423. A master-oscillator power-amplifier transmitter has been operating normally. The antenna ammeter suddenly reads zero. All filaments are burning and plate and grid meters read normal currents and voltages. What happened?

The antenna ammeter reads zero while plate and grid meters remain normal, indicating a fault in the metering circuit rather than the transmitter itself. Both a shorted antenna ammeter and an open connection in the remote reading meter can cause this zero indication without affecting the transmitter's operation, making 'both a and b' the correct answer.