Date   : Tue, 16 Oct 2007 21:40:22 +0100
From   : pete@... (Pete Turnbull)
Subject: Issue 4 freezes in old age

On 16/10/2007 20:28, Alex Taylor wrote:
> I was wondering about all this. I've got a bunch of faulty Beebs, one
> of which seems to work - except an odd video ULA problem.
> 
> It's an issue 7 that the original ULA caused random flickering cursor
> widths and non-functional 20K modes. My spare ULAs result in an
> inverted mode 7 but the other modes work fine - and these spare ULAs
> seem to work fine in an issue 4 machine. None of them have ever had
> heatsink paste on them.

That sounds like the replacement ULAs are *not* ULAs at all, but VTi 
video processors.  As I wrote in an earlier post, the original version 
didn't automatically handle the teletext mode change correctly -- and 
the symptom is that you get inverted video.  None of the VTi devices 
ever need a heatsink, only the original Ferranti ULAs did.

> Does this mean I can modify something on the iss.7 board to make mode
> 7 work properly with the spare ULAs I have? Or do I need a different
> ULA?

See my previous post :-)  If the video processor is an early one, add 
the wire and remove S26.  If the video processor is a later one, remove 
the wire and re-instate S26.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York