Apple schematic in PDF form for logic board 820-00939, revision A, carrying the J174 project code given in the file name. The drawing set lets a technician follow power rails from the input stage through the regulators, trace signal paths between major chips and find the reference designators needed when chasing a short or a missing voltage. Any PDF reader opens it, and searchable net names speed up tracing. Apple 820-xxxxx numbers are printed on the logic board itself, so confirm the number and revision on the PCB match before trusting the values.
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 @notebookschematic. 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.