Season 1 Season 1 NBC , 1973 1973 N.B.C. Episodes. Season 2 Season 2 NBC , 1974 1974 N.B.C. Episodes. Season 3 Season 3 NBC , 1975 1975 N.B.C. Episodes. Season 4 Season 4 NBC 1976 N.B.C. Episodes. Season 5 Season 5 NBC 1977 N.B.C. Episodes. Season 6 Season 6 NBC 1978 N.B.C. Episodes. Season 7 Season 7 NBC 1979 N.B.C. Episodes. Season 8 Season 8 NBC 1980 N.B.C. Episodes. Season 9 Season 9 NBC 1981 N.B.C. Episodes. Season 10 Season 10 NBC 1982 N.B.C. Episodes. Season 11 Season 11 NBC 1983 N.B.C. Episodes. Season 12 Season 12 NBC 1984 N.B.C. Episodes. Season 13 Season 13 NBC 1985 N.B.C. Episodes. Season 14 Season 14 NBC 1986 N.B.C. Episodes. Season 15 Season 15 NBC 1987 N.B.C. Episodes. Season 16 Season 16 NBC 1988 N.B.C. Episodes. Season 17 Season 17 NBC , 1989 1989 N.B.C. Episodes. Season 18 Season 18 CBS 1989 C.B.S. Episodes. Season 19 Season 19 CBS 1990 C.B.S. Episodes. Season 20 Season 20 CBS 1991 C.B.S. Episodes. Season 21 Season 21 NBC 1991 N.B.C. Episodes.