Date   : Sat, 08 Feb 2014 15:09:27 -0800
From   : fozztexx@... (Chris Osborn)
Subject: Problems with BBC Micro

On Feb 3, 2014,@8:29 AM, J.G.Harston <jgh@...> wrote:

> 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.

It was IC27, the 7438. I ended up replacing IC39/IC36/IC40/IC27 because 
it was really hard to tell which one was the bad one, even with a logic 
analyzer. Once I had it opened I figured I?d just socket them all since 
it?s not too hard to desolder them with a desoldering gun. After I socketed
them I put all the original chips back in and then swapped them one at a
time. Of course it was the 4th chip I swapped that was the bad one. :-)

Thanks for the help!

Now to see if I can find a good terminal emulator to burn to eprom and stick
inside?