Today I repaired one of mine PC-9821 machines (a PC-9821AP2/U8W), you can read repair log here hoping to help other people with similar issues
http://www.jammarcade.net/nec-pc-9821ap2u8w-repair-log/ (http://www.jammarcade.net/nec-pc-9821ap2u8w-repair-log/)
Great read! Thanks for sharing :)