The 2SK3121 is covered by this 126 KB PDF, which the index lists as a schematic although it serves as a part datasheet. In JEITA nomenclature, 2SK designates an N-channel FET. Open it in any PDF viewer to look up pin assignment, voltage and current limits, thermal data and the package drawing. It is most useful when testing a suspected shorted transistor or searching for an equivalent; match the body marking on the PCB to the sheet first.
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.
Any PDF readerSumatraPDFFirefox / ChromeFlexBV (links PDF to boardview)