Dropzone [Model DMG-AD4P-EUR]

Cheat sequences:
All cheats should be entered at the title screen, after DROPZONE ADVERSARIES has been displayed. If entered correctly, the game will start immediately, so you couldn't use more than one cheat at once.
Up, Down, Right, Right, Left, Left, Up, Down - The Cloak timer 4 times slower.
Right, Left, Left, Right, Down, Left, Up, Right - Extra 25 lives and bombs.
Down, Left, Up, Down, Up, Down, Up, Left - Start on Wave 5.
Source: gamefaqs.com
Left, Left, Right, Down, Up, Left, Right, Down - Start on Wave 10.