Sansui 9090db powers up but no sound

Why not buy one that works? Seems like you're asking for trouble. 

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

Before sending to service, I would make sure the pre-out/main-in jumpers are there and installed correctly. If they are missing or installed incorrectly, that will result in no sound. See lower left side of this picture,

https://www.iavscanada.com/wp-content/uploads/Innovative-Audio-Sansui-9090-DB-back.jpg

Also, if either of the "Tape Monitor" switches is pushed in, that will prevent anything else from playing.

Also google Terry Dewick mcintosh repair. I belive he is in TN but a friend of mine had him do a 9090db and had very good things to say.

Welcome aboard! There are several possibilities that could be the cause from power supply, protection circuit, speaker relay, output transistors etc. These old amps can be great sounding pieces and good bargains on the used market IF you can do the work.

Vintage recievers are ticking time bombs and sooner or later they will need repair, and if you have to send them to a tech the expenses can rack up pretty quickly. Having said that if you come across one in very good condition cosmetically that  you may want to condiser spending the money To have it completely rebuilt including the boards, caps and solder joints etc.

i did that with a marantz 2285b and its been problem free for 8 years.

Best of luck