Problem on a Yamaha PSR-620

Hello i have a yamaha PSR-620. The keyboard he has presented 2 fails. The first is that he doesn’t call at first. It takes a few seconds or even minutes for it to turn on. And the sounds in some moments stay normal, and start a lot of noises, and the the voices mix more in a very confusing way, as if it were an interference of timbres. Does anyone have any idea which components might be defective on the motherboard? I am sending a link with a video showing the defect:
and a link to a repository where I managed to find the electrical diagram but I can’t even imagine what the defect could be. I still have basic knowledge in electronics:
PSR-620 - Google Drive

That keyboard is 29 years old. If you inspected the main board for obvious defects (cracks, burnt components, corrosion,etc) and don’t find anything suspicious, then time for a new (or newer) keyboard.