Ugh, PC0 + PDP-8I problem.

At 10:11 PM 11/24/99 -0500, you wrote: Just a guess: are the "working" tapes the opaque grey DEC (un-oiled) paper, while the tapes that fail were punched on oiled (probably yellow or green) tape? We had problems with illumination on our 300cpm reader when reading the more "transparent" oiled yellow and green tape (the kind you punched on your ASR33.) Seems as the bulb aged, the filament got smaller as the tungsten boiled off and the brightness went up until it failed. I guess as the resistance went up a constant current source just kept cranking up the voltage, and the color temp went up... Anyway, for whatever reason... The reader eventually "saw right through" the tape. The source of illumination was an "automotive" style end-pin clip-in bulb with a single in-line filament and its aging characteristics, even at approximately 1/2 intensity, were not good. The reader needed tweaking every few months. A good design would have had a feedback cell controlling the light source... But I digress... We diagnosed the problem by creating a loop of tape with the pattern 10101010 01010101 10101010 01010101 ... and running a dumb "reader loop" program to just cycle the bytes into (and ignored by) the computer. Then you adjust the illumination and gain of the photo detectors until the pulses are shaped correctly. I suppose you could also check the results on the computer and update a counter in the AC (I just love having real control panels...) so you had some additional feedback the problem was being solved. This wasn't on a DEC machine, but the principles may be the same. Just a suggestion. -Gary