Some musicals debase the material they're adapted from. Raisin, which won the 1974 Tony for best musical, actually improves A Raisin in the Sun, Lorraine Hansberry's seminal drama about various members of a black family on Chicago's south side and how their aspirations are blunted by racism. The book by Robert Nemiroff and Charlotte Zaltzberg conserves the play's pointed dialogue and strengthens its compelling story, and songwriters Judd Woldin and Robert Brittan use music not to divert the audience but to show us the depth of Hansberry's characters. When Peabo Bryson, excellently cast as the angry, defeated Walter Lee Younger, realizes his dreams have been too long deferred and sarcastically sings "You Done Right" to his mother, you can feel his bitterness in your bones. New Regal Theater, 1645 E. 79th, 721-9230. Through February 26: Thursday-Friday, 8 PM; Saturday, 2 and 8 PM; Sunday, 2 and 7 PM. $22.50-$30.