Date   : Mon, 16 Apr 2012 12:55:14 +0100 (WET-DST)
From   : BBCMICRO@... (Peter Coghlan)
Subject: MODE 7 video corruption

>
>this is a different machine to the one you describe. mode 0 - 6 are fine. 
>
>just found out my user VIA is dead! does anyone want to part with a good one?
>

Last years poster also reported a dead user VIA. I suggested he check it again
when the main fault was fixed as loading on the data bus from the original
problem may have made the VIA appear to be faulty. The machine then developed
PSU problems and I don't know if those were ever fixed so we never found out if
the user VIA had really been faulty or not.

>
>the
> problem doent appear to be a bit thats stuck rather 'floating' - the 
>long dashs can appear all over the screen intermittently like noise.
>
>
>i will try the program and replacing the 74LS273 (ic15) in the next week.
>
>any other ideas?
>

Looking more closely at the circuit diagram, I see that D6 is not directly
connected from IC5 to IC15. It goes via IC36 and IC37 so these are suspect
too as well as the connections between them.

If none of the ICs involved are socketed, it may be worth verifying the
connections between their pins with an ohmmeter before going to the trouble of
unsoldering them. They are:

IC5  pin 10 to IC36 pin 12
IC36 pin 1  to IC37 pin 2
IC37 pin 1  to IC15 pin 19
IC15 pin 18 to IC14 pin 3 (for example)

It might also be worth checking:

IC36 pin 2  to IC15 pin 3
IC36 pin 13 to +5V

Another thing to do program-wise would be to see what happens when you try
to display other characters, such as a screenful of long dashes or screenfuls
of question marks, @ signs and letter As for example.

Another thought: is it possible that the lines you are seeing are cursors
rather than displayable characters?

Regards,
Peter Coghlan.