The 2SC3901 transistor is documented in this 88.3 KB PDF. Following the JIS convention, the 2SC prefix denotes an NPN bipolar device, so the file is most plausibly its datasheet; no maker, package or revision appears in the name. Use the absolute maximum ratings, electrical characteristics and outline drawing to check pin assignment, test junctions in circuit and select a cross-reference part with adequate voltage and power headroom. Confirm the component marking matches before substituting.
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.
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The standard format for schematics, block diagrams, power-sequence charts and service manuals.