These old receivers usually need a good cleaning. Go to Radio Shack and get their best electical contact cleaner. Spray all of the controls and switches and while they are wet with the cleaning solution be sure to operate the controls and switches numerous times. Just one dirty switch can cause the kind of problems you are having. Do this first before taking it to a repair shop. Most of the Japanese receivers have good longevity. I own several receivers from the 70s and they all work flawlessly.
The statement of old receivers not working well with new speakers is somewhat true. Most of the old receivers were optimized for driving very efficient 8 ohm speakers. I have tried these receivers with low efficiency 4 ohm speakers and the sound becomes very hard and closed in indicating the amp is not happy with the load at the other end. However, any 8 ohm speaker with at least medium efficiency seems to be fine.
The statement of old receivers not working well with new speakers is somewhat true. Most of the old receivers were optimized for driving very efficient 8 ohm speakers. I have tried these receivers with low efficiency 4 ohm speakers and the sound becomes very hard and closed in indicating the amp is not happy with the load at the other end. However, any 8 ohm speaker with at least medium efficiency seems to be fine.