Check the speaker wires are not shorted.
But bear in mind that Sansui is a defunct brand and your amp will be quite old now.
Japanese stuff from the 1970's was built like a tank but some of the later stuff was not so solid. May be on the way out and not necessarily economic to repair.
There are lots of old-hifi enthusiast sites and you may find someone on a forum, who has experience of your model.