Karen Sunde has chosen an elaborate device with which to dramatize episodes from the life of Benjamin Franklin. In ''Balloon,'' at the CSC Repertory, Miss Sunde imagines a play-within-a-play in which some of Franklin's French friends act out fragments of his personal history within the context of their own more immediate concerns and philosophical debates. The overall effect of the double image is more confusing than clarifying.
Franklin's (Christopher Martin) distant and more immediate past is recaptured in a series of ''pantomimes.'' The recollected incidents from the sage's life include the historic kite-flying experiment, various events from his up-and-down diplomatic career in England and France, and particularly his prolonged estrangement from his son, William.
After being discredited by the British government as an emissary of the American colonies, Ben Franklin committed himself to the cause of freedom and the revolution. William - the royally appointed governor of colonial New Jersey - remained loyal to the crown. True to the facts of the case, ''Balloon'' records the ultimate father-son reconciliation.
Miss Sunde's complex comedy of period manners and historical matters makes strong demands on the impressive ensemble resources of the CSC cast. Mr. Martin, the codirector (with the author) and designer of the production, plays Franklin as a vigorous mentor, an American original who commands the affectionate respect of his Gallic friends and who is resistant only when it comes to discussing his relationship with William. Gary Sloan's William challenges ''Papa'' sharply and accusingly.
As Madame Helvetius, the widow Franklin wanted to marry, Mary Ellen O'Donnell treats the often contentious guests with ameliorating grace, solicitude, and hospitality. Also cast in the play's mostly multiple roles are Barry Mulholland, Howard Lucas, and Walter Williamson.
Mr. Martin's scenic design features a visual symbol of freedom and its fragility - a model of the graceful balloon which has created a Parisian sensation. The decor includes white umbrellas and a draped parachute plus a bust of Franklin as a reassuring reference point. The time is about 1783 which, Miss Sunde reminds us, was the year of the first manned balloon flight and which places the action between the American and French revolutions. The occasion for the gathering at Madame Helvetius's is Franklin's imminent departure for America.
According to an author's program note, ''Because as it unfolds in the salon, among people who tried to share lives and loves and ambitions, and to make them serve their nations (this story of a man and his son) seems to have something to say to us too.'' How much is debatable. As stage fare, ''Balloon'' succeeds at times in stimulating thought and stirring the imagination. It seldom grips the emotions, partly because the shifts of role-playing create an identification problem for the audience. A more straightforward treatment might have produced a more comprehensible play.
''Balloon'' joins a repertory that includes Goethe's ''Faust'' (Parts 1 and 2 ), Strindberg's ''Ghost Sonata,'' O'Keefe's ''Wild Oats,'' and Buchner's ''Danton's Death.''