While no board schematic there is quite good selection of guides hosted iFixit (Valve links you there from their website so it is the "official" source) https:

While no board schematic there is quite good selection of guides hosted iFixit (Valve links you there from their website so it is the "official" source) https://www.ifixit.com/Device/Steam_Game_Console https://www.ifixit.com/Device/Steam_Game_Console