IBM Monochrome "Black and White" Parallel card double iimage
billdeg at degnanco.com
Thu Mar 25 07:42:46 CDT 2010
> 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.
> 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.
> Along with Phil's suggestion of memory addressing issues resulting in a
> double-scan of the memory, the staggering of the images suggests
> might be occurring when it shouldn't be. Sometimes it is possible to
> V/H-hold controls enough on monitors to end up with interlacing
> 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
> NTSC monitor would sync up or sync down to half the scan rate.
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!
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