I have a Casio CTK 551 Keyboard, when the AC adaptor is connected and power is turned on, the Keyboard lights up for a few seconds then turns off. I noticed no red light lights up when the AC adaptor is attached and turned on. Batteries are connected at the same time.

The adapter has a problem... likely a poor connection of input power that briefly contacts building up some voltage that is then stored in the capacitors in the adapter which then discharge quicklly when you turn on the Casio. The adapters with FIXED blades OFTEN have problems contacting the connections in wall receptacles and extension cords. You may need to bend the blades slightly or use a better receptacle like connector. IF your adapter has a three wire cord from it to the wall receptacle, those SOMETIMES don't seat well...