If the Tempo is resetting to 120 BPM or the DB-30 is shutting down after being powered on for a few seconds, this indicates that the battery is dying.
Replace the battery with a fresh new Alkaline type 2032. These batteries can be found at most stores that have a battery section.
On the back panel of the DB-30, there is a small phillips head screw. Remove it, and slide open the small "door" to expose the battery.
When installing the new battery, make sure the flat side (+) is facing up.