Digital keyboards rely on sensors underneath the keys to relay information to circuit boards and speakers. The majority of the problems you will encounter involve these sensors. Damaged or dirty sensors can cause individual keys to quit functioning entirely. In addition, malfunctioning sensors can reduce the volume level of individual notes on your keyboard.
If the keys are completely unresponsive, the synthesizer patches you are using may be the source of your problem. Control parameters such as polyphony and equalization filtering may reduce the number of keys that can be played simultaneously or completely mute tones on the extreme ends of the keyboard. You must use a patch that allows several keys to be played at once. Other mechanical issues may include damage to the internal key sensors or dirt and grime accumulation between keys.
Online databases of keyboard parts and repair shops such as Synthzone (synthzone.com) can direct you to a S80 vendor or repair technician.
Always unplug your keyboard before you attempt a repair. The risk of electric shock is continually present when equipment is connected to an outlet.