Re: Weird video on Apple IIc

[rubywand@aol.com (RUBYWAND)]

In article <53m3fi$8vt@news.ycc.yale.edu>, macrl@pantheon.yale.edu (Ryan L
Macarthur) writes:

>....
>     I recently got my hands on an old Apple IIc [? PEEK(64447) ==> 255]
and 
>it seems to have a couple of problems, the most disturbing of which is 
>the video display.  It's the normal "Apple //c" screen when the power's 
>turned on, but there are a couple of rapidly blinking @ symbols staying 
>in the same place on the screen. ....
>

     This sounds like an error in memory associated
with bits 5-7 of the first 64k block of memory. A text blank
should be in memory as $A0 (1010 0000).  Instead, in a
few places, you seem to be getting a $40 (0100 0000).

     One check is to try POKE-ing the value 160 ($A0) into
a screen location displaying the flashing "@". You can
find the location by going to ESC mode and cursoring to
the upper left corner; then, count X and Y moves to an
"@" and look up the location in your Apple manual's map
of the TEXT screen.

     For example, if the first char on the seventh line from
the top is an "@", you would enter POKE 1792,160.
If it does not change to a blank (space), you can be pretty
sure that RAM or some gate or controller which feeds
RAM is messing up. (If it does change to a blank, one
suspect is some kind of screw-up in system firmware.)

     Edhel Iaur's idea of re-seating the RAM IC's may fix the
the problem. IIc's are supposed to be portable; so, they
banged around more than most computers.

     Since you are getting several "@"'s but not
huge numbers of them, a bad connection to an address
pin is a fairly good suspect. (In fact, scootch-up and re-seat
all of your IC's-- several have the potential for making RAM
look bad.)

     If the problem remains, try switching RAM chips (swap
high bits with low bits in the same bank). If the problem
changes to mess-ups with low bits, you can be pretty
sure that a new RAM chip is needed.


Rubywand