PDP-8/I memory problem

Somehow I missed Robert Krten's original post, but I saw two lines quoted, and since minimum-information puzzles are fun I'll quote it for reply. Forgive me if I'm missing obvious clues that were presented in the original. :) In _both_ core stacks? Well, there goes the obvious inhibit-driver answer... If it were me, I'd be tempted to stick a probe onto the output of the MB00 flip-flop (M220, E/F 34, pin U2 of the E side) and see if it were spitting out usable information or nonsense. If it's nonsense, you've probably found your culprit. If it's being sensible, I'd follow the signal to the inhibit drivers (and perhaps to the front panel as well) to verify it's going where it needs to go. (Does that signal go to the inhibit drivers directly? It may go to the memory extension control and get rebuffered there first, but alas I don't have my extended-memory-control drawings handy... If the MB00 signal _does_ get rebuffered between the original F/F and the inhibit drivers, that rebuffering device would be an even more likely place to look for a problem.) Let me know if this helps. I'm curious now. [I suppose it's possible that the bit 0 inhibit drivers of _both_ stacks could independently be locking themselves on, but it seems unlikely. :) ] [I suppose your MB00 panel light could have burned itself out at an inconvenient moment as well, but that seems entirely too simplistic an answer. :) :) ] -O.-