Toshiba HX-MU900
This page was last modified 10:26, 17 February 2020 by Rderooy. Based on work by Mars2000you and Gdx and others.




The Toshiba HX-MU900 is a crippled MSX-AUDIO extension. It lacks built-in Sample RAM, MSX-AUDIO BASIC/BIOS and a sound input to be complete.

The product name is "FM-Synthesizer Unit". It was made in Japan, but only sold in Europe.

Brand Toshiba
Type HX-MU900
Year 1985
Region Europe
Official price IT: L. 340,000
NL: ƒ399 (incl. HX-MU901)
Package content


This cartridge has a built-in music program, called 'MSX Music-System' that boots up automatically, but can be skipped by holding the ESC key. This firmware is also available on the Panasonic FS-CA1 cartridge, but slightly modified.

The menu of this utility gives access to three MSX-AUDIO songs and a music editor. You can notice that the menu screen creates some confusion with MSX-MUSIC, which is a light version of MSX-AUDIO!

From MSX-BASIC, it can be launched by entering CALL SYN or _SYN


  • On the Philips NMS 8220 pressing ESC on startup to skip the integrated Designer firmware will also prevent the MSX Music-System software from starting at the same time. In this case, start it from MSX-BASIC with CALL SYN.
  • Upgraded versions made by the MSX scene inverse the procedure as you need then to press the ESC key while booting to launch the firmware.



Toshiba HX-MU900 box
Toshiba HX-MU900 right side
Toshiba HX-MU900
Toshiba HX-MU900
Toshiba HX-MU900 back
Toshiba HX-MU900 with 32kB Sample RAM
Toshiba HX-MU900 upgraded with 32kB Sample RAM (back)
Toshiba HX-MU900 upgraded to full MSX-AUDIO with 256kB Sample RAM


Toshiba Music - intro screen
Toshiba Music - menu
Toshiba Music - editor
Toshiba Music - save/load screen


The HX-MU900 is a 'basic' MSX-AUDIO cartridge:

  • Yamaha Y8950 soundchip
  • 32kB ROM with firmware

It can be upgraded to a full MSX-AUDIO. (See links below)


  • Connector for a standard MSX-AUDIO music keyboard (usually the Toshiba HX-MU901)
  • 2 RCA mono Audio-out connectors


Whatever the upgrade, the microphone jack will be missing.