Firing up the pdt11
cz at alembic.crystel.com
Mon Oct 12 10:58:54 CDT 2020
Curiouser and curiouser....
On a lark I got a 47 ohm (actual reading 50.1 ohms) resistor from the
box, put it on the anode supply line of the sector pulse LED on the
drive that is in the PDT11 (not the one with the blown LED) and hooked
it up to the power supply.
With the resistor in I fired up the supply, set the voltage to 3v, and
checked the current. Now the LED only draws about 30ma measured at the
supply instead of over 120ma without the resistor. So, different.
Then I put this assembly into the actual PDT11 circuit: Now when I check
voltage at the resistor point I see 1.9v across the diode instead of 5v.
And when I put a disk in the PDT11 does drop the solenoid down and tries
to "read" the disk (but fails, but it is 8 slow failures, and not 8 fast
ones). So I think the LED is now "lit".
Then I looked closely at the wires on the actual floppy drive to board
interface. I see on the *other* plug (the one that runs the track zero
detection) that there *is* a small resistor in line on the harness
itself. Haven't checked the value but I'm guessing it's 68 ohms or so.
But there is no similar resistor on the line to the sector pulse LED. I
just checked the broken drive, same deal.
Maybe they "forgot" the resistor on the sector pulse LED? They didn't
put it on the board or I wouldn't see +5 with the LED lit. More
interestingly maybe the actual disk assembly for a PDT11 is
*deliberately different* from an RX01 so you would have to order the
"right" specific part. I don't know; I haven't taken apart my RX02 to
see if that resistor is in line on the disk drive itself for either of
It could also have been an oversight, I'll check the top disk to see if
the resistor is visible. Maybe the LED works at full +5v brightness for
awhile, then opens at +5 but is still closed at lower voltages and lights.
This brings up another issue: As these are soft-sectored, does the
amount of light the LED produces matter? Maybe my LED+resistor is bright
enough to trigger sector pulses, but they are now not where the data is
anymore? How critical is sector pulse alignment for an 8 inch floppy drive.
This is progress, and this is interesting. I wonder how deep this rabbit
On 10/12/2020 11:11 AM, Tony Duell wrote:
> On Mon, Oct 12, 2020 at 4:04 PM Chris Zach <cz at alembic.crystel.com> wrote:
>>> In a 'real' RX01, each sensor LED (track 0 and index) has the cathode
>>> grounded and the anode connected to +5V via a 68ohm series resistor.
>>> If you measure the voltage across the pins with the LED disconnected
>>> (or if the LED is open-circuit) with any reasonable meter, it'll read
>>> 5V. The meter will not draw enough current for the resistor to drop a
>>> noticeable voltage.
>> Ok, that makes sense. Is the resistor on the board or on the drive wire
>> harness somewhere?
> I'm pretty sure it's on the PCB.
>> I'll jumper the disk 0 sensor into my test board here (actually just the
>> extender board from another PDT11) and see what the voltage looks like,
>> then compare it to the same spot on the second drive.
> I'd look at the voltage on each pin of the LED in the 3 other sensors
> (index and track 0 for both drives)
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