Only one of the 10 best weekly comedies in TV history is still on the air -- in prime-time first run -- and that show is "M*A*S*H." Since 1972, when it began, and despite the defection or musterings-out of cast members and producers, it has maintained a consistency, quality and civilized disposition rare in television. Rare anywhere.

"M*A*S*H" constitutes "proof that the vast wasteland can be made to bloom," says the prologue to "Making M*A*S*H," a glossy-flossy but justifiably admiring 90-minute documentary airing as part of fund-raising week on Channel 26 tonight at 9:15-ish (a mere six weeks after it was shown nationally on other PBS stations). Most of the stars, past and present, and many of those behind the cameras are interviewed about their work and why it's generally so good; the narrator is no less than Mary Tyler Moore, in her cold, "Ordinary People" voice.

A tribute to a current commercial network show may seem an unusual undertaking for public TV, but if "Making" had been done for CBS, it would have been done under CBS control, and therefore would have turned out considerably less candid.

From what the creators of the series say, you don't get the feeling that the CBS Television Network has been terribly instrumental in keeping "M*A*S*H" fresh. This comedy about the life-and -death adventures of the 4077th Mobile Army Surgical Hospital stationed in Korea would probably never have made it to the air if it hadn't first been a hit movie directed by Robert Altman. And Burt Metcalfe, for years a "M*A*S*H" producer, says, "I think probably if 'M*A*S*H' were a pilot today, you couldn't sell it."

Network brass consistently opposed ideas that made the show as distinctive and exceptional as it is. They didn't want scenes taking place in the operating room, for instance, because they thought that would be too grim for a comedy show.

And CBS rigorously opposed killing off Lt. Col. Henry Blake when McLean Stevenson, who played him, recklessly elected to bolt the series at the end of the 1974 season. But the amassed "M*A*S*H" forces prevailed, and Blake's death was announced at the end of a memorable episode.

One story around Hollywood was that Stevenson was so disliked on the set that Blake's death was written in to make sure there was no way he could ever come back; the documentary doesn't get into anything this irreverent in chronicling its irreverent subject. According to producer Gene Reynolds, among those interviewed on film, the death made sense because "M*A*S*H" tries to deal with realities and the reality of wars has been that "a lot of guys didn't get back to Bloomington" afterwards. "And so, we killed him," says Larry Gelbart, the genius-or-so who developed "M*A*S*H" for TV and guided it through its first 97 episodes as producer and frequent writer.

The producers didn't always get their way with CBS. Because of what Gelbart calls "the prehistoric thinking that goes on at the networks," the show is encumbered with the electronic gurgle of canned laughter. The operating room scenes are played without electric titters, however, and one or two complete episodes played, Gelbart says, "as in the actual war, where there was no laugh track."

But Wayne Rogers, who left the show and the character of Trapper John in 1974, says the network does deserve credit for keeping the program on the air even during its early so-so ratings. In particular he credits executives Robert Wood, now with Metromedia Producers Corp., and Fred Silverman, who in his days at CBS went to bat for some of the best shows in the network's history.

The documentary pokes around the "M*A*S*H" set and catches such privileged peeks as star, occasional director and so-called "creative consultant" Alan Alda looking through a camera lens with his hand still caked in makeup blood from the OR. Lorettta Swit is seen breaking up all over again when, during an interview, she recalls a touching line from a show that gave new poignance to the character she plays, Hot Lips: "Didja ever once offer me a lousy cop of coffee?"

Jamie Farr, who plays Cpl. Klinger, conducts a tour of the women's wardrobe he wore when Klinger used such ploys to opt for a discharge. "Your earlobes get numb" after a day of wearing earrings, he complains. Cast member Mike Farrell says he is filled with feelings of "how incredibly lucky I am to be here." A scene from a highly celebrated episode, "The Interview," is intercut with the actual Edward R. Murrow wartime newsreel that inspired it.

Patterson Denny, director of the documentary, says from Chicago's WTTW-TV, which produced the program, that he and producer-writer Michael Hirsh spent seven working days on the "M*A*S*H" set in December '79 to to catch the cast and crew behind scenes and between shots (several line fluffs and out-takes are included). "They were slightly nervous, initially," says Denny, "and not sure they were glad they agreed to do this. But the more we got into it, the more they trusted us."

The "M*A*S*H" set is known as one of the most impenetrable in TV. It may hardly be the happy family unit depicted in this documentary (though Alda, for all his attempts to come off as a high-minded idealist, still seems more like a pompous ego-maniac), but the illusion can be justified as something that will keep "M*A*S*H" fans happy. "M*A*S*H" fans probably deserve to be happy.

Now that "M*A*S*H" reruns are in syndication, "M*A*S*H" fans can be positively delirious. The harder-core fans can catch as many as a dozen episodes a week in some markets -- including Washington, where "M*A*S*H" is so popular that even the reruns aired in fringe times (7:30 and 11 p.m. on Channel 5) show up in Arbitron's monthly list of the top 25 TV shows here.

And one "M*A*S*H"-o-maniac told me that thanks to cable TV and its importing of distant stations, he was once able to catch four episodes in a single day.

For a TV program to command that kind of loyalty, something has got to be being done right. In the documentary, Reynolds decries the "unfelt violence, unfelt trauma" that characterize most TV shows. "M*A*S*H" may be many things, but it is never unfelt, and neither is Hirsh's passionate paean.