PDP-10 KI10 panel - repairing the light show

Parent Category: Stories Category: The PDP-10 KI10 console panel Written by Administrator

After having dealt with the cleaning disaster for several weeks, I went on with the big goal: The KI10 should be reanimated by adding "BlinkenBone" electronic and connecting it to a PDP-10 SimH simulator.

Rebuilding the missing LED board

While looking quite perfect from the outside, the KI10 panel was not complete: the board assembly holding all the LEDs behind the lower acryl plate was missing. On this photo, taken from behind, you look directly onto the front plate, with nothing in between:

PDP-10 KI10 fresh from eBay, LED board missing


If the panel should blink, it needed the LEDs. So I had to

  • drill the positions of all 133 LEDs into a white light conducting plastic board.
  • make boards with all the LEDs and mount them against the light conductor.
  • mount a steel plate into the panel to hold the light assembly.

PDP-10 KI10 panel, home made LED board

Light conductor and LED boards - home made

 PDP-10 KI10 panel, home made LED board carrier

Steel plate to hold LED assembly


I have never seen a real KI10 LED assembly ... I hope my work is not too far from the "DEC style" of building such a thing!

About LEDs:

For my LED assembly I had to find apropriate LEDs. They should have the dark glow of those 1972 originals. I tested a few types, and finally used "KingBright L-53HD" 5mm LEDs. These are quite dark (2-8 mcd) and emit a wonderfull 700nm dark red.

From historic photographs I found that they need a 150 Ohm resistor, if connected to +5V ... then they seem to have the correct restro-style glow!

I used the L-53HD also for repair of a PDP-11/70 panel in the meantime and found that 700nm is a good vintage color. But of course they look a little bright if assembled in mix with original 1977 LEDs in the same panel.

About light bulbs:

In each of the 113 push buttons is a little light bulb. When testing these lamps, I found them half dead, so I decided to replace them all.

It's quite normal that much of the light bulbs in those ancient console panels are dead. Even LEDs often failed on my initial tests. Light bulbs are not very long living, and my KI10 had over 20800 hours on its hourmeter. Suppose a typical lamp lasts 1000h and is ON half the time, the early administrators had to change every bulb 10 times over the life span of the PDP-10. No wonder they finally simple let them burn out, when the end of the machine was announced.

But something was irritating: not only ADDRESS and DATA lamps were dead, but also the OVERTEMP ALARM indicator was burnt out. So this machine run 1000h in an overheated environment? Or the ALARM lamps are run with much more voltage and fail earlier? Or somebody took the ALARM bulb as replacement for other ones?

* * *

Originally the panel lamps are driven with 15Volts. Because 15Volt lamps were not available, and those PC power supplies do have lot of +12V current, I decided to go +12V with the panel.

The lamps I bought are called "midget flange lamps" (Farnell 1466393). Bulb size is T-1 3/4, power rating is 480mW, so current is 40mA each. This gives a total current of 4.6 Amperes ... and much more, if all lamps are lightened together from cold state (exactly the situation when you do a lamp test after first switch-on!). At least one of my PC power supplies tends to break down in this situation.

Changing over 100 light bulbs can take a while too ...


Finally, after connecting the panel to the BlinkenBone installation, the big day came:

 pdp10 ki10 all lamps-2

This was worth the work!