28 Mar
2005
28 Mar
'05
11:46 a.m.
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.