pdp8/e problems

Ian S. King isking at uw.edu
Sun Apr 24 21:18:09 CDT 2016

On Sun, Apr 24, 2016 at 1:06 PM, Mark G Thomas <Mark at misty.com> wrote:

> Hi,
> So far, the only M8330 prints I have found are extremely fuzzy, so I cannot
> make out the IC identifiers or match them up to the equally fuzzy
> schematic.

You do know about bitsavers.org, yes?  Al has lots of very clear scans of
DEC prints.  Keep in mind, though, that these are hand drawn prints -
sometimes you have to zoom to 150% or 200% to make out particular

> By board-inspection I identified the 74S74 "run" flip-flop, and in fact I
> can un-stick the RUN state by briefly grounding CLR, but still cannot
> DEP or EXAM from the front panel -- it gets stuck back with RUN on if I
> hit either DEP or EXAM again.
> The power supply voltages are right-on. I've reseated and repositioned
> the boards. The connectors are very clean. Wiggling boards doesn't change
> any of the symptoms. Wiggling switches doesn't affect anything. This is
> a very clean 8/e.

Check not only your voltages, but your power waveforms.  Also, I remember
having an issue involving a race condition with the DC OK line, which is
not often considered when one is checking power supply voltages.  If the +5
rail isn't solid *before* DC OK comes up, this will lock up things.  Have
fun -- Ian K7PDP

Ian S. King, MSIS, MSCS, Ph.D. Candidate
The Information School <http://ischool.uw.edu>
Dissertation: "Why the Conversation Mattered: Constructing a Sociotechnical
Narrative Through a Design Lens

Archivist, Voices From the Rwanda Tribunal <http://tribunalvoices.org>
Value Sensitive Design Research Lab <http://vsdesign.org>

University of Washington

There is an old Vulcan saying: "Only Nixon could go to China."

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