Re: I found it! A very sorry looking MK14.
While we await the outcome of other avenues of investigation, earlier on, you said that you had tried 'exercising' the upper address lines and scoping the _CE signals for activity, which you did see.
I suggest that you now need to specifically check whether address A8-A11 state =nnnn actually results in the expected _CE signal being activated, so,
xxxx1111xxxxxxxx (0Fxx) should activate both _CE pins on the standard RAM ICs
xxxx1011xxxxxxxx (0Bxx) should activate both _CE pins on the extra RAM ICs
xxxx1000xxxxxxxx (08xx) should activate _CE on the RAM I/O
xxxx110100000000 (0D00) should activate the strobe signals to the display output hardware. For this last one you will probably need to assert NWDS low as well once the address is established.
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