If it does this with ALL the speakers disconnected then one of the
output channels has a problem. The unit needs troubleshot to the
component level and repaired. Any good technician with the service
manual in hand will be able to diagnose and repair the unit.

Until you get an accurate diagnosis and estimate, there is no way to
know if you think the unit is worth fixing.

Call any Sony authorized distributor for parts or service manual, like
TriTronics www.tritronicsinc.com. etc.