Looking For A Motherboard For An Electronic Piano 5meke Yamaha, Model : Pf70)

Hi !I need help to repair a vintage electronic piano (probably dating from the nineties). The make is Yamaha and the model is a PF70According to the tests I made, the problem is very likely to come from the motherboad. Indeed, The power supply unit is ok. The speakers seem to be ok too (you can hear them gently purr when you, switch on the piano) pressing the keys doesnt produce a single sound. Besides the digital screen doesn't work well and when you press the various switches supposed to change the sounds available, nothing happens : the displayed figure on the screen remains the same. But the main problem is that, even though the speakers are not damaged,, no sound comes out of this piano when you press its keys. My diagnosis is that there's something wrong with the motherboard, so I would like to know where I can get one in order to replace the probably damaged motherboard that is in the keyboard I have. Unless you know sb who is capable of finding the problem and changing the componets that must cause the problem on the mother board. I couldn't find any any electrical circuit diagrams which could have helped me find the defective component or componets on the mother board.Thank you so much in advance for taking time to read my email and for you help.