I learn long ago that Yamaha products like the Electones, PSRs, Clavinovas have a "test mode". This will test the LEDs, Contact in buttons, tone generator, EQ, Amps, etc..... simply,
Hold the C# chord (C#-E-G#)in the second octave while turning on the instrument, to press every button to be check, for the midi test you have to conecto a midi cable from the in to the out...I hope it helps...that may tell you what is wrong before you take it to have it then with the start button activate the check on feature that will display in screen, (like LED check) in witch you have service....I hope it helps you.

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mdorantes