DEC PC04 Paper Tape Reader/Punch

Rick Bensene rickb at
Mon Mar 28 14:12:01 CST 2005

I have verified that the phototransistor element itself is dead.
Everything else through the interface to the Omnibus is fine.
I've even swapped the wiring at the phototransistor array so
that a different channel of the amplifier board was hooked up
to the phototransistor, and it was still dead.

It seems odd that a phototransistor would fail, but it's pretty
clear at this point that it is, which puts me in the bind of trying
to find another phototransistor array.

Thanks for the help, though.


> -----Original Message-----
> From: cctalk-bounces at 
> [mailto:cctalk-bounces at] On Behalf Of Heinz Wolter
> Sent: Monday, March 28, 2005 11:47 AM
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: DEC PC04 Paper Tape Reader/Punch
> 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.
> >

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