Date   : Mon, 03 Feb 2014 16:29:38 +0000
From   : jgh@... (J.G.Harston)
Subject: Problems with BBC Micro

Chris Osborn wrote:
> Yes, the problem occurs in MODE 0 through 3, and not at all in MODE 4
> through 7.

That means it's definitely the adder circuitry, then. If the adder 
circuitry is working in MODE 4-6 then it's not IC3 or IC32, it's either 
the NAND gate decoder IC36/IC40/IC27 which isn't decoding the "screen 
size 3" signal, or IC39 which isn't adding the address from the CRTC and 
IC36/40/27. As IC39 is working for all the other screen modes, I'd 
suspect IC36/40/27. If they're socketed, carefully remove them and plug 
them back in again.

-- 
J.G.Harston - jgh@...      - mdfs.net