Date   : Tue, 16 Oct 2007 00:10:56 +0100
From   : pete@... (Pete Turnbull)
Subject: Video ULA compatibility query

On 15/10/2007 23:22, Daniel Beardsmore wrote:
> I imagine most Beeb fans will be aware that the original Ferranti video ULAs
> on the Issue 4 and earlier would cook themselves and end up displaying all
> sorts of weird visual defects like snow, stripy colours, bad default palette
> entries (entry 3 in mode 2 showing red instead of yellow) and just general
> garbage on the screen.

Yes, they're supposed to have a heat sink -- and with that, they're OK 
(until the white gooey heat sink compound dries out or oozes away, and 
needs replaced).

> My first ever Beeb had this, and when I'd sold it to my cousin I later tried
> replacing the Ferranti ULA with a VTI 2069 from an Issue 7. While modes 0-6
> were fine, it seemed to be incompatible with the SAA5050 causing Teletext
> text output to be horribly smeared on his colour television and barely legible.
> 
> I'm presently running an Issue 4 that also had a fried ULA, replaced again
> with one from a (broken) Issue 7. I have no TV; I'm using a Microvitec Cub
> screen and mode 7 displays perfectly.
> 
> According to Jon Ripley's Wiki that led me here, the VTI VC 2069 is a
> straight drop-in replacement for the Ferranti predessor, but clearly
> something changed between the Issue 4 and 7 that makes the VC 2069-based
> ULA, or the software therein, incompatible somehow. I'm just curious as to
> whether anyone knows what this is about?

It was supposed to be a drop-in replacement, but the early ones (VC2023) 
had a problem in that they didn't correctly switch settings between 
teletext and non-teletext modes.  To fix that, you had to remove a 
jumper (S26) and solder a small wire between the VidProc pin 27 (=centre 
of S26) and one of the select lines for the memory buffers (IC10 pin 1, 
which is the inverted TTX teletext signal).  The official fix put that 
wire under the board, which caused some headscratching when people who 
didn't know that swapped one VidProc for another (or for a ULA).

The difference is not the BBC board issue, but the VidProc version.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York