By Japanese JIS numbering the 2SB643 is a PNP bipolar transistor, so this 44.2 KB PDF labelled as a schematic should be its datasheet, with _E probably marking an English edition. It should document terminal layout, package dimensions, breakdown voltage, collector current, dissipation and hFE groups. Open it in any PDF viewer when diagnosing a failed stage: test the junctions with PNP polarity in mind, and confirm that a cross-referenced substitute matches pinout and at least meets each rating.
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 @cyberlanelectonik. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.