IBM Monochrome "Black and White" Parallel card double iimage
pinball at telus.net
Fri Mar 26 16:43:39 CDT 2010
B Degnan wrote:
> > 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
> > 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...
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