PDP-12 Restoration at the RICM

Cory Heisterkamp coryheisterkamp at gmail.com
Sun May 31 09:47:55 CDT 2015


Sounds like you're making some real progress. Next time you're near the ASR33, check the rubber hammer for the print cylinder. These have a tendency to self destruct and in doing so, destroy the cylinder itself...and they can go at anytime. There's a fellow on the Greenkeys that has tooled up and is producing replacements; same profile as the original and easy to install. Cheap insurance, really. -C

On May 31, 2015, at 8:08 AM, Michael Thompson wrote:

> 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
> week.
> 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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