Season 1 NBC , 1972 - 1973 Season 2 NBC , 1973 - 1974 Season 3 NBC , 1974 - 1975 Season 4 NBC , 1975 - 1976 Season 5 NBC , 1976 - 1977 Season 6 NBC , 1977 - 1978 Season 7 NBC , 1978 - 1979 Season 8 NBC , 1979 - 1980 Season 9 NBC , 1980 - 1981