Date   : Mon, 16 Apr 2012 16:56:34 +0100
From   : jgh@... (J.G.Harston)
Subject: MODE 7 video corruption

J.G.Harston wrote:
> Peter Coghlan wrote:
>> Do they appear below the character positions as cursors would or
>> half way up the character positions as long dashes would?
>
> On the video they appear horizontally aligned in the character
> positions, but vertically aligned at varying distances within

Ooops. I've freeze-framed the video, zoomed in, and stepped frame by
frame and the lines are all midway within the character cell, ie they
are character 95 - which is teletext character 96 - and they are
flickering on and off in all 2000 character cells including in the
cells where non-space characters are displayed.

So, every character cell is flickering between what it should display
and byte &60. It's not just b6 and b5 flickering to high, as if that
were the case then "BASIC" would flicker to "basic", "BASIC" is
flickering with "-----".

So, it looks like the data bus connection to the teletext IC is
dropping it's connection, and when disconnected happens to be
floating at &60.

Is the SAA5050 and the 6845 socketed? Have you tried gently pushing
them to ensure they are seated properly - or carefully unplugging
them and re-inserting them. Also, the data buffers, the IC number
I don't know off the top of my head, something like 74ls157 ish.

-- 
J.G.Harston - jgh@...      - mdfs.net/jgh