Apple schematic in PDF form for logic board 820-3023, revision A judging by the trailing letter, about 1.29 MB. The older four-digit 820 numbering identifies the specific logic board design, so check the silkscreen on your PCB matches before trusting it. Use the sheets to trace power rails, locate the source of a missing voltage, follow signals between chips and find which components share a shorted net. Any PDF viewer opens it, and searching net names speeds up navigation.
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 @one5technology. 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.