YDP-121 Yamaha Digital Piano - very low sound volume problem

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vprasit

Joined May 6, 2020
1
I have a 10 years old digital piano model YDP-121. It was working well until last month the sound come out very low even it increased the volume to the highest. I've tried some testing and found that
1) if i play the demo song from the Piano, the sound will gradually louder but still not as loud as it used to be
2) even it plug in the head phone, the sound also not loud

From these symptom, i think there is something wrong with the amplifier circuit. I have visually check the main circuit board and power supply, i cannot see any burnt or broken capacitor.
I just have little knowledge about the circuit, but can check the capacitor, resistor, or transistor.

I've attached the service manual for this Piano model which include the diagram.
Please kindly help guiding me how to further check to solve this problem.
 

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