Date : Fri, 03 Jun 2011 23:15:46 +1030
From : station240@... (paul aslin)
Subject: Model B with video corruption
> Date: Thu, 2 Jun 2011 19:23:33 +0100
> From: BBCMICRO@...
> To: bbc-micro@...
> Subject: Re: [BBC-Micro] Model B with video corruption
>
> >
> >My theory is there is a fault around IC 23, 24, 26, which is causing data bus
> >conflicts between chips. Either that or a faulty chip somewhere.
> >
>
> According to my circuit diagram, IC23 is an eight input NAND gate which
> (I think) is responsible for slowing the clock to 1MHz when particular
> peripherals are addressed and IC24 and IC26 are address decoders. I doubt if
> IC23 is responsible. I suppose something could be selecting more than one i/o
> device at the same time. However I think it would be easier to try to
> eliminate the actual devices themselves which could have faults of their own
> as well as causing problems if the get selected when they shouldn't.
>
> I would suggest pulling all non-essential i/o chips such as the ADC, the user
> VIA. the serial ULA (assuming not soldered) and any extra paged ROMs. As you've
> noticed, removing an 8271 or 68B54 requires further steps to be taken to avoid
> hanging the machine.
Pulling the Serial ULA (and ADC, 68B54) finally got rid of most of the screen
corruption. Except sometimes it comes back as bad as it ever was.
Also discovered the UserVIA is dead, after swapping it for the system one.
> If the corruption is still present, I would suggest trying to figure out what
> could cause the particular corruption being experienced. For example, if
> incorrect characters are appearing in MODE 7, compare the ascii values of the
> characters present with what is supposed to be there. It may be that the
> problem can be narrowed down to a specific bit or bits which may in turn point
> to a problem with particular memory chips or bus tranceivers.
>
> Does the machine respond to commands?
Yes, except rarely hangs.
Vidproc is getting hot though, not a good sign.
> Is the contents of the screen memory
> wrong or is it only what is on the screen that is wrong? If the screen memory
> is corrupt, is non-screen memory corrupt too? Is there corruption in all
> screen modes?
I don't think its the memory itself, noise is rather odd to say the least.
http://s1092.photobucket.com/albums/i405/station240/bad%20B%20video/
View from 1 to 5.jpg and that is the order the noise appears in.
Of note is this is a 20k screen mode, and the top potion is never corrupted.
By my calculations this is &3000-&3FFF. Something screwy going on with the
addressing.
Also of note, despite the removal of IC15 74LS273, which disconnects the
teletext chip from the databus, mode 7 isn't dead but rather a mass of noise.
> Any random noises from the speaker?
Some sort of faint staticy noise from the speaker.