Vortex

Type in the word VORTEX and press and hold the reset button on the machine. After a couple of seconds, an animation sequence with all the cool game robots will show them being sucked into a vortex. This sequence will loop until the reset button is released.

The following codes are all entered at the "NEW NAME" entry screen:

At the "NEW NAME" screen press L (the button on top of the controller), C-RIGHT, and C-DOWN to bring up the secret characters.
Enter the name "CREDITS" to view the credits.
Enter the name "LINES" to play the hidden game.
Enter the name "G(alien head)MEBOY" for hidden tunes.
Enter the name "(saturn)(spaceship)(rocket)(heart)(skull)" for access to all levels.
Enter the name "VORTEX" for ???.