The 2SD1879 is documented in this 94.5 KB PDF, indexed as a schematic yet most likely a manufacturer datasheet for the part. Its 2SD prefix, per Japanese JIS numbering, denotes an NPN transistor rated for low-frequency operation. When a transistor in a power or audio stage tests shorted, a sheet like this gives the voltage, current and gain figures needed to choose a correct replacement and the pinout for in-circuit diode testing. The part may carry only D1879 on its case.
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.