Secret Quest Cheats

To enter a password, start a new game, enter the status screen (by pressing the black/white switch) and then press the game select switch.
If done correctly, the cursor should be blinking on the bottom symbols.

Now, to change the symbols, press Up the following number of times:
On the first symbol press Up 7 times.
On the third symbol press Up 7 times.
On the fifth symbol press Up 9 times.
On the ninth symbol press Up 9 times.

Press the game select switch again to apply the password. If done correctly, the status screen should change, you should have all the weapons and a full energy/oxygen bar.