Yamaha Electone Me 300 Problem

My ME 300 Yamaha electone, no light indicator on to all menu button when switch on, only power on indicator is light on near the power button, checked 2 pcs fuse are ok. Checked the DC supply from rectifier board to main board is appear 6.5 VDC. I checked all pasif component on main board are oke ( transistor, capacitor, resistor, dioda). I presume the problem from integrated circuit, but very difficult to find it. Would you like to help me which ussualy failure IC. More information, when switch on and off all indicators menu are flicker just a moment.Thank you for your attention and help. I'm waiting your answer and good news.