Might be a broken socket.
My dad fixes keyboards from a local high school with broken headphone sockets fairly regularly. The MIDI socket could easily have suffered the same fate. A simple soldering task would fix that.
There's an opto-isolation chip in the MIDI circuit that could have gone bung. Problem is that ISTR such a thing lives in the receiving circuit, not the sending one. If that's the problem, it, too, should be easy to fix by one capable of weilding a soldering iron.
Wade.
