Re: Weird video on Apple IIc

[nathan@visi.com (Nathan Mates)]

In article <53m3fi$8vt@news.ycc.yale.edu>,
Ryan L Macarthur <macrl@pantheon.yale.edu> wrote:
>      I called 1-800-SOS-APPLE on this, and was referred back to Nathan 
>Mates's home page,"If you can't find the answer there, I don't know 
>where else you could," the tech support guy said when I told him it's  
>where I started from. 

   Ummm, Apple's sending folks to *ME*? Yeow. If this is true, this is
a real vote of confidence in my stuff. I did try and make the best
Apple II site on the web, and it looks like I've halfway succeeded.

> 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.
{snip}
>up an external drive and try booting off that), the symbols are still there, 
>and when I hit CR the @ symbols start forming a column up the screen:

   The fact that they're going up the screen seems to indicate that
this problem is in the actual RAM, not merely the character generation
and display to the screen. The scrolling is done by copying up
characters in memory, row by row, so it looks like a bad value is
being propagated around. Thus, something's's being written to RAM that
shouldn't be, or the RAM chips are defective in places.

>where the *'s are the rapidly flashing @ signs, and the ones that scroll 
>up the screen are normal.  Applesoft BASIC seems to work fine, and as the 
>screen is filled with text, text that is scrolled up past the flashing 
>@'s will be changed to a different character.

   However, this is more confusing. As far as I recall, Applesoft
should be scrolling the same way that just pressing return does. Does
'changed to a different character' mean a 'different, but incorrect'
character?  If so, it's a sign of which bits are bad. [If it's
'differnt from @, but correct', then that pushes the problem to the
character generator chips.]

   Can you provide a short list what gets changed around? [Specifying
what '~' and '1' get changed to will isolate most of the bits.]

>"Sounds like some weird memory problem or something," 1800SOSAPPL Apple 
>II Tech Support Guy says.
>"So I guess the only solution is to try and get my hands on a new 
>motherboard or something," I says

   A new motherboard, or possibly replacing the RAM inside.

>    When I switch over to HGR, there are about a half a dozen pixels on 
>the screen which appear in the same place every time (as do the @'s), and 
>a little program written to draw 10 random lines on the screen, refresh, 
>and repeat works fine but doesn't in HGR2, but maybe I don't understand 
>Applesoft BASIC that well.  Lines will draw over the pixels on the screen 
>(which also blink a little) and will leave the pixels unaffected (a white 
>line over the pixel has a black spot on it, and the pixels will remain after 
>CLEAR).

   If you get blinking pixels on the HGR screen when you're not doing
anything, that's an intermittent fault-- there is zero hardware
support for that kind of thing naturally. I'd recommend opening up the
//c (sorry, never done that and don't quite know the procedure
offhand) and making sure all chips are properly socketed (push down on
all of them in case they've worked their way out). If that doesn't
help, find an electrical engineer friend to look for bad solders
joints and fix those.  Failing that, you may need a new //c or
motherboard-- I've seen them for fairly cheap on
comp.sys.apple2.marketplace.

>    When the keyboard is pulled off after power-up (it goes into 
>self-test otherwise), the problem is still there, which brings me to my 
>final question: what am I supposed to expect from the Apple //c self-test?

   That first version of the //c may have an incomplete self test. If
it doesn't stop after a few minutes, then it's not useful.

Nathan Mates
--
<*> Nathan Mates http://www.visi.com/~nathan/      <*>
# What are the facts? Again and again and again-- what are the _facts_?
# Shun wishful thinking, avoid opinion, care not what the neighbors
# think-- what are the facts, and to how many decimal places?  -R.A. Heinlein