What was found
(CAN) THE AIRCRAFT WAS EN-ROUTE TO THE DESTINATION WHEN THE AUDIO RADIO CONTROL DISPLAY UNIT (ARCDU) P/N: CDU3933AE05 MALFUNCTIONED WHEN THE TUNING KNOB INTERMITTENTLY SWITCHED CHANNELS. THE AIRCRAFT LANDED AND MAINTENANCE REPLACED THE MALFUNCTIONING UNIT IAW AMM 23-81-01 WITH A SERVICEABLE UNIT. OPERATIONAL CHECKS SHOWED THAT THE UNIT WAS WORKING. ON TAKE-OFF, THE SERVICEABLE ARCDU DISPLAYED A FAIL MESSAGE. THE UNIT WAS TURNED OFF AND ON BUT THE ISSUE WAS NOT RESOLVED. THE AIRCRAFT RETURNED TO THE DEPARTURE AIRPORT WHERE MAINTENANCE REPLACED THE UNIT AGAIN IAW AMM23-81-01. THE AIRCRAFT RETURNED TO SERVICE WITHOUT ANY FURTHER INCIDENT. (TC# 20190515014)