Use your Service Tag on the Dell Support portal to find the official manual for models like the OptiPlex 790 Go to product viewer dialog for this item. or Go to product viewer dialog for this item. .
In addition to a PDF schematic, look for a "Boardview" file (.asc or .brd). This software provides a visual, interactive map of the board, allowing you to click a component and see everywhere its traces lead. E93839 Motherboard Schematic