The keys on my Technics SX-KN750 doesn't work

[jubjunior]

The keys on my Technics SX-KN750 doesn't work. Everything else works for example the manual percussion, demo, etc. So there is no problem with the speakers, power or processor.
When the keys are played no sound can be heard. All the cables are connected.
I am searching for the service manual please send me If you have.
Can anyone help?

Thanks.