Corroded and dirty switches can cause this. With the unit off operate every switch and knob 30 times. They can sometimes clean up enough to work for a while. Switches that you don't use are the most likely to corrode.
You can check to see if there are any speaker fuses that are blown. Might be on the back or inside. If you replace them with the same amp rating fuse and they blow don't try again. It can cause additional damage.
If the amp has an internal relay that can also have corroded contacts that will cut off the output.
You can get the service manual here https/www.hifiengine.com/manual_library/sony/ta-1150.shtml