Turn knobs to ensure they don't "crackle" or feel gritty.
Ensure speaker wire clips or binding posts aren't snapped. The "Golden Era" Brands Receivers: Marantz, Pioneer, Sansui, and McIntosh. Turntables: Technics, Thorens, and Dual. Speakers: JBL, Klipsch, KEF, and Bowers & Wilkins. buy used stereo equipment
Check if the foam rings around woofers are crumbling. Turn knobs to ensure they don't "crackle" or feel gritty