Apple //c repair help!

[mkuhn@bolivar.cs.uiowa.edu (Martin Kuhn)]

Hello.  Recently I picked up a //c at a hamfest.  It was quite beat up looking,
but hey, for $15 including the power supply, it would be a nice way to get
a machine to run all my old programs from my ][+ and //e days without taking
up oodles of space doing it.

It works quite well, actually, except for one thing-- it has a sticky bit
throughout the display memory/bus (bit 6 to be exact).  This causes characters
of ASCII values of <64 to be shifted up by 64, vertical lines to appear in
the hires screens, etc.  However, since all programs seem to work correctly
otherwise, and since the //c has 64k chips (rather than the 16k ones in the
old ][ and ][+) this can't indicate a memory problem or a problem with the
memory bus.  So, I figure, the problem must lie in the bus to the video
generator.  The question is, which chip(s) is/are responsible for video
generation on the //c, and where might the video bus lie?  (board coordinates
would be helpful here)  Or might I be barking up the wrong tree altogether--
does the //c do something funny with video ram by mapping it somewhere else
for faster access?  Sorry about the odd questions, but most of my days of
using Apple ][s was back in the pre- //gs days, and most of it was spent
on a ][+ and a (non-enhanced) //e, and this was quite some time ago...
Gee, I remember back when I was lusting after the then-new //gs and seeing
those funny little "Macintosh" things at the Apple dealer and asking why
anyone would buy one if you couldn't get a color monitor for it...  Then
a little later of course, was "Apple ][ Forever!" :-( 

BTW, I know there were at least a couple of Apple //c revisions-- does anyone
have a list (or can point me to one) of how to identify these revisions and
how the bugs/enhancements differ between them?

Sorry if any of this is a FAQ, but I haven't been reading this group much
at all of late...

                      --Marty