Check for shorts between the outputs of the 74LS145 would be my first choice. This could cause the effect you describe. Looks like a short between the lines that drive the A and F keys or the keyboard cable not inserted correctly into the mother board. If you have a diagram of the keyboard matrix it should be easy to find.
keyboard matrix is given at
http://www.6502.org/users/andre/peti...rds.html#graph