This video is from "Electronic Service Vlog" channel https://www.youtube.com/channel/UCBJEaReomdyPytLPr48JOmg/videos
You can get an idea how to disassemble the keyboard and access various parts like keys and rubber contacts.
12 key rubber contact:
http://artaudioparts.com/v341360r-rubber-contact-12key-for-yamaha-psr-gx76-ypt-300-dgx-202-500-50513 key rubber contact: