New subject: Restoration of a PDP-8a

5 Dec 2013

...
  Message: 9
 Date: Wed, 4 Dec 2013 12:46:27 +0100
 From: anders at abc80.net
 To: cctech at classiccmp.org
 Subject: [Fwd: Restoration of a PDP-8a] 
...

 Hi,

 I have started restoration of a PDP-8a model 8A420-CN. I doesn't have any
 schematics for this machine and I have searched internet for sources
 without any luck. The sets on
 http://bitsavers.trailing-edge.com/pdf/dec/pdp8/pdp8a does not include my
 version.

 My machine came with 16K*12bits core memory, 20 slot backplane and only
 the "Limited function board" frontpanel (don't now if this machine ever
 had the "Programmer's console").

 This is how my "Power distribution board" looks like:
 http://www.abc80.net/docs/pdp8/pdp8a/DSC_2118.JPG

 Some questions:
 1. Is there anybody that could help me with schematics on this machine?
 Scans are preferred, but I can pay for paper prints. I'm especially
 looking for a diagram that matches the "power distribution board" that I
 got.

 2. If I plug in the machine with only the regulator boards (2xG8018) and
 the limited function board in it, should the lights and fans come to life?

 3. I suspect that my model shouldn't have "Power distribution board
 control", G8019 since my "Power distribution board" misses the connector
 for this board. Can that be correct?

 Any help is appreciated. PPD-8a is new to me, but I have some knowledge in
 the PDP-8 architecture since I restored a PDP-8/L that was in really bad
 shape.

 /Anders

What chassis is your -8A in -- is it the BA8-C as listed in
EK-0PDP8-SP-001 - PDP-8 Family Configuration Guide, April 1978 at
http://bitsavers.trailing-edge.com/pdf/dec/pdp8/omnibus/EK-0PDP8-SP-001_PDP…
 The exploded view of the BA8-C is on page 41 of 217 of the MP00415 -
8A Family (Semi) Field Maintenance Print Set, April 1981 found at the
same source.  Pages 50-56 show the "Power Distribution Board."   The
Limited Function Board is page 57.  The "Power Distribution Board
Control" is shown on pages 58-61.  The 20-slot Omnibus is there too.

Can you list your modules/boards/etc. for us?  You may have to pick
through the different drawing/schematic sets to find your individual
components.

The PDP-8 Family Configuration Guide will give much information about
your system and proper placement of modules.  Page 2-10 (Worksheet,
8A420, 8A425) discusses your machine's layout.  It says: "The BA8-C
power supply consists of two G8018 regulators; one provides 25 A at +
5 Vdc, 2 A at + 1 5 Vdc, and 2 A at -15 Vdc for the top 10 slots,
while the other provides the same current for the bottom 10 slots."
Figure 2-1 "PDP-8/A Computer Designations" breaks down the 8A420 as:
4 = CPU is the KK8-A
2 = CHASSIS is the 20 slot BA8-C
0 = CORE memory

The PDP-8 Family Configuration Guide doesn't explain the "CN" suffix,
but that looks similar to the suffixes listed in some of the
Engineering Drawings/Field Maintenance Print Sets for "Unit
Variations."  There is a "CN" Variation of the 8A425 listed in the 8A
Family (Semi) Field Maintenance Print Set, but I haven't found a
description of it.  ...  I just found some of it.  The 8A425 Parts
List on pages 24-25 lists the parts used in that version of the
semiconductor-memory 8A42x system.  The parts that aren't obviously
semiconductor-memory-related may apply to your system too.

The pictures of your system show only the Limited Function Board, but
I _think_ that all -8As used the same Front Panels/Consoles, whether
they were the Limited or Programmer's versions.  Perhaps you could
pick one up somewhere.  I've got two or three Programmer's Consoles,
but all have one or more problems.  The one on my 8A400 has a bad gate
on one of its quad-[I forget]-gate ICs, but I haven't replaced it yet.

Bob