Firing up the pdt11

Chris Zach cz at
Mon Oct 12 10:04:23 CDT 2020

> 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'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.

> 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.

Not many places for the connection to be open: The LED most certainly 
did work and did provide IR light on the bench and in-circuit. Weird.


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