Originally Posted by Rusty the Scoob
What are the common things that would cause this? I'm not even sure where to start looking.
I do not dispute the wise words from the pro's, but...
IMLE as a hobbyist, the most common causes for a DB to go flat while playing are:
#1 Slipping strings on the tuner barrels, ie. incorrectly strung.
#2 If using guts or weedwhackers, the strings stabilizing.
#3 the tail-wire slipping.
Especially on the basses from the lower end of the price spectrum.
Since the bass was free, I'd make sure that at least the sound-post is at its correct location, if not a full and thorough once-over.