Teens in the Universe(1975)

Original title: Отроки во Вселенной

Interstellar expedition equipped by "pioneers"(soviet scouts) reaches Alpha Kassiopea and finds that smart robots took control of whole planet. Their only goal - to make happy, as they understand, their masters. Happiness was, actually, satisfactions of primitive needs, and removing "disturbing" emotions like love, responsibility etc. A many years ago they succeeded in reaching this goal and all the people died out. A very few people escaped from "total happiness" and their descendants orbiting the planet in a big orbital station for many generations. Brave soviet pioneers land on a planet, destroy the robots and return it to the people