To get the desired result, pause the game (press Start), then push and hold L+R. Now enter the following:
God mode: A, A, B, A, A, A, A, A
Disable God mode: A, A, B, A, B, B, B, B
Computer map: B, A, A, A, A, A, A, A
Radiation suit: B, B, A, A, A, A, A, A
Invincibility: B, B, B, A, A, A, A, A
Berserk: B, A, B, A, A, A, A, A
All weapons, items, keys: A, B, B, A, A, A, A, A