"Seventh Avenue," the latest of NBC's serialized "Best Sellers," proves to be palpable but basically inculpable pulp! It's the sort of enjoyable "mindless melodrama" that a network executive recently said "roots" will liberate television from.

In the meantime, "Seventh Avenue," which premieres with a two-hour episode at 9 o'clock tonight on Channel 4, is casually diverting and a model of adequacy, dignified by a cast that includes extremely engaging and industrious young actors.

Location filming in the New York garment district where the story is set (in the late '30s and early '40s) gives "Avenue" a certain added class, but what may turn out to be its chief improvement over previous "Best Sellers" is its more judicious length - three weekly chapters of two hours each. In the time it took to watch all of "Captains and the Kings," which began the series and dragged on for weeks, you could not only have read the book, you could have written it.

The success of last year's "Rich Man, Poor Man" led the networks to believe they could stretch thin material even beyond the breaking point and still keep viewers coming back to see how the story came out. Undoubtedly the success of "Roots" will lead to similar abuses along other lines. This year's hit is always next year's glut.

Norman Bogner's 1966 potboiler about sex and striving in the garment biz is negligible pop literature at best, but "Seventh Avenue" has just the ingredients networks look for: a hero struggling to succeed and succeeding, a period setting, a little love, a little lust, and a simplistic approach to life than can be made still simpler. All it lacks is a multi-generational time span. That's probably why it's shorter than the others.

Adapting a book for television is a little like tearing down a building and putting up a parking lot. However ugly the building or sleek the parking lot, the effect is to level the landscape, smoothing out psychological complexities and replacing them with recognizable cliches, so as not to startle viewers. In television, the hum of familiarty often passes for the ring of truth.

Writer Laurence Heath and producer-director Richard Irving made some intriguing alterations along "Seventh Avenue." They toned down the sex, almost to the inaudible point, and puched up the violence. Hero "Jay Blackman" is still Larry the Louse, but not quite so irredeemable His wife, Rhoda Gold, assertive and even pushy in the book, has been made meek and submissive. Her sister, Myrna, a mere nervous wrieck in the novel, is now an intellectual 1938 feminist.

The sterotypical Jewishness of the book has been tactfully muted. Rhoda's father no longer has a thick accent and no longer says things like, "See, from mine mouth the food he'd asteal." A loveable old duffer played by Eli Wallach has been added to the cast of characters, presumably for pathos appeal. In the first chapter, he commits suicide in his bankrupt clothing factory with some kind of cutting and stitching machine. Perhaps he sews himself to death; it isn't made quite clear.

Changes in little details tell us something about the mentality of TV writers. In the book, Rhoda goes to see an abortionist; Bogner eschewed the cliche at this point and made the abortionist a kindly lady who like to knit. TV writer Heath throws that out and leaps right to the stereotype: a huge, intimidating butcher of a woman who operates out of a filthy hovel.

What carries the program is Steven Keats as Blackman. Keats made an impression in the low-budget film "Hester Street." Here his slick and surly vulgarity makes him compulsively watchable; he is trying to flesh out a character critically underwritten, and he often succeeds. As Rhoda, Dori Brenner is a genuine and gratifying discovery, beautiful but substantial and able to surmount the mawkishness of the scenes invented for her.

The Blackman character has been rationalized into a variation on Sammy Glick and all the guys who grabbed for all the gusto they could and didn't care who they stepped on as they clawed their ways to tops. "I take," snarls Keats, "because nobody gives."

The TV version is certainly keener about establishing time and place, even though it's done in such pedestrian ways as having secondary characters remark, "Aw, Hitler's bluffing - you think he's gonna risk war with Poland" This is still more trouble than novelist Bogner went on.

"Seventh Avenue" is compromised, contrived, and overripe, but it is also unashamedly entertaining. It's in the television tradition of giving us the best mediocrity money can buy.