswitch, again
the old Tandons use a microswitch. It provides a signal back to the
controller and also disables thw write electronics (a write-protected
disk cannont be written to, no matter what the controller may try!).
6) The write circuit, enabled by the write gate signal from the
controller, provided the disk is not write protected. Theis takes the
write data line from the controller and drives the read/write head so as
to write a flux transition on the disk fro every pulse on thw write data line
7) The read circuit, which takes the signal from the head if the drive is
not writing, and oputputs a pulse to the controller for every flux
transition on the disk
8) The ehad swtich cirucit, used to select between the 2 heads ian
double-sided drive. A signle-sided drive won't have this.
Now, you have a problem with the spindle motor, since you say the disk is
not spinning. In some dirves, but not AFAIK, this Tandon unit, the motor
will only start if there's a disk in, etc. But in the Tandon, it should
run whenever the controller tells it to. So that's a definite problem.
Get tyhe drive out -- from what I rememebr you have to remove the top
drive wrist, and to get tho the mounint screws for the lower drive, you
have to remove the PSU on the side of the drive tower. Refit the PSU and
top drive (screw the PSU in place, if it shorts out to something, the
results are expensive as there's live mains on that PCB!), and find some
way of cabling up the drive externally.
Trun on the machine and check the supply voltages _at the drive_. A
missing power line will casue all sorts of 'interesting' problems. Then do
something that should get the second drive started. If you have one of the
drives with a separate motor control PCB, look as the motor-on line on the
cable to that board. If that line is asseted, there's a problem on the
spidndle motor cotnrolelr, if not, then you need to check back through
the enable logic on the main drive logic board. Either way, I can help
you sort it out.
-tony