Roland KR5000 buttons and keys play dead but still on power

Hi there, I have a 90’s Roland KR 5000. At times, there was one note that would suddenly play extremely loud when I used the sustain pedal. Some time ago, all keys suddenly reacted the same: playing extremely loud and incorrect. After shutting down and restarting, My KR5000 would play braindead: I hear the activation/power click, see lights, see the metronome light, but none of the keys nor the buttons respond. I tried playing through midi (XV console), which seemed to lift the problem and bring the piano back to life, however, next time I tried: braindead again and even the midi connection would not give any result (in both ways) Dusting off and checking connections did not make a difference. Could somebody tell me where to look and what I need to replace, please? Or should I consider it a personal death of an era?

Thank you very much in advance for any helpful comment…

Bob