Yamaha Clavinova CLP-50 very low sound

Hello together, 
i got hands on a old Clavinova CLP-50 from the 80s. The instrument is in a very good shape, but there's a problem with the sound. Unfortunately the sound is extremely low, even though the volume is on max. Also the sound seems to be a bit distorted. I already checked the output voltages of the powersupply. They're good. I also connected my Nintendo Wii via Aux In and the sound played was good. So the whole amplifier path should be good. Resistence of the volume adjuster measured from almost 0 to 10kOhm. That's according to the schematics. The Piano is playing the right sounds but verly low so i would guess the RAMs are ok. Is this correct? Does anyone have an idea or have fixed the same problem already? Thanks in advance