A low-frequency hum or buzz is audible through one or both speakers.
Safe first checks
Turn the volume down before changing cables.
Disconnect all input cables; if the hum remains, it is likely inside the unit or related to speakers.
Reconnect sources one at a time to identify a source, cable, or turntable ground issue.
Use the turntable ground terminal only for the turntable ground wire; do not defeat the power plug safety ground.
Next step
Record whether hum changes with the volume control and whether it is present with all sources unplugged.
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.
Separate input, preamplifier, power-amplifier, and speaker-path issues by testing one stage at a time with known-good sources and cables.
Inspect grounding connections, power-supply filtering, board connectors, and solder joints using the exact schematic as a reference.
Do not defeat a safety ground or make unapproved ground changes merely to reduce hum.