EAST ST. LOUIS, Ill. (AP) _ A federal judge who had to face the music to make a decision has awarded $15,500 to a woman who filed suit when she found out that her violin wasn't 150 years old, as she had thought when she bought it.

''The sale may be over, but the warranty lingers on,'' U.S. District Judge William D. Stiehl said Wednesday in ruling in favor of Karen Bentley, 23, a graduate student at Indiana University.

Ms. Bentley filed the civil lawsuit in 1985 against Charles Slavik, a retired music teacher from Edwardsville, Ill., who sold her the violin for $17,500 in 1984.

He gave her a certificate authenticating the violin as having been made in 1835 by Auguste Sebastien Philippe Bernardel, a 19th century Parisian violin maker. The certificate was from Robert Bernard Triplle, a violin maker and appraiser from Mount Vernon, Ill., who has since died.

Ms. Bentley said she later learned her instrument was a $2,000 German violin made this century. She originally asked for $20,000 in damages.

In hearing the two-day trial last month, Stiehl listened to the violin played in court and compared its sound to the tones of a $300,000 Stradivarius.

The judge said he based his award on the violin's original price, minus the $2,000 one expert testified it was worth.

Stiehl ruled the Slavik ''did not wilfully misrepresent'' the violin's worth, but actually believed it to be authentic. Nonetheless, the contract was breached, he said.

A man answering Ms. Bentley's telephone in Bloomington, Ind., said she was in New York and could not be reached for comment. Slavik's wife Rosemary said today they would have no comment.

In the 16-page order, sprinkled with references to music, Stiehl commented on the case's unusual nature.

''It is clear that this dispute concerned more than a simple commercial transaction,'' Stiehl wrote. ''The defendant felt his integrity attacked; the plaintiff felt victimized. While sympathetic, the law is ill-equipped to sooth such emotions. ...

''Just as many classic works of music are based on a simple melody, the law of this case is based on a consistent rule: that a seller's description of an item amounts to a warranty that the object sold is as described,'' Stiehl wrote.