Date   : Mon, 16 Apr 2012 07:46:28 +0000
From   : jackleather@... (jack leather)
Subject: MODE 7 video corruption

Hi Peter and thanks for the reply...

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?

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?

> Date: Sun, 15 Apr 2012 23:07:20 +0100
> From: BBCMICRO@...
> To: bbc-micro@...
> Subject: [BBC-Micro]  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.
> 
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