Part reference for the 2SD1669, supplied as a 107 KB PDF and filed in the schematic section. Japanese JIS numbering assigns 2SD to NPN bipolar transistors aimed at low-frequency and power circuits such as drivers, regulators and audio stages. A technician uses the sheet to look up pin order, collector-emitter voltage, current rating and gain grouping. Those values matter when picking a substitute. Check the marking on the faulty part, often shortened to D1669, matches before relying on the data.
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.