P180 FLT CREW HAS A CABIN PRESSURIZATION PROB; FOLLOWED BY A L MAIN GEAR UNSAFE LIGHT; RETURNED TO DEP ARPT.

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DEPARTED DEN; REPOSITIONING TO PICK UP PAX. WE HAD LEVELED AT FL240; COMPLETED CRUISE CHKS AND WERE GIVEN DIRECT ETL. WE HAD A RED CABIN PRESSURE ANNUNCIATOR COME ON. THIS COMES ON WHEN THE CABIN ALT REACHES 10000 FT. WE CONFIRMED THE BLEEDS WERE ON AND CHKED CABIN ALT. SURE ENOUGH IT WAS AT 10000 FT. WHAT WAS UNUSUAL WAS THE CABIN DIFFERENTIAL NEEDLE WAS AT THE SAME SPOT -- USUALLY THERE'S A SPLIT WHEN PRESSURIZED. EVERYTHING WAS OK ON CLBOUT. THE COPLT HAD SET 9000 FT FOR LNDG AT TEX BUT OBVIOUSLY THE CABIN WENT HIGHER. WE PUT THE MASKS ON AND TOLD ATC WE HAD A PRESSURIZATION PROB. WANTED LOWER AND TO RETURN TO DEN. I LET THE AUTOPLT FLY SO WE COULD TAKE CARE OF CHKLISTS. I INITIATED A STEEP (GREATER THAN 20 DEG) BANK AND CALLED FOR ABOUT A 3000 FPM DSCNT. THE AUTOPLT DIDN'T RESPOND SO I CHANGED SETTINGS. NOW; MIXED TRAINING CAUSED A PROB. I HAD BEEN TAUGHT TO GO TO SPEAKER WITH OXYGEN MASK BUT MY COPLT USED HER HEADSET. NOW I WAS TOTALLY OUT OF THE ATC LOOP AND COULDN'T HEAR HER (I'VE BEEN SINGLE PLT MOST OF MY CAREER). I HAD LOOKED OVER AT HER AND SHE STARTED WRITING NOTES TO COMMUNICATE. NEITHER OF US NOTICED THE AUTOPLT HAD DISCONNECTED TILL WE HEARD THE OVERSPD HORN. I IMMEDIATELY LOOKED AT THE INSTS (WE WERE 45 DEGS BANK; 25 DEGS NOSE DOWN AND 5 KTS OVER BARBER POLE. I CORRECTED AND SLOWED. RETURNED TO A NORMAL DSCNT AND LEVELED AT 16000 FT; DIRECT TO DEN. ONCE AGAIN ON AUTOPLT AND EVERYTHING (EXCEPT ANNUNCIATOR) NORMAL. I PUT ON THE HEADSET AND WE RAN CHKLISTS AND DID SOME BASIC TROUBLESHOOTING. NO CLUE. WE WERE PRESSURIZED BUT STILL HAD RED CABIN PRESSURE LIGHT AT 10000 FT CABIN ALT. BUT NOW WE HAD A RED L MAIN GEAR UNSAFE LIGHT ILLUMINATED. WERE WE IN THE SIMULATOR? I SLOWED THE ACFT AND WE TOLD DEN WE NEEDED TO HOLD TO BE SURE OUR GEAR CAME DOWN. I SLOWED TO 20 KTS BELOW GEAR EXTENSION SPD AND WE LOWERED THE GEAR HANDLE. 3 GREEN; DOWN AND LOCKED. WE RESET THE PRESSURE FOR LNDG AND THE CABIN CAME DOWN; CABIN PRESSURE LIGHT WENT OUT AND THE CABIN DEPRESSURIZED FOR LNDG. WHILE WE DISCUSSED DECLARING AN EMER; WE ELECTED NOT TO ONCE EVERYTHING APPEARED TO CLR UP. DEN TWR DID CALL OUT THE EMER VEHICLES BUT THEY WERE NOT NEEDED. MAINT COULD NOT DUPLICATE PROB ON GND AND CHKED OUTFLOW AND SAFETY VALVES. WE DID A TEST FLT THE NEXT DAY DUPLICATING CONDITIONS (ONLY NOT OVER THE MOUNTAINS) AND WE HAD THE SAME EXACT THING (CABIN PRESSURE LIGHT) HAPPEN.