PDP-11/70 debugging advice

Josh Dersch derschjo at gmail.com
Sun Jan 31 16:31:20 CST 2021

Hi all --

Making some progress with the "fire sale" PDP-11/70. Over the past month
I've rebuilt the power supplies and burned them in on the bench, and I've
gotten things cleaned up and reassembled.  I'm still waiting on some new
chassis fans but my curiosity overwhelmed my caution and I decided to power
it up for a short time (like 30 seconds) just to see what happens.  Good
news: no smoke or fire.  Voltages look good (need a tiny bit of adjustment
yet) and AC LO and DC LO looked good everywhere I tested them.  Bad news:
processor is almost entirely unresponsive; comes up with the RUN and MASTER
lights on, toggling Halt, and hitting Start causes the RUN light to go out,
but that's the only response I get from the console.

I got out the KM11 boardset and with that installed I can step through
microinstructions and it's definitely executing them, and seems to be
following the flow diagrams in the engineering drawings.  Left to its own
devices, however, the processor doesn't seem to be executing
microinstructions at all, it's stuck at uAddress 200.

In the troubleshooting section of the 11/70 service docs (diagram on p.
5-16) it states:

- uADRS = 200 (ZAP)

Which seems to adequately describe the symptoms I'm seeing, but as far as I
can tell the AC and DC LO signals are all fine.  (This system has a Setasi
PEP70/Hypercache installed, so there's no separate memory chassis to worry
about.)  I'm going to go back and re-check everything, but I was curious if
anyone knows whether loss of AC or DC would prevent the processor from
executing microcode -- from everything I understand it should cause a trap,
and I don't see anything in the docs about inhibiting microcode execution.
But perhaps if this happens at power-up things behave differently?  And the
fact that the troubleshooting flowchart calls out these exact symptoms
would seem to indicate that this is expected.  But I'm curious why the KM11
can step the processor, in this case.

I'm going to wait until the new fans arrive (hopefully tomorrow or tuesday)
before I poke at this again, just looking for advice here on the off chance
anyone's seen this behavior before.

Thanks as always!
- Josh

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