Doug I. wrote: 82? 3 Data Field 3 Instruction Field 12 Program Counter 12 Memory Address 12 Memory Buffer 1 Link 12 Accumulator 12 MQ 1 AND 1 TAD 1 ISZ 1 DCA 1 JMS 1 JMP 1 IOT 1 OPR 1 FETCH 1 EXECUTE 1 DEFER 1 BREAK 1 ION 1 PAUSE 1 RUN Rick Bensene The Old Calculator Museum http://oldcalculatormuseum.com

I said this one is seeming easy and I meant it. If you look at the photo on Wikipedia Desktop Straight 8 <http://upload.wikimedia.org/wikipedia/commons/6/6d/PDP-8.jpg> You can count on the horizontal lines of bulbs labeled: Program Counter 18 Memory Address 12 Memory Buffer 12 Accumulator 13 Multiplier Quotient 12 On the right side there are two vertical rows of bulbs labeled as follows: AND FETCH TAD EXECUTE ISZ DEFER DCA BREAK JMS JMP ION IOT PAUSE OPR RUN This gives 82. However there is a secret bulb in the space between the BREAK lamp and the ION lamp below it. There is a hole in the fiberboard mask for the bulb, the driver transistor and resistor are on the lamp board, the bulb was in place on my machine and there is a wire running to the backplane connected to it. But there is no way to see it if it ever turned on with the glass face plate in place as the silkscreen over the bulb position is opaque. Based on this I would say that the actual count is 83 but 82 are useful. One of these days I will trace the wire to see where it is cabled into the back plane. It does not show on the schematics I have looked at. I stole that bulb at one point to use as a replacement for a burned out one. At the moment I have three early vintage white LED's in the front panel of my machine. These LED's are a cool blue white light and look out of place for a couple of reasons. In addition to the color, they are on dimly whenever the machine is running. Cross talk in the wires is enough to trigger an LED. They also turn on and off too quickly. I calibrated the brightness at just under a ma as being the correct brightness. The original bulbs draw about 30 ma when on. Anyone who has ever taken apart the front panel of a straight 8 knows that it is such a pain to replace bulbs that I have decided the next time to just replace all of them with an LED, two resistors and a capacitor to somewhat simulate the delays and persistence of a filament bulb. I may even use a soft white LED and a yellow LED in order to get better color temperatures but this will take too much room. One of my friends says this would be sacrilege and I should only use filament bulbs. Of course he also says it would be OK to use a tri-color LED and a microprocessor for each bulb to generate the correct color temps and delays. Seems like too much work to me. The next trivia question will be next year so Happy New Year everyone! Doug Ingraham PDP-8 SN 1175 (Running OS/8 from DECTape) On Fri, Dec 26, 2014 at 2:57 PM, Doug Ingraham <dpi at dustyoldcomputers.com> wrote: