Schematic for Apple logic board 820-1550, supplied as a PDF inside a 1.34 MB RAR archive. The four-digit form of the 820 number marks it as one of Apple's older boards, from before the switch to the five-digit 820-0xxxx format. The diagram lets a technician follow power rails, check voltage regulator circuits and trace signals between chips during board-level repair. Extract it with any RAR-capable archiver, and confirm that the board number silkscreened on the PCB matches before trusting its references.
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 .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.