DEC PC04 Paper Tape Reader/Punch

[Heinz Wolter]

Rick, see if you can track down the schematics and verify that it's
actually the photo-transistor element that's dead. It could
merely be a loose wire, or a bad component in the buffer/amplifier
comparator/filters in each of the channels. The surest way to be sure
would be to put a scope right on the lead of the sensor array's output.
Typically phototransistor would be pulled up to +V supply  by a resistor
and optionally biased by another resistor on the base. Check those two
resistors and the make sure nothing is shorting the  collector's output -
then
trace that bit back all the way to the interface buffer to the omnibus.
good luck! -h

Rick Bensene wrote:
> I've got a DEC PC04 punched tape reader/punch that I've been working
> on getting running on my PDP 8/e.
>
> The punch seems to work well.
>
> The reader, on the other hand, has a problem.  The 2^2 bit is always stuck
> low.  So, I started tracing things down, and found that the
phototransistor
> in the read head at that bit position is 'dead'.
>
> Problem is, the reader uses an array of 10 phototransistors, (8 for data
> holes,
> one for sprocket hole detection, and the last for detecting an "out of
tape"
> condition)
> arranged in a linear array with 0.10 spacing, with built in lens bubbles.
> There's no way to substitute just the one failed phototransistor.
>