Archive for Apple board 820-2523, RAR format, 1.85 MB, with no revision given in the name. A separate archive for the same number is labelled revision A and differs in size, so compare both against your board. Contents are not listed; expect a schematic and perhaps a boardview. After matching the 820 marking on the PCB, use the documents to trace power rails, follow enable signals and locate components and test points while diagnosing faults.
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 @Mac_Shematic_Santale. 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.
7-ZipWinRARThe Unarchiver (macOS)