The DGX-530 and YPG-535 models by Yamaha^® are virtually identical instruments.  This particular DGX-530 had all its lights, buttons, and display functioning properly but there was no sound from the built-in speakers or the phones output.

The speakers and phones are driven by an audio amplifier IC, the LA4625.

An electrolytic capacitor connected to pin 2 of this chip was faulty, causing the IC to shut down.

Result:  Lights, no sound.

The troublesome capacitor is designated C207 in the above photo and also in the following schematic, showing the pinout of the LA4625 chip.

The following PDF file is a full schematic of the "AM" circuit board, containing the DGX-530/YPG-535 power supply and audio power amp:

YAMAHA^® AM Board Schematic