The 2SD1198 is the subject of this 56.6 KB PDF, filed as a schematic although the name points to a component datasheet; an _E suffix follows the part number with no stated meaning. In the JIS system, 2SD designates an NPN bipolar transistor for low-frequency work. Read the absolute maximum ratings and pin assignment against the part on the board, since lookalike packages can have different pinouts. Compare any substitute on voltage, current, dissipation and gain rather than on case shape alone.
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.