PDP-11/70 debugging advice

[Josh Dersch]

On Thu, Feb 4, 2021 at 2:42 PM Fritz Mueller via cctalk <
cctalk at classiccmp.org> wrote:

>
> > On Feb 4, 2021, at 1:51 PM, Josh Dersch <derschjo at gmail.com> wrote:
> > [RC maintenance clock] Yeah, you'd think... but it doesn't work :).  I
> mentioned earlier in this thread that it's stuck in single-step mode.  From
> the schematics it looks like the crystal clock needs to be running to clock
> some of the flip flops used in the "Source Synchronizer" to select the
> proper clock source.  My guess is that since the clock isn't running, the
> flip flops are left in a weird state.
>
> Oh, interesting!  Noted for future reference.  The TIG is not exactly
> straightforward...


So I realized today that, hey, I have a burned-out 11/45 in the garage, and
it has a TIG (albeit an M8109), and it also has a 33.33Mhz crystal on it.
And because I'm impatient and can't wait for my Digi-Key order to come in,
I went out and borrowed the crystal from the 11/45's TIG and installed it
in the 11/70's.  The 11/70 is now somewhat operable.  I can examine and
deposit memory, at least for a short while -- after the system warms up for
30 seconds or so the Load Address switch stops functioning.  (Happens
gradually, too... for 10 seconds or so it works every other time you toggle
it, then it stops entirely.)  Everything else seems to work properly after
this happens, so I'm kind of curious if there's an IC on the front panel
that's gone bad, though I suppose it's still completely possible it's
something on the CPU end.  I'll have to do some debugging, but it'll have
to wait until this weekend.

As an aside... The M8109 is very similar looking to the M8139... any ideas
what the functional differences are?  There seems to be an extra IC and
some ECO wiring on the M8139.  I'll have to take a closer look someday.

- Josh