Re: My monitor only displays strange characters....HELP!

[yakowenk@cs.unc.edu (Bill Yakowenko)]

In article <49sovv$2jme@majestix.uni-muenster.de>,
Arne Voigtlaender <mr.pixel@uni-muenster.de> wrote:
>Hi there!
>I've been given an old Apple //e... The power supply was damaged but
>I've repaired it. But now, after the reparing of the power supply, the
>monitor only displays strange characters like @ (the whole screen!).
>Does anybody know of this problem? Does anybody know what is damaged
>or what's to do? Please mail!! Thanx. -PiXEL
>
>  -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
>  =       Arne Voigtlaender (PiXEL)          -
>  -    email: MR.PiXEL@uNi-mUeNsTeR.dE       =
>  =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

It occurs to me that the ASCII codes for the at-sign is nearly
the same as for the space;  hex 40 for at, and hex 20 for space.
In binary, you can see these are similar:  01000000 vs 00100000.
Is it possible you got a bit crossed somewhere, like on the way
in to the character-generator ROM?  I don't know much about
Apple Innards, but that's one guess.  If the screen shows mostly
at-signs, with splotches of other stuff in the right patterns
(like where the non-space characters would be on a normal boot-up
screen), then this is likely.

(Similarly, the character-generator input patterns might not be
strictly ASCII codes, so the at-sign might correspond to hex 00,
in which case the problem could be a single bit stuck low, which
could easily happen; rather than crossed bits, which seems kinda
improbable.)

Another guess: it's using memory-mapped video, right?  Maybe
there is some problem with the video circuit's address lines,
so that when it asks for bytes to display, they are actually
coming from a different area of memory.  This could be caused
by an addressing bit stuck low or high.

Either of these could happen if someone was mucking around with
the hardware a lot.  Are there signs of that, like obviously-
amateur soldering, random jumper-wires, or cut traces on the
board?

	Good luck,
	Bill.
-- 
--
Bill Yakowenko      http://www.cs.unc.edu/~yakowenk        yakowenk@cs.unc.edu
~{SH?KND~}              "Too much reality can kill you."