B Degnan wrote:
It looks something like this:
-------------------------------------------------
-128K 128K -
- -
- -
- -
- -
- -
- -
- -
-------------------------------------------------
I'd be looking for a stuck address bit, or possibly a failed RAM chip
(it's theoretically possible for an ESD-protection diode in a CMOS chip
to fail shorted, which would effectively pull that signal line high or low).
If I had a circuit diagram I could probably tell you which parts / pins
to test, but either way you'll need a good logic probe (one that has a
pulse detector in addition to the hi/lo comparators) or an oscilloscope
to pin down the fault.
My money's on a 74xx chip (possibly a counter, buffer or multiplexer)
that's had an output fail open.
--
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/