There are notes on the digital schematic of which parts are not present on the 128K board. The two boards are alike except the RAM array on the 512 obviously has 256K chips instead of 64K, and there is an extra address demultiplexer. If you have a Mac later than about the 30th week of '84, it is the same PC board as a 512. The schematic should help you with either. My experience is that digital boards rarely fail, and a classic Mac that doesn't boot nearly always has analog problems. In particular, 5V being low is common, since it isn't regulated. Cracked solder joints on parts that run hot are VERY common. There are several revisions of the analog board, so yours may be slightly different than mine.