PDP-12 Restoration at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sun May 31 08:08:26 CDT 2015

We spent some time on the console Teletype that came with the PDP-12. The
platen was nearly impossible to move, so the Line Feed did not work. We
removed the platen, and found that the plastic in the bearing area had
swollen and was binding. We sanded, cleaned, and lubricate the bearing
surface and the platen now turns freely. On reassembly we found that none
of the Control Characters like Line Feed or Bell would work in Local Mode. We
fiddled for quite a while, but did not find a problem. We speculated that
something got bent when it could not move the binding platen.

We found a bad SN7474 E13 on the M706 Teletype Receiver flip-chip from the
PDP-12. We will repair and test it next week.

We borrowed the M706 Teletype receiver from the PDP-8/I and connected the
Teletype to the PDP-12. We loaded and ran a toggle-in program that echos
the keyboard to the printer. We were a little surprised when everything in
the Teletype worked OK. We were even more surprised when the Teletype now
worked correctly in local mode.

We borrowed the console cable from the PDP-8/I and connected my laptop to
the PDP-12. The terminal emulator worked correctly and echoed characters to
the PDP-12 and back.

We toggled in the RIM loader and then loaded the LBAA BIN loader from my
laptop. We ran the BIN loader and loaded and ran the PDP-8/I Instruction
Test #1. It actually works OK!

We tried twice to load MAINDEC-8I-D02B-D Instruction Test #2, but failed
both times. Running that diagnostic and others will be the project for next

Al Kossow posted LOTS of PDP-12 manuals to Bitsavers. One manual includes
the allowable ripple for the power supplies. They allow 3,000mV of ripple
on the -30V supply for the core memory, so I guess that the 180mV that we
measured two weeks ago is OK.

On Mon, May 25, 2015 at 8:07 PM, Michael Thompson <
michael.99.thompson at gmail.com> wrote:

> Today we pulled all of the M113 flip-chips and tested them because SN7474
> and SN7400 ICs seem to be a problem in these early DEC systems. The ones in
> slots J33 and K30 were bad. Replacing them fixed the problem with the JMP
> instruction. We did some more testing with the toggle-in programs and found
> that ISZ cleared the AC. Replacing the M119 in slot H28 fixed that. All of
> the toggle-in tests pass, so the processor is substantially functional.
> Core memory in field 1 with addresses X5XX didn't work. We replaced the
> G221 in slot D10 to fix that.
> We tried the ASR33 Teletype that came with the system. The mechanics were
> sticky from not being used for 30 years, but we got most of it free and
> working. We could send characters to the Teletype, but could not receive
> anything. The M706 receiver failed in the board tester. The spare is also
> broken, so we need to fix both.

Michael Thompson

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