repairing a dead PDP-11/35

Henk Gooijen henk.gooijen at
Tue Jan 2 09:09:00 CST 2018

Best wishes for 2018!
I have been busy trying to repair my dead 11/35.
The system was working, but there was one screw loose that mounts the
system units in the BA11 box ... that screw created a short circuit :-(
After powering the 11/35, ENA/HALT on HALT, and toggling the LOAD ADRS
switch, all DATA lamps go on, and after that the machine is totally

I installed the KM11 replica from Guy Sotomayor (at last, after 7 years,
I have soldered one of the kits that I had since 2011-2012!). With the
KM11, I can step the microcode right from power up.   When I toggle
LOAD ADRS I see that the SWITCH signal (via the 7474 flipflop) is set,
but when the microcode checks *which* toggle was activated it decides
that none was activated. I measured the signals that play a role here,
and all looks fine.

Now, what gets me puzzled.
If I toggle the LOAD ADRS switch *and hold it pressed down*, then, when
I step the microcode, the branch that handles the LOAD ADRS switch
actually does get executed, and the ADDRESS lamps on the console show
the switch register settings.

Anybody for clues how to proceed?

If you are interested, I made a write up of my testing in (way) more detail:<>

Thanks for any advice,

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