Re: Getting rid of the infamous "magenta line" on a Rev. 7 II+ board.

[mwillegal <mike@willegal.net>]

On Feb 25, 7:59 pm, Linards Ticmanis <ticma...@gmx.de> wrote:
> Hi,
>
> it looks like quite a few Apple programs, especially those released
> after the //e became available, produce ugly artifacts on a ][+ board.
> What I mean is the "magenta line" at the left of the screen that appears
> when you extend the last bit of undisplayed memory into the display
> range by having bit 7 set in the leftmost byte of a line of hi-res data
> while the "undisplayed" bit is also set; this seems to be fixed on the
> //e hardware.
>
> Now this Apple II+ board that I have, a Rev. 7, was modified already, a
> while ago, to include the change marked as "Rev. 8 or Rev. 9" in
> Sather's "Understanding the Apple II" Figure 8.7.
>
> Thus I have two unused NOR gates still left (the LS02 introduced during
> the former mod has four of which only two are used). I think they could
> be used to ensure that the hi-res delay flip-flop (74LS74 at A11) is fed
> zeros during horizontal blanking, so that no undisplayed bit can make it
> through to the display via the delay. My idea is to feed the term (NOT
> (NOT hires-data OR hbl)), i.e. (hires-data AND NOT hbl), to that
> flip-flop instead of plain hires-data, abusing one NOR gate as an
> inverter by tying the inputs together.
>
> The problem is that the copper trace to pin 12 of A11, which I need to
> cut at a suitable point without destroying the connection between B4 pin
> 15 and A9 pins 2 and 15, is on the component side of the PCB and is
> hidden below the socket on my board. The only usable image of the
> Apple's PCB layout I could find on the net is from the Rev. 0 replica
> project by Mike Willegal, and of course the Rev. 0 doesn't have that
> flip-flop yet.
>
> So my question is, does anybody have an unpopulated Apple II Rev. 7
> board or an image of one, or maybe a board that uses sockets that don't
> hide everything below them, as those on my board do. And could either
> send me an image (of that board area) or check for me where this trace
> runs when it comes out from under the socket and whether it can be cut
> easily without destroying the other connection. Otherwise I'd have to
> remove or destroy the socket which I'd rather not do if it can be avoided.
>
> By the way, do you know if there is any software that needs the "magenta
> line" to be present?
>
> Thank you!
>
> --
> Linards Ticmanis

Looks like the net goes from a via under A10  on the top of the board
between A10 pins 2 and 3 to A11- pin 12 and never really goes under
A11 at all. However, why not just lift pin 12 of A11 out of the socket
and wire to the leg on the chip directly?  You could at least check
out your mod this way without altering the PCB and do something more
elegant later on, if it works out for you.

Regards,
Mike Willegal