One side has no sound or substantially less sound than the other.
Safe first checks
Swap the left and right speakers at the receiver end.
If the problem moves, the speaker or its cable is the likely cause.
If the problem stays on the same receiver channel, test another input source.
With the volume low, gently operate the balance control and input selector; do not spray cleaner into the unit while powered.
Next step
Record whether the fault affects every input and whether headphones have the same missing channel.
Bench Technician Steps
This is the model of work a qualified electronics technician follows using the exact service manual and suitable test equipment. It is not a do-it-yourself instruction to work on energized equipment.
Use a known-good signal source and trace the signal path methodically from the input stage toward the output, following the model-specific block diagram or service manual.
Compare left and right channel voltages and signal behavior at equivalent points where the design has mirrored channels.
Verify switches, relay contacts, connectors, and board interconnects before replacing active parts.