Season 1 NBC , 1970 Season 2 NBC , 1971 - 1972 Season 3 NBC , 1972 - 1973 Season 4 NBC , 1973 - 1974 Season 5 NBC , 1974 - 1975 Season 6 NBC , 1975 - 1976 Season 7 NBC , 1976 - 1977