Check the rectifier diodes in the power supply for shorts and look at all the filter capacitors in the power supply. Look for puffy discolored and domed tops. All filter capacitors must have flat tops. Check the windings of the transformer for darkened or burnt spots and test for shorts in both the primary and secondary windings
If the speakers are all good and wiring too, then it's likely that a fault has developed in the main amp section. Till this is cleared the protection will not go away.
You may be lucky enough to find a rubber band that fits. But the quality rubber in a household rubber band is not as good as a belt made for the purpose, and variations in stretch may not affect your broccoli but may be audible in a cassette player. Watch tension and don't put on too tight because it could hurt delicate bearings. I have repaired many items this way also. Bon appetit and good listening! HOW TO REPLACE CASSETTE DECK BELTS INEXPENSIVELY OR FOR FREE
