1) Remove the communication cable from the CDU when it is normally powered up and there is no CDU communication alarm.
2) Turn off the power amplifier of the TRX corresponding to the faulty CDU to stop power transmission.
Remove the jumper from the CDU antenna port.
3) Use a multimeter to measure the DC voltage between the inner and outer conductors of the transmit antenna connector of the CDU. Check whether the measured voltage is in the normal range of 11 to 13 V (inner conductor connected with the positive pole).