Good intentions clash with limitations of time and budget in tonight's segment of WJLA-TV's observance of Black History Month. A half-hour program, "Black Americans Making It Happen" (7:30 p.m., Channel 7,) looks like a table of contents for a full treatment of its vast, complex subject.

It ranges from prehistoric African explorers (who may have discovered America centuries before Columbus) to the black astronaut, Maj. Robert H. Lawrence Jr., who died accidentally before he got the chance to go into space -- a lot of territory to cover in half an hour, particularly when the topics also include black entertainers, cowboys, soldiers, educators, scientists and athletes.

A large portion of black history is hard to handle in a television documentary -- there is no available footage, for example, on Lewis Latimer, a colleague of Edison who helped to perfect the electric light bulb. So the viewer sees only a still photo and a picture of a patent application. Dozens of great names receive similar treatment.

The visual interest picks up notably when the subject gets to black athletes -- on whom film footage is available. Jesse Owens is shown winning Olympic medals in Nazi Germany, Althea Gibson at Wimbledon and on her triumphal homecoming. Jackie Robinson in his days of baseball glory and, earlier, scoring a touchdown for UCLA. But this is precisely the part of black history that is already best-known.