Re: Revision of VGC Chip

[Mitchell Spector <mitch2gs@hotmail.com>]

dempson@actrix.gen.nz (David Empson) wrote:

>Bryan Parkoff <nospam@nospam.com> wrote:
>
>>     I notice that there are revision of two or more VGC chip for Apple IIgs.
>> Newer VGC chip corrects DHGR problem on older VGC chip.  What is this
>> correction for?  I am curious.
>
>My first IIgs was a relatively early ROM 00 with a faulty VGC. The VGC
>got replaced, so I no longer have one.
>
>I believe the fault had somewhat variable symptoms between different
>machines, so mine might have been better or worse than another one.
>
>My recollection is that there were two major display issues on mine:
>
>1. In monochrome double hi-res mode, grey regions on the screen (e.g.
>rows with alternating black and white pixels, such as the MouseDesk
>desktop background) appeared pink instead. I think it might have been
>some kind of alternating line thing: one line was grey, the next line
>was pink. There were also pixels which flickered betweeen grey and pink.
>
>2. In 80-column text mode, with certain combinations of foreground and
>background colour (I specifically remember it happening with white text
>on black background), some white pixels would also flicker to pink.
>There were similar effects with different colour combinations.

    My old high school had an affected Apple IIgs with the faulty VGC,
so I'm quite familiar with the symptoms of the problem.

    As mentioned above, it only caused glitches with monochrome
Double-Hi-Res and 80 columns text (both emulated video modes). 
The only thing to add is the tiny pink bits looked like fringing that
would was either inversed or flashing. I don't recall any specific
patterns, it seemed more random. You'd see the pink actually within 
certain text characters (e.g. the gaps between "e","m", "a", "w").
I didn't look at Double-Hi-Res much but it also had the same stray
bits of inversed/flashing pink.

    It would typically be noticeable in AppleWorks or MouseDesk.
It was less apparent with certain background/foreground text
colors set in the Control Panel, true, but that never completely 
resolved the issue, it just better masked it. It never went away.

>I don't recall seeing any problems in 40-column mode, standard hi-res or
>super hi-res modes. I didn't use lo-res, double lo-res or colour double
>hi-res modes enough to pay attention to them.

    It caused absolutely no problems with native graphic modes. Also
it had no ill affect with color Double-Hi-Res or any other emulated IIe
video modes.

    The problem is sever enough where you can spot it instantly looking
at the screen, but if you have a disconnected machine the faulty VGC 
chips ended with a "-1" or "-A" suffix. I'd consider them on the rare 
side since it was only very early GSes that shipped with them, and 
Apple replaced the faulty chips free of charge.

Mitchell Spector