With the public's appetite for new cable networks apparently reaching the saturation point, the trend this season will be for existing networks to do whatever they can to stay healthy.

More and more, that means shifting toward more original programs, especially weekly series.

The reason: Weekly series ensure regular viewing. For ad-supported networks, that reassures advertisers. In the case of pay networks, it helps slow the rate of "churn," the constant turnover of subscribers who decide to drop one network for another.

Here are some of the new series scheduled to start on cable networks during the 1990-91 season:

"Big Brother Jake" (The Family Channel), a sitcom starring Jake ("Body by Jake") Steinfeld as a former Hollywood stunt man who becomes foster dad to a houseful of kids in Brooklyn.

"The Adventures of the Black Stallion" (Family) with Mickey Rooney reprising his trainer role from the original "Black Stallion" movie.

"The Avengers" (Arts and Entertainment) isn't new, of course, but America has never seen the so-called "lost" episodes starring Honor Blackman as Mrs. Catherine Gale, predecessor to Emma Peel (Diana Rigg). Premieres Oct. 6.

"Expedition Earth" (ESPN) is a regular adventure series with emphasis on the earth's environment. Among the adventures are two programs devoted to the "Earthwinds" global balloon flight. Premieres in late 1990.

"Liquid Television" (MTV) is a combination of animation, graphics, short films and acted segments that make up a bizarre variety series. Premieres in December.

"Comedians on Comedy" (HA!) is an interview show hosted by director Paul Mazursky ("Down and Out in Beverly Hills"), who talks with people with rare comedy gifts. Premiere: to be announced.

"E.N.G." (Lifetime), a one-hour drama series from Canada, is set in a TV newsroom where E.N.G. means "electronic news gathering." Stars Sara Botsford, Mark Humphrey and Art Hindle.

"Kurt Vonnegut's Monkey House" (Showtime) is an anthology series based on the stories of Kurt Vonnegut Jr., who will be the host. Premieres in early 1991.

"HBO Storybook Musicals" (HBO) is a series of animated specials based on classic children's books.

"Maniac Mansion" (Family) is a wacky sitcom, the first from Lucasfilm ("Star Wars," etc.) and starring Joe Flaherty as a goofy inventor, surrounded by the sad results of some experiments in genetics.

"Invention" (The Discovery Channel) is a series on "the spirit of innovation" that profiles the people and concepts behind inventions from the paper clip to spy satellites. Hosted by Lucky Severson and co-produced by the Smithsonian Institution. Premieres Oct. 2.