Apple schematic for logic board 820-02536, a 4.06 MB PDF. The index associates this board with related Apple numbers 820-03160 and 820-03286, which are covered by sibling files. Apple uses 820-xxxxx numbers for its logic boards, printed on the PCB itself, so read that marking before working from this document. Technicians use these schematics to trace power rails, check the power-on sequence, find the correct value of a failed part and identify which IC drives a given line. Searching net names in the PDF speeds this up.
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 @cynetitsolution. 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.