Public Broadcasting plans to right inroads of cable

PBS is striking back at the encroaching danger of cable TV . . . with love and expansion, Monitor correspondent Arthur Unger writes. Public Broadcasting president Lawrence Grossman, in a speech to PBS program managers in New Orleans Wednesday, stressed that public television needs to change in ''this era of multiple TV channels and reduced federal funding.''

He called for a new three-part plan for the future:

First, he suggested expansion of the ''MacNeil/Lehrer Report'' to one hour every night.

Second, he called for co-financed co-productions on arts and sciences with cable companies. The shows would air on cable first to small audiences and then on PBS to a much wider audience.

Third, Mr. Grossman wants to strengthen children's programming. He says children of 6 and over are not being well served anywhere. He would like to arrange a consortium of PBS stations to sponsor more after-school programming on PBS.