A B737-300 FLIGHT CREW WAS UNABLE TO SELECT ANOTHER FREQUENCY AFTER TAKEOFF ON EITHER VHF RADIO; SO THEY RETURNED TO LAND AT THEIR DEPARTURE AIRPORT.

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ALL GND OPS WERE NORMAL. WE WERE CLRED FOR TKOF. AFTER TKOF; WAS TOLD TO CONTACT DEP. FO SWITCHED TO DEP AND ATTEMPTED CONTACT. TWR RESPONDED; 'YOU'RE STILL ON TWR.' WE VERIFIED SWITCH POSITIONS AND TRIED AGAIN. SAME RESPONSE FROM TWR. I INSTRUCTED FO TO USE RADIO #2. HE TUNED RADIO #2 AND TRIED AGAIN. TWR ONCE AGAIN RESPONDED. WE TOLD TWR WE MAY HAVE A RADIO PROB AND TO 'STAND BY ONE.' AT THIS TIME WE WERE ON ASSIGNED HDG 285 DEGS FROM TWR AFTER FIRST RADIO APPEARED NOT TO WORK I ENGAGED AUTOPLT #1; APPROX 8000 FT. I LEVELED AT 10000 FT ASSIGNED ALT AND SLOWED ACFT TO 210 KTS IN ORDER TO TRY AND REMAIN IN TWR'S AIRSPACE. I REQUESTED A TURN TO THE N TO BE ON A DOWNWIND FOR RWY X; THE ACTIVE LNDG RWY. AFTER CHKING ALL SWITCHES AND CIRCUIT BREAKERS; WE DETERMINED RADIO WAS LOCKED ON TWR FREQ. I INSTRUCTED FO TO CALL FLT ATTENDANTS AND LET THEM KNOW WE ARE RETURNING TO ZZZ -- NO EMER; JUST A RADIO PROB. I THEN TOLD HIM TO MAKE A PA WITH SAME INFO. DURING THIS TIME; I COORDINATED WITH TWR FOR A VISUAL TO RWY. WE WERE SET UP ON DOWNWIND; SLOWED TO 180 KTS; FLAPS 5 DEGS. FO NOW BACK WITH ME; I HAD HIM ENTER ZZZ AS DEST AND ENTER ILS AND 'CLEAN UP THE BOX.' I BRIEFED THE APCH AND WE RAN APCH DSCNT CHK. WE THEN HAD A QUICK DISCUSSION TO ENSURE WE WERE BOTH ON THE SAME PAGE WITH THE PLAN AND HAD NOT MISSED ANYTHING. GROSS WT WAS 99K. I WAS CLRED VISUAL AND INTERCEPTED FINAL AT 9 MI. WE TOLD TWR WE MAY NEED THEM TO COORD WITH GND AND COMPANY FOR CLRNC TO GATE. TWR THEN OFFERED A DIFFERENT RWY AND I ACCEPTED. WE LANDED UNEVENTFULLY. ONCE ON THE GND; RADIOS AGAIN WORKED NORMAL. MAINT DETERMINED A REMOTE AIR/GND SENSOR HAD FAILED LOCKING THE RADIO ON THE CURRENT FREQ.