ACFT EQUIP PROB RADIO COM PANEL.

Narrative

SHORTLY AFTER TKOF ON ASSIGNED HDG OF 205 DEGS AND LEVEL AT 5000 FT; COMPLETE VHF COM FAILURE OCCURRED. INDICATIONS ON GND WERE THAT THERE WAS AN OPEN MIKE IN THE COCKPIT. WE SUSPECTED SUCH; BUT WERE UNABLE TO DETERMINE WHICH. IN THE TROUBLE-SHOOTING PROCESS; WE REGAINED COMS FOR 1 ATC XMISSION FOR US TO TURN TO 105 DEGS (I AM NOT POSITIVE THAT IT WAS 105 DEGS). WE AGAIN LOST ALL CONTACT; SO I INTENDED TO SELECT 7600 ON THE XPONDER; BUT INADVERTENTLY SELECTED 7500 INSTEAD. SUBSEQUENT TROUBLE-SHOOTING ALLOWED US TO FIRST REGAIN THE CAPT'S VHF COM AT WHICH POINT ATC ASKED US IF WE WERE SQUAWKING 7500 TO WHICH WE REPLIED IN THE AFFIRMATIVE. WE ASKED FOR A NEW CODE AND FURTHER CLRNC ENRTE. THE FO'S VHF COM WAS SUBSEQUENTLY RECOVERED. WE THINK THAT THE FO'S COM PANEL MALFUNCTIONED THUS BLOCKING ALL INCOMING XMISSIONS TO ALL 3 VHF COM RECEIVERS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: WHILE TROUBLE-SHOOTING; THE FLC RESET THE CIRCUIT BREAKER FOR THE FO COM PANEL; WHICH ALLOWED THE RPTR CAPT TO REGAIN THE #1 VHF COM. SUBSEQUENTLY; THE FO ALSO REGAINED HIS RADIO BY RESETTING CIRCUIT BREAKERS. CAPT SUBMITTED RPT BECAUSE OF THE ERRONEOUS XPONDER CODE HE SET OF 7500; WHICH ALERTS ATC THAT THE ACFT IS BEING HIJACKED INSTEAD OF THE CORRECT CODE OF 7600 ALERTING ATC OF LOST COM.