When you play a key on an electric keyboard, the key touches a contact board. That contact board turns the impact of the key strike into an electrical signal. The electrical signal is sent to a circuit board, which processes the electric current and turns it into a sound. The method of processing depends on the type of keyboard you have. Some keyboards are analogue, some are digital. If your keyboard is malfunctioning or failing to operate at all, an electrical component inside may be "fried". Fried is a slang term for an electrical part that has shorted or burnt out.
Turn the keyboard off. Remove the keyboard power adaptor and try to power it with a different adaptor of the same voltage. Power ratings vary between different models and manufacturers. You can find out the power rating of your keyboard by examining the power adaptor. It is written as an AC voltage. For example, the Roland Fantom-G8 has a variable power rating of AC 117, AC 230 or AC 240. Turn the keyboard on with the replacement adaptor. If it still fails to power up it is likely that an internal component is damaged and not the adaptor.
Check the power adaptor on a known working device of the same voltage. If the device works, you can eliminate the power supply from your troubleshooting process.
Place the keyboard face down. Use a Phillips screwdriver to remove all of the screws holding the base cover in place. Remove the cover to expose the circuit board and wiring. Some instruments may have two circuit boards. Circuit boards are typically screwed into the keyboard chassis and connected by plastic plugs that you can unclip. Some keyboard circuits are soldered to the contact board, other circuits, such as the Roland D50, are connected with a circuit board plug. If yours is soldered, melt the solder connection with a soldering iron and remove the circuit board.
Replace the fuses in the IEC power supply socket. The socket is typically located at the back of the instrument, wherever the adaptor is normally plugged in. Unclip the fuse from the fuse-holder and replace it with one of an equal type and rating.
Inspect the circuit board. Look for any component that has a brown discolouration, especially transistors. A fried transistor may have an accumulation of soot on it. Transistors are side-mounted on the board. Flip the board over and melt the solder connection holding the transistor connector legs in place and remove it. Side-mount a replacement transistor of equal rating and solder the connectors to the metal strip running along the underside of the board.
Clean the underside of the key assembly with a dry cloth. It's smart to carry out routine maintenance alongside repairs.
Check the user manual to make sure that all of the fuses are the correct rating. If your electric keyboard is pre-owned, it's possible the that the previous owner has installed the wrong fuses. Fuses of the wrong rating will blow out frequently.