EclipseEye
Well-known member
- Joined
- Oct 18, 2010
- Messages
- 1,594
- Reaction score
- 102
Interesting one this, graphics were doubled up, wrong tiles used and the text was garbage;
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%;
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%;
