Before sending it off, you should check a few things.
Inspect the BNC connectors (scanner and antenna); if the center conductor connection is poor, your reception will suffer.
Remove the battery cover, battery pack and rear case (4 screws); check the connection from the BNC jack to the RF board. Poke it with a (non-metallic) stick to see if it's broken. Reconnect the battery pack, tune to a decent signal and poke it some more. If you get scratchy noises, something's wrong.
While monitoring a known good signal (like a control channel), adjust the tuning cap for best reception. The top of the cap looks like a Phillips screw head and it's at the bottom of one of the round holes (the one furthest from the battery pack) in the big silver RF shield.