unlikely to be IC14 as I assume the rest of the IO functions are working. Look at IC11 (7407) these are the output buffers to the pads or IC5 (74LS09) which act to debounce the pad signals coming into the cpu board.
It could be dirty contacts or bad tracks or dry solder joints somewhere in the connections to and from these IC's
It could be dirty contacts or bad tracks or dry solder joints somewhere in the connections to and from these IC's