Brushless excitation systems are widely used in marine synchronous generators due to their high reliability and reduced maintenance requirements. In these systems, the rotating rectifier converts the three-phase AC output of the exciter into DC current for the main field winding. However, faults in the rotating rectifier, particularly a single diode open-circuit fault, can degrade excitation performance without immediately triggering protective devices, making early detection difficult. This paper experimentally investigates the effects of a single rotating rectifier diode open-circuit fault on the excitation system and voltage formation of a brushless synchronous generator under no-load operating conditions. The no-load condition minimizes the influence of armature reaction and load current, allowing fault-induced excitation behavior to be clearly isolated. A brushless excitation system was implemented using three synchronous machines of identical rating, and a single diode open-circuit fault was intentionally introduced in the rotating rectifier. Excitation-related electrical quantities related to the excitation system, including DC excitation current and voltage, exciter armature currents, and generator terminal voltage, were measured and compared before and after the fault. Experimental results demonstrate that the single diode open-circuit fault causes reduction in the average excitation current and introduces low-frequency ripple components in the excitation current waveform while the terminal voltage reduction remains limited under no-load conditions. These results indicate that excitation-related electrical signals can serve as effective indicators for the detection of rotating rectifier diode faults in brushless synchronous generators.