If you have managed to destroy U2 on your Ni-Wumpf Series 1 MPU then good news! You can fix it if U2 (GAL or PAL) has been damaged...
Typically U2 fails (this is rare!) if you connect solenoid power to Return0 or Return1, or you short the outhole switch to solenoid power, or the Slam switch to solenoid power.
A simple test is to remove the Ni-Wumpf board from the game then with one lead of your ohmmeter connector to ground on the MPU touch the other lead of the ohmmeter (lowest setting or Diode Test) to pins 4, 5, 6 or 8 of U2. All four should show some much the same reading, and not a short to ground!