PDP-8/e

Hi I recently aquired a complete PDP-8/e. As it had not run for a while I took the usual precautions of reforming the electrolytics in the PSU as well as dismantling the whole system and cleaning everything. I was rewarded with a working system. I started to run some simple toggle in programs and got to the stage of driving a VT 320 with a display of whole the character set at 110baud. I was about to toggle in the next test which would have been a keyboard echo routine when I discovered I could no longer load anything into memory. Load address works but setting the data to load into memory in the switch register and raising DEP no longer writes the data to memory. . RUN works so it looks like the timing plus the RUN/STOP flipflop are running ok. However one shot operations like DEP and EXAM are difficult to diagnose as is hard to see a one off 500nS pulse. I don't think its the memory itself ( 4 x 4K of core) as I have tried each 4K bank on its own with the same result. I do have the CPU manual and a decent Tektronix scope. Has anybody any suggestions? Rod Smallwood