A very sad PDP-8/S

Ethan Dicks ethan.dicks at gmail.com
Sat Nov 3 03:02:40 CDT 2018

On Sat, Nov 3, 2018 at 1:12 AM Lyle Bickley via cctalk
<cctalk at classiccmp.org> wrote:
> On Sat, 03 Nov 2018 15:13:16 +1100
> Guy Dunphy via cctalk <cctalk at classiccmp.org> wrote:
> > This is rather sad.
> > I've received a PDP-8/S. It turns out to be missing so much, that
> > it's probably beyond restoration. Anyone who can think of potential
> > ways to find the missing parts, please speak up.
> >
> > http://everist.org/pics/PDP-8S/
> >
> > Missing:
> >
> >  - Front panel PCB,
> >  - Case top AND bottom,
> >  - Power supply.
> >  - Some flip-chip slots are empty. Not sure if supposed to be.
> I have a restored and running PDP-8/S.
> It looks to me that you're not missing modules - the open slots are
> for the I/O cables.


Start by looking at http://www.ricomputermuseum.org/Home/equipment/pdp-8s

At a quick glance, I see you _might_ have 2-3 cards in wrong slots
near some intentional gaps.  Check that.

The open slots at the back are for the I/O cables.  The open slots
near the memory stack are supposed to be empty with the one possible
exception where I see in your photo you have a single-height amber
card and the RICM PDP-8/S appears to have a dual-height amber card.  I
don't have the printset up so I can't tell exactly what's going on
there.  Could be normal variation as they made changes across the run.

> The P/S is a standard DEC linear P/S and can be easily substituted until
> you come across the real thing.

Yes.  You just need enough amps at +10V and -15V to fire it up.

What I don't know off the top of my head is the difference between a
table-top and rack-mounted system.  RICM has pictures of a
rack-mounted system.  Mine is table-top.  It's entirely possible that
yours was pulled from a rack and the PSU stayed behind.  Yours might
never have had covers like the desktop unit.

> You need to get the schematics for the version of the PDP-8/S you have
> and check the modules in your system against the module chart.

Yes.  "for the version you have" is important.  I don't think there's
exactly one way they built it across multiple years.  There can easily
be a module or two different between different manufacturing dates if
there was a large ECO.

> When you say the front panel PCB is missing - do you mean that there
> is not a large circuit board with incandescent lamps in the system?
> That would seem unusual to me - given that all the switches are on the
> front panel.

Unlike every other machine I've worked on, the -8/S has *two* front
panel boards.  One is very short and has just the switches and sits in
slot 1.  The other is full-sized and has only the lamps, no switches
and sits in slot 2.  I see from your photos, that board is missing.
Given that it's fairly simple, you might be able to find a Unibus
hex-height protoboard and fabricate a replacement.  The lamps might be
interesting to source - ISTR they have 28V filaments, underdriven to
give them a long lifetime.  I can't recommend the original lamps
anyway - they are not the bi-pin bulbs used in later models (PDP-8/L
and newer).  They have these brittle leads coming out of the glass and
no plastic base.  I have several in mine that had the leads break off
right at the glass.  Even if the filaments are intact, these bulbs are
essentially dead.  I do not presently have any replacement bulbs of
any type for mine.

> > Also there's no sign of any I/O circuitry.
> There is NO I/O on a PDP-8/S. The I/O cables connected to any I/O that
> existed.

Yep.  All the I/O was external on the Negibus, even the console TTY
(there's a PT08 in the base of an ASR-33 that was standard for this
model).  I didn't get the TTY with mine (it was long gone, to a Ham
club, I was told) so I don't have a console SLU either.  Once I get
the CPU working, I'll have to fabricate a PT08 replacement.

There just wasn't room in the box for any I/O.  The backplane on this
model is close to half the size of the backplane on a Straight-8.

> Please get a PDP-8/S service manual and schematics...



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