Oct 052018
 

Interesting one this, graphics were doubled up, wrong tiles used and the text was garbage;

I quickly and correctly diagnosed that Bit 0 is stuck low on reads/writes from screen/object RAM.  You can tell by looking at the scoreline;

00001000 = 08 = H … becomes 00001000 = 08 = H
00001001 = 09 = I … becomes 00001000 = 08 = H
00000111 = 07 = G … becomes 00000110 = 06 = F
00001000 = 08 = H … becomes 00001000 = 08 = H
 
00010011 = 19 = S … becomes 00010010 = 18 = R
00000011 = 03 = C … becomes 00000010 = 02 = B
00001111 = 15 = O … becomes 00001110 = 14 = N
00010010 = 18 = R … becomes 00010010 = 18 = R
00000101 = 05 = E … becomes 00000100 = 04 = D

 

I started to look at the video RAM addressing circuit, found a LS157 Multiplexer at 4G used to access a bank of video RAM with pin 9 stuck low.  Removed, fitted socket and new LS157;

Now working 100%;