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The Restoration of the PDP-8/I Minicomputer (PAGE 5)

Repair Front Panel -- 2003 06 09 - 2003 06 10

So then I unscrewed the front panel, and looked at the bulbs. The person who repaired this the last time used the wrong type of replacement bulbs! Almost all of the "different" bulbs were in spots that I had marked with a yellow sticky. The "normal" bulbs were black base -- these were a white base (numbered CM7380) -- obviously the wrong type because they burned brighter than the black-base bulbs. The only spare ones I have are white-based CM7381 -- these burn even brighter :-( I have some spares arriving tomorrow...

Front panel of PDP-8/I detached from faceplate S M L XL This picture shows the single red LED and the other black-based and white-based bulbs. The black-based bulbs are the 15 V bulbs and are the correct value for use in the 8/I front panel.

What I plan to do when I find spares is insert gold "pins" into the hole where the bulb goes. This means that the pin is soldered into the circuit board, and the bulb just plugs into the pin (think of it as a one-pin IC socket). This isn't "stock" or "vintage" but it certainly is convenient, and doesn't really disrupt the look and feel of the machine.

As it turns out, I don't think this will be possible due to the clearance -- the pins are rather tall (i.e., their base doesn't go through the hole, only the very thin end of the pin goes through) so it will be very tight. I was thinking about soldering them on from the back, but that just smells like "kludge on top of kludge".

What I ended up doing was drilling (UGH!) out the hole in the PCB to make it just a hair bigger so that the pin would go through. I felt kind of bad about this, but then I recalled that my original PDP-8/I that I had as a kid had the exact same (pins instead of soldered-in-place) modification done to it, and it may even have been original in the factory.

Using LEDs

If you do decide to replace your light bulbs with LEDs, then the resistor you need is 920 ohms (a 1k will do just fine):

E = I * R


R = E / I = (15 - 1.2) / 0.015 = 13.8 / 0.015 = 920

Be sure to note the orientation of the cathode. It's the one that's usually marked with a notch on the LED. The cathode should go on the transistor side of the socket, and not the +15V side.

I consider it tacky to have some white and some red lights -- but you can find white LEDs these days. If the white LEDs (or any other LEDs you use, blue could be rather nice) have a different voltage drop (1.2 V) and current (15 mA) you'll need to recalculate the series resistor value.

Detail of red LED used in PDP-8/I front panel S M L XL A detail of the red LED and its series-mounted 1k resistor. To make things easier to remember, I always mount the resistor on the cathode side -- that makes identification simple. The cathode is the negative voltage side; since the bulb (or LED in this case) goes between +15V and the output of a transistor, it means that the cathode side connects to the transistor, and the anode connects to the +15V.

One important note to keep in mind is that the LEDs with soldered in-line resistor will be slightly taller than the lamps. You'll want to cut the leads on the LED and the resistor very short.

Front panel of PDP-8/I showing red LEDs replacing burnt out bulbs S M L XL This is the tacky red (LED) and white (bulb) version of the front panel. I did this because I wanted to continue debugging. Since the LEDs are socketed, I can very easily replace them with the bulbs when I get them later. Notice the pieces of yellow sticky note on the front panel -- these indicate lights that I have still not yet seen light up. The one with the dot, bit 10 of the MQ register, indicates that the bulb has been repaired; but I still haven't seen it light up. It is possible that some of the transistors driving the bulbs/LEDs are damaged as well -- there are one or two transistors that have been replaced.
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