Crappy socket used at Z11.....i would replace it or them if more are used. Z11 is only buffering switchmatrix strobes and the board will boot without it. In Z12 also a few ports are used as addressline buffers, without that chip your board won't start. Switchmatrix RIOT U4 contains RAM which the program needs in order to run (U5 also and U6 provides memory for the stack). If you are familiar with test ROM's you are welcome to use mine. Z11 usualy fails when something wrong happened under the playfield and the solenoid voltage enters the switchmatrix. Check also the switchmatrix diodes under the playfield after something happened like this.

Edit: Andy was a bit faster about the crappy sockets

Marco