Judge was sent a public part of the phone have a problem, see circuit circuit local feeding as shown in Fig. Open the Cabinet check found no element has exceptions. Directly with tables measuring the unit voltage of the Zener diode on the circuit board, Z2 voltage is 0. 6V,Z3 voltage is 4. 4V, then form two transmitter voltage measurement 0. 1V, obviously microphone operating voltage is too low, can't produce enough audio signals. From the circuit diagram is not hard to see outside sent 48V operating voltage via four Dl~D4 diode bridge rectifier. Composed of transistors Q2 and Q4 adjusted (composite) tube, the diode after Z3 regulator, output DC 4. A voltage of 4V, work for the entire circuit. Part of this voltage, resistors R16, R57, R60, R59, Zener diodes Z2 regulator and the capacitor C9 filter, D31, D32, adding to the two ends of the transmitter. According to experience, Zener diode Z2 and the capacitor C9 damaged most measured capacitance welding, Zener diode Z2 voltage is 0. 6V unchanged; Z2 Zener diode measurements under found its forward and backward is a low resistance State. C9 voltage rises to 4. 1V, hands-free transmitter voltage is 1. 6V, handle transmitter voltage is 2. 0V. Then call to and receive sounds properly. In order to ensure stability, after using the same model Zener diode replacement, troubleshooting.