The speakers and headphones come off the same feed. Prime candidate is the speaker selector switch, liberal spay of deoxit followed by working the switch.
Other possibility would be wiring from speaker selector to speaker posts. Maybe try connecting speakers to the "B" set. With Rx powered off/unplugged
you can check continiuty by measuring the resistance from pin 27 left power amp and left A positive speaker post, selector set to A. Repeat for Pin 26 on
right power amp and right A speaker post.