What PMU should be referenced for transmission systems?

The PMU-73 is the correct reference for transmission systems because it specifically contains detailed guidelines, maintenance requirements, and troubleshooting protocols related to transmission systems. This includes specifications on system operation, component checks, and standards for ensuring the effective functionality of transmission equipment. By utilizing PMU-73, technicians and personnel can ensure they are following the most current and relevant procedures necessary for maintaining and servicing transmission systems effectively.

In contrast, the other PMUs mentioned may refer to different components or systems that do not pertain to transmission systems specifically, which would limit their relevance and usefulness in this context.