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
An old trick for testing DRAM in circuit is to simply press over the
suspect RAM a known-to-be-good RAM with the pins tightly touching. In
many cases DRAM failures relate to failing pull-downs and thus a good
DRAM in parallel will take the load and provide good results.
This doesn't work for Static RAM though as far as I know.
And I don't know if 9114s are Static or Dynamic, I suspect they are
Static, but if DRAM then the test above may help avoid desoldering a
pack of them...
John :-#)#
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