Sony MBX-189 motherboard documentation for project M760, revision 1.0, packed as a 2.30 MB RAR, larger than most files in this series and perhaps holding extra pages or files. It likely contains a schematic for tracing power rails, checking signals between chips and locating components and test points. Open the extracted drawings in a PDF viewer. Before measuring or replacing anything, confirm that the PCB carries MBX-189 and revision 1.0, since later boards may change component values.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @generalschematics. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.