I was a specialized Navigational Electronic Technician for the Central Navigational Computer(CNC) on submarines. The CNC was built in the 1950s. We didn't have any solid state components or circuits boards. We had to know how to diagnose down to each component (i.e. resistors, capacitors). The best way to prepare would be knowing how to read schematics and what each type of component does. We had to learn (cross-train) what regular ETs did (i.e. radar). We had to compete with them for advancement on tests.