NBC's revamped magazine series "Monitor" begins quietly but intriguingly tonight. The music with which it opens, Prokofiev's first piano concerto, was written in D-flat major, but the program that follows it sounds more like B-flat minus. Still, there are plenty of encouraging signs.

Lloyd Dobyns, who once presided over the dearly departed "Weekend" show, anchors "Monitor," at 10 on Channel 4. This may be the only published preview of the program that does not compare it one way or another with a certain magazine show of 60 minutes duration that has long been running to great success on another network; comparisons between the two are irrelevant. However, it is fair to compare "Monitor" to "Weekend."

"Monitor" is not as good as "Weekend."

Dobyns is as good as ever--as direct, unaffected and craggy--but the program has problems with tone. You don't quite realize that the second piece, "Spice of Life," is supposed to be humorous until you are well into it. It's a drawn-out version of the traditional funny kicker that often ends network or local newscasts, all about the horseradish trade in Tulelake, Calif.

One of the best of the four pieces on the show is correspondent Rebecca Sobol's story about a bizarre law passed in Louisiana in 1970 ("That's right," Sobol says with punch, "nineteen seven zero.") that classifies people by race and declares people white or black on the basis of as little as 1/32 of their blood. Thus was a woman who thought she was white for 48 years told she was black by a state agency that poked into her ancestry. This is a touchy story, and Sobol handles it very well.

Dobyns narrates a piece about a tribe of 5,000 American gypsies who consider it a part of their heritage to survive and prosper via con games and scams and to whom, according to a cop, "victimizing people is a way of life," practically a theology. It is estimated their annual bilk tally comes to $50 million. Few will talk on camera, so the story has to be reported somewhat tentatively, and while Dobyns wants us to be moderately outraged, the fact that the gypsies manage to avoid paying taxes does give them some status as rogue heroes.

The opening piece is about people who harass other people on the telephone, sometimes because they are fanatically in love with them, or think they are. The focus of the segment is not made clear quickly enough; it starts out telling about an 85-year-old woman who harassed a man in Chattanooga for 40 years with obscene calls, but then the bulk of it concerns one David and one Emily on whom the one David is fixated and whom he refuses to stop telephoning.

Although the Prokofiev is a classy touch, the "Monitor" team (Sy Pearlman, executive producer, Peter Poor and Thomas Tomizawa, producers) has not incorporated it very smoothly into the opening tease, which includes an embarrassingly clunky shot of Dobyns and does not roll smoothly. The "Monitor" set is handsome, a large trapezoidal grid with a chroma-key screen behind Dobyns, but the show's logo includes that eyesore, that abomination, that wretched thing, the NBC "N" with the peacock in the middle of it. Shoot it, strangle it or smother it, Grant Tinker, but kill that thing somehow, puh-leeze.

The name "Monitor" previously was used by NBC radio for its pioneering weekend news and feature service, the forerunner of today's all-news and all-talk radio stations. Listeners would hear electronic boops and beeps and be told by an announcer, "You're on the Monitor beacon." No matter when you tuned in, you had an 85 percent chance of catching something worth catching. With the new TV "Monitor," the odds have fallen to about 35 percent. They well may improve, but the first program fails to bring home the beacon.