Firing up the pdt11. Dec put 5 volts on an LED?????

Chris Zach cz at
Sun Oct 11 23:20:53 CDT 2020

Maybe. The part number is partially missing, the drive itself has a part 
number of 70-13077-02, DEC badged. That maps to a print set of 
EK-13077-IP which doesn't seem to be anywhere online. Drat.

Looking at Wikipedia these seem to be in TO-18 metal cases. Maybe they 
are Gallium Arsenide LEDs?

However if it was current limited then running it up to 9v should not 
have killed it. Is it possible one can drive an LED hard enough to get 
it not to light after awhile at 5v levels but still have it light at 
lower voltage levels?

This should be simple to test with the other non-working drive: Put a 47 
ohm resistor in series with the LED and see if it works. If so great, 
I'll ECO the other drives. If not then I can replace this with a 
standard IR LED, put in a damn resistor, and get it online again.

Still, it means the track zero LED can't be far behind assuming they did 
the same trick. Hm. Need to do more research.


On 10/11/2020 11:50 PM, Doug Jackson wrote:
> Could it have been a 5V LED with integral current limit?
> That would explain the odd behaviour.
> Kindest regards,
> Doug Jackson
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> On Mon, 12 Oct 2020 at 14:33, Chris Zach via cctalk 
> <cctalk at <mailto:cctalk at>> wrote:
>     Ok, this is weirder: I put the "bad" floppy drive on the bench and
>     started to take a look at it. First I checked the LED (yes, it's an
>     LED). With a bench voltage of 1.5 volts and a 100ma draw it lit up
>     nicely in the IR (detected by phone camera, so nice they can see the
>     light) and the photo transistor also seemed to work fine (at the sector
>     hole resistance went from infinite down to about 500 ohms). That's
>     good,
>     so what is wrong?
>     I noticed I could crank the LED higher current-wise to 150 ma and the
>     voltage was still <2 volts. Interesting. Then I hooked a break-out
>     harness to the pdt11 to see what kind of voltage it was putting out to
>     the LED.
>     It's putting out +5v whenever the unit is on. Maybe it's current
>     limited? To check I hooked up the drive's plug to the breakout to see
>     what the LED was seeing.
>     +5v. And even weirder, the LED was not lit.
>     What the heck is going on here?
>     So I put the LED on the bench for a bit of a destructive test.
>     Disconnected the PDT11 from the breakout cable, hooked up the power
>     supply, turned up the voltage and the LED came on, then went *off* at
>     around 3v. At 5v it was dead off, no IR light as measured by the
>     camera.
>     Turn the voltage down, and it comes on again. Up and it goes off.
>     And unfortunately at 9v it died (CRAP!) as I turned up the current
>     limit
>     Yes, I forgot to set the voltage limit on the power supply, my bad,
>     I am
>     boo boo the fool...
>     But this is weird: It looks like DEC put an LED in there with no
>     current
>     limiting, and a straight +5 volts. And the LED is always on at this
>     high
>     voltage? With no current limiting resistor? This does not make sense,
>     but the volt meter don't lie. I'm going to check the working drive to
>     see if it is limiting the voltage somehow. I'd say there was a resistor
>     in the LED assembly limiting the current, but if that's true my
>     cranking
>     the voltage to 9v should not have blown it up, and it should not
>     turn on
>     at low voltages then off at 5v.
>     Maybe the solution is to insert a resistor in series with the second
>     drive at around r=e/i or r=5/.1 (100ma) or 50 ohms.
>     Does this make any sense?
>     C

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