THE FALL TV SEASON : HOW RETURNING TV SERIES ARE CHANGING THEIR ACTS

Do not adjust that fine-tuning dial.

Producers of returning prime-time series have done that for you--honing, refining, revamping and, where necessary, revitalizing those familiar shows that will make their new-season debut in the next few weeks.

Anyone who’s ventured through a supermarket checkstand recently knows that Patrick Duffy is returning to “Dallas,” and “Miami Vice” is beefing up its macho image with an injection of Testa Rossa--that’s the new model of Ferrari that Don Johnson as Sonny Crockett will drive.

But the likeliest candidates for retooling are often lesser-watched freshman shows lucky enough to make it to their sophomore years. For example, three of last year’s mystery men--”The Equalizer,” “Spenser: For Hire” and “MacGyver”--have gotten emotional face lifts for 1986-87.

Robert Urich as Boston-based private eye Spenser says goodby this season to Barbara Stock as girlfriend Susan Silverman, an ever-present voice of reason in the Robert Parker novels where the characters originated. Instead, he’ll become friends with a new character, assistant Dist. Atty. Rita Fiori, played by Carolyn McCormick.

“Their relationship was sort of drifting; it wasn’t changing in any way,” said Juanita Bartlett, the series’ new executive producer. Bartlett herself represents a change in the show: The former executive producer of “Greatest American Hero” and “Scarecrow and Mrs. King” has been specifically charged by Warner Bros. Television and ABC with giving Spenser a lighter touch.

“The character of Spenser in the books has a great deal of humor and we want to mine that a little more,” she said. New episodes begin Sept. 27.

Robert McCall, alias “The Equalizer,” alias actor Edward Woodward, was perhaps the most secretive of last season’s new heroic figures. During his first season on CBS, writers intentionally gave the global spy turned local do-gooder very little “back story,” that is, personal history for his actions to draw upon. He was a bare-bones hero.

But he was also a little-watched hero, the series often finishing third in its time period against “St. Elsewhere” and “Hotel.” This season, McCall becomes potentially more accessible to viewers by revealing more about his past exploits and by having “interesting women in his life,” said executive producer James McAdams.

Viewers also will finally learn McCall’s mysterious source of income. He apparently made some wise investments when he was a government operative, privy to knowledge about foreign regimes about to topple and the like. “For the most part, it’s pretty clean,” McAdams said, referring to his character’s life savings. (Starts Oct. 8.)

“MacGyver,” portrayed by Richard Dean Anderson, last year shared the Equalizer’s free-lance status, but this year the ABC operative goes to work full time for the Phoenix Foundation, a fictitious L.A.-based think tank. He’s also moved to a loft in the Larchmont district over a hardware store.

Both changes are designed to “broaden the scope of storytelling,” according to co-executive producer Stephen Downing. Last season he was limited to “spy-type stories,” Downing said. This season his assignments will range from checking out security at a military complex to tracking the progress of a pair of golden eagles in the Utah wilderness.

MacGyver, like Spenser and the Equalizer, will get a new love interest this year, but only briefly. She turns out to be a nun. But he will have a permanent sidekick in the form of Dana Elcar as Pete Thornton. Last year, Thornton gave MacGyver his orders; this year, he’ll travel with him as director of field operations for Phoenix. (Starts Monday.)

Last season, much was made of the new trend in anthology series. But neither “Amazing Stories” nor “The Twilight Zone” was a stunning success, and they are both changing their time slots and target audience for Season Two.

“The show is going to be basically more adult-oriented,” said “Twilight Zone” executive producer Philip DeGuere. Moved from Friday at 8 p.m. to Saturdays at 10 p.m. on CBS (beginning Sept. 27), the show is going for the yuppies.

Examples: The season premiere will have a segment about an Elvis impersonator who goes back in time and impersonates the real Elvis Presley. Another storyline has “a yuppie couple lamenting the loss of innocence from the ‘60s,” only to find that their ‘60s selves reenter their lives.

Similarly, “Amazing Stories,” in its move to Mondays at 8:30 p.m. (as of Monday), will pick up some more mature themes, a program executive at NBC said.

And what about the new car on “Miami Vice”--not to mention the much-written-about new duds in purples and maroons?

Those changes sound like a stylish girding-of-the-loins in preparation for the battle against “Dallas” Fridays at 9 p.m. But it isn’t so, said producer Richard Brams. “The total palette of the show is meant to work in harmony with what we see as contemporary change,” he explained. The pastels still will be in view on the beach, but much of the show will now have a “nighttime” look, Brams said--one that is “sexier” and has “a more deeper color saturation.”

The Testa Rossa similarly replaces Crockett’s beloved undercover mobile, the Ferrari Boxer, simply because it was a newer model.

“We sat down and said, ‘How do we stay contemporary?’ ” Brams said. “This is not a formula show.”

Here’s how some other shows returning for the fall are changing their formulas:

“Scarecrow and Mrs. King”: After three years of hinting around, viewers tonight (at 8 p.m.) will see the bloom of a much closer relationship between Kate Jackson’s and Bruce Boxleitner’s characters. But it will be a “pretty wholesome relationship,” said a Warner Bros. TV spokesman. Fans might not necessarily go for their being lovers, he said. The relationship nonetheless will remain a secret from everyone else at the Agency, where Mrs. King will now be a full-fledged freshman agent.

“The A-Team”: The guys get a new leader next Friday when Robert Vaughn joins the cast as a mysterious government official who brings the Team out of hiding and into official covert work. Eddie Velez also joins the show as a special-effects expert.

“Dynasty”: In the best tradition of the daytime soaps, this show is plunking a new face--belonging to actress Karen Cellini--into an old role, that of Amanda Carrington. Catherine Oxenberg, who originated the role, is no longer with the series, which begins its new season Wednesday.

“Gimme a Break”: Cast and setting undergo big changes when the new season starts Wednesday. Nell (Nell Carter) moves to New York with best friend Addy (Thelma Hopkins), but leaves the Kaniski clan behind--except for Grandpa (John Hoyt) and Joey (Joey Lawrence). Lawrence’s real-life younger brother Matthew joins the cast.

“The Facts of Life”: NBC’s longest-running current series gets a boost from Oscar- and Emmy-winning actress Cloris Leachman, who joins the cast as Beverly Ann, sister of Edna Garrett (Charlotte Rae), who will no longer be a regular. New episodes start Sept. 27.