Ensure the "Tape Monitor" button isn't engaged. This is a common culprit for "dead" units.
Do not rely on guesswork. Do not "poke around" inside the chassis without the schematic. And do not waste hours trying to figure out the Sigma Drive switch by trial and error. Kenwood Amplifier A-5j Manual
The story of the Kenwood Amplifier A-5J is one of Japanese precision engineering from the late 1980s and early 1990s. Originally released as part of high-end component "stack" systems like the Kenwood M-93 Ensure the "Tape Monitor" button isn't engaged