Firing up the pdt11

Tony Duell ard.p850ug1 at
Mon Oct 12 09:46:30 CDT 2020

> One is for the solenoid that lowers the pad onto the disk to allow
> reads/writes.
> One is for the sensor to detect track 1 (LED and detector)
> One is for the sensor to detect sector pulse
> One is for the servo motor that moves the head in and out
> One is for the read/write head
> One is for the 110v power to the drive motor itself that spins the belt
> and the disk via the center spindle.
> I did not move the 110v power wire because that is from a common harness
> between the drives and it's pretty obvious the disk is spinning. :-) The
> other five were switched between controller channel 0 and 1 (PD0: and PD1:)
> At this point I think I'm onto the sector pulse LED not working due to
> 5v being applied to it. Will be working on that more this morning with a
> better/more precise power supply.

Jumping in at this point...

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.

Of course there is no reason why the resistor couldn't be between the
cathode and ground (with the anode connected to +5V). It's a simple
series circuit, the current is the same everywhere.

The thing that puzzles me is why you read 5V across an LED that seems
to work on the bench. I wonder if a connection is open somewhere.


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