I have a Sansui 8080 and the answer about the jumpers is brilliant. The 9090db & 8080 are extremely similar. One thing that happened to mine is the selector switch for inputs had issues a few times. We are talking 30+ years ago, but I believe the tech had to re-solder the contacts back on a couple times. Also the speaker selector switch had the same issues. But as posted prior it could be a lot things. Good luck. My amp rocked my world for many years. So much so that my daughter used in in her college years in the early 2000's.
-John
-John