Double check the plug for the apple base to the MFU, make sure none of the sockets have backed out and you have a good connection. Also could this be the SW3 switch enigma? I remember years ago when I built my JT it was acting strange and somehow the SW3 switch got moved. I can't remember what exactly wasn't working but it solved the problem. As EX Pat pointed out, putting it front of a mirror and seeing if you take a hit will help to proceed.