Most of the topics discussed here was cut & pasted while following a discussion between Don, Ola, Tom and Ton. Thanks for your efforts!

 Your SY99 Music Synthesizer is equiped with a 240x64 pixel Liquid Crystal Display (LCD) with backlight for better readability.

 It is difficult to observe, but some people think, that old LCD displays get darker with the years.

Current theory is that this has something to do with the internal lamp getting weaker.

Contact Yamaha for LCDDisplay Replacement.

Francisco Jiménez was kind enough to send me a link to a web site by Christopher Baxter describing how to replace a LCD display on your own.

See your SY99 manual to adjust the settings of the LCD contrast.