Because this model was never officially sold in many Western countries in large numbers, finding an original paper can be a challenge.
The manual specifies grounding points. Ensure the turntable ground wire is attached to the screw marked "SIGNAL GND" shown on the rear panel diagram. Also, check the power supply capacitors (C901, C902) – the manual lists their values (often 12,000µF/50V) for easy replacement. Kenwood Amplifier A-5j Manual