Pause the game and enter the following combinations:
500 Credits: Right, Left, Right, Left, L, R, Up, Up, R, R, L, L.
1000 Credits: Up, Down, Up, Down, Left, Right, L, R, L, R, R, L.
5000 Credits: Up, Right, Down, Left, L, L, Right, Left, R, R, L, L.