Date   : Sun, 15 Apr 2012 23:07:20 +0100 (WET-DST)
From   : BBCMICRO@... (Peter Coghlan)
Subject: MODE 7 video corruption

>
>A post from last year said;
>"The teletext chip may be serialising random junk due to having its inputs
>floating. If you plug IC15 back in and check what characters are displayed in
>
>mode 7 when the screen is supposed to be clear, this may give a hint as to
>which bit or bits are having problems."
>

I wrote that about a machine where the owner had removed IC15 and was seeing
random junk in MODE 7. I was trying to point out that if you remove IC15,
seeing random junk in MODE 7 is a likely result.

>
>Is there any relationship? what should i try next?
>

Are we talking about the same machine as last year or is this a different
machine?

I don't have any way of viewing your youtube video.

If the MODE 7 screen should be clear and you are seeing all long dashes
(as opposed to minus signs which are shorter. There aren't any underscores
in MODE 7) then it looks likely that the teletext IC is seeing 96 where it
should be seeing 32, ie bit 6 stuck on or appears to be stuck on to the
teletext IC.

If nothing at all is displayed incorrectly in modes 0 to 6 then that would
seem to strongly suggest that there are no memory problems.

To further rule out memory difficulties, you could try something like the
following program, assuming you can use the tape or network filing system.
(Type it in using MODE 6 or some other mode where you can see what you are
doing).

10 MODE 7
20 *SPOOL "SCREEN"
30 CLS
40 *FX 3,2
50 FOR I% = &7C00 TO &7FE7
60 PRINT ~I%, ?I%
70 NEXT
80 *FX 3
90 *SPOOL

RUN it and check what values you get in the file called "SCREEN". All
addresses should contain the value 32 ie ASCII space. If not, post the
values you do get and what appears on the screen at the same time as running
the program.

If the memory is ok, I would suspect problems with IC5 (SAA5050) or IC15
(74LS273).

Regards                                          
Peter Coghlan.