I had similar issues with an old 500 mobo many many moons ago. I had to steal the 74x157 from another board to fix the issue.
Come to think of it, there was also a component blown on the board which fed power to the socket. I would have to find the board to know for sure, but I do remember that the two were related. The previous owner had somehow shorted out the port and it blew the component (I think it was a resister) and the multiplexor chip.
