#1
I just recently purchased a Yamaha FGX720SCA acoustic guitar. It has a pickup in it so you can plug it into an amp or computer or whatever you want.

I've been trying to record with it, but after a minute, the preamp/tuner shuts off. It's done it every single time and has become increasingly frustrating.

Anybody know what's wrong? Maybe I'm turning it on incorrectly or something, all I press is the tuner button...

http://guitars.musiciansfriend.com/product/Yamaha-FGX720SCA-FG-Series-Acoustic-Guitar?sku=513387
(I've got the natural finish one...)
#3
It could very likely be the battery as nathan said above.

Also, some preamps have a feature built in that mutes the pickup when you turn on the tuner. You don't need to push the tuner button to play plugged in. All you need to do is plug the guitar in and play.