DEC PDP-8/S #4 Restoration

DEC PDP-8/I Restoration

We did an initial survey of the system when it was donated and were pleased to see that it was in really good condition.

Detailed schematics and documentation for the custom built I/O section was included with spares and DEC documentation.


We reformed the capacitors in the CPU and I/O power supplies. Both power supplies are working well.

We tried to read and write memory, but didn't get far.

We wiggled and reseated all of the modules and saw some results from memory.

We found that MA bits 0-5 worked OK, but bits 6-11 were always 1.

Since this is a serial architecture system we suspected that the MA bits were not getting transferred from the flip-flop for bit 5 to the flip-flop for bit 6.

We swapped the R202 modules in slots E17 & E18 and saw the problem move between bits 3 & 4.

We replaced the R202 module from slot E18 with a NOS spare and the MA worked correctly for all bits.

After some more experimenting we found that bit 1 in the MB was always 0 and could not be set to 1.

Replacing the R202 module in slot D11 with a NOS spare fixed that problem.

When read, bit 10 of the MB was always 0 even when it looked like a 1 was written.

We replaced the G803 modules in slot D27 with one borrowed from the OMD8S for PDP-8/S #2.

Now we can read and write memory.

We entered a few small toggle-in tests and found that most of the front panel instructions work.

The CONT switch does not work. That may be the switch, or circuitry. We will investigate.

We successfully ran the Increment Accumulator toggle-in program.

We compared the speed to the PDP-8/L and found that the 8/L is 16x faster than the 8/S.