Thanks, both of you.
@SiamJai: The PSR 550 is actually newer than my PSR740. The first number is the series, the second is the version.
I have a 7 series PSR, version 4.0, you have a 5 series version 5.0.
Yours is newer. For instance, your version is more directed to the mainstream, having a songbook to select a popular song (from that time) and it will automatically prepare you to play that song (set the rythm, and tones)