This is certainly not a capacitor problem...
I think this board uses 2144s as the video memory
So my first guess is to replace the RAMs.
-tony
----
I'd be looking for a stuck address bit, or possibly a failed RAM chip
My money's on a 74xx chip (possibly a counter, buffer or multiplexer)
that's had an output fail open.
Phil
-----
Along with Phil's suggestion of memory addressing issues resulting in a
double-scan of the memory, the staggering of the images suggests
interlacing
might be occurring when it shouldn't be. Sometimes
it is possible to
tweak the
V/H-hold controls enough on monitors to end up with
interlacing
occurring when
it shouldn't. Can you discern whether each of the
scan images
contains a full
set of lines vs half the number of lines?
I hate to ask, but what sort of monitor is this being displayed on?
IIRC the scan rate for MDA was higher than NTSC and I'm not not sure
whether
an
NTSC monitor would sync up or sync down to half the
scan rate.
Brent
-----------------
Thanks for the feedback. I am using a standard IBM monochrome monitor.
I tested the monitor with another known-working card, and I also tried
another monitor. The problem is with the display card.
This is one of the original black connector versions of the IBM 1904057
XM 407 display cards. It has 9114 RAMs in it, not socketed of course,
so I think if I can probe each RAM chip first to ID the bad chip it'd be
more efficient. I would similarly have to check the 74L chips. oy!
Bill Degnan