The guy that created the term "Simasimac" has a discussion on how to troubleshoot/repair this one:
http://www.biwa.ne.jp/~shamada/fullmac/repairEng.html
Just jump down to "Bright Horizontal Line"
http://www.biwa.ne.jp/~shamada/fullmac/repairEng.html
Just jump down to "Bright Horizontal Line"