One of the most troubling problems an owner of the S90 ES may encounter is the absence of any sound coming through the speakers. Yamaha recommends that owner's first check the control sliders to ensure that they and the volume are at proper levels. Next, owners should ensure that the S90 has power and that all external equipment is properly connected and powered. Ensure also that the foot controller is not depressed, that the external MIDI controller settings are not too low, and that the element switch, note limit, and velocity limit parameters are appropriately set. If a lack of sound persists after all of these checks and fixes, the S90 may be damaged or faulty.
Another problem that S90 owners may run into involves sound that is present but distorted. This problem could arise from the inadvertent application of effects. Check the "VOICE", "PERFORM", and "SEQUENCE PLAY" buttons to ensure that they have no applied effects. Additionally, any filters set to excessive resonances can cause distortion. Finally, a very high volume can also cause distortion and clipping. Try turning the volume down to avoid a distorted output.
It is important that any synthesizer have accurate representations of every pitch. If the S90 ES is misrepresenting certain pitches, there may be some incorrectly set parameters. Check both the Master Tune and Note Shift parameters and turn each to a value of zero. Turning down the LFO Pitch Modulation Depth also can get the synthesizer back to standard pitch.
Another potentially frustrating problem with the S90 ES involves a lack of intended effects. If this is the case, check the "EFFECT BYPASS" button, and make sure that it is turned off. Additionally, turn the control sliders for "REVERB" and "CHORUS" up from zero to hear effects fully. Finally, make sure that each individual effect has not been turned to "thru" or "off," which will negate that effect.