sd_snatcher has released a guide on how to upgrade the Toshiba HX-MU900 FM Synthesizer Unit (a.k.a. Toshiba MSX-Audio) with 256kB Sample RAM and MSX-Audio BIOS. This cartridge was, up to now, the most limited Y8950-based device for MSX. The DIY upgrade guide is available in both Portuguese and English on Hans Otten's DIY Enhancements page and, in the near future, on MSXPro.

The article also documents a third, unfinished, upgrade that would allow the HX-MU900 to record audio as well. As the completion of this upgrade requires analog electronics knowledge beyond sd_snatcher's skills he hopes that the information in this document will encourage someone else with analog electronics knowledge to solve the currently existing background noise problem.

Relevant link: Hans Otten's DIY Enhancements page