A SAAB 340A ON APCH AT 3000 FT DECLARED AN EMER DUE TO NO RIGHT MAIN GEAR LIGHT AND RED UNSAFE LIGHT ON. CAUSED BY CREW REPLACING ONE BULB IN RIGHT GREEN LIGHT.

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WE DEPARTED THE GATE IN PFN. ON TAXI OUT THE FO NOTICED THAT A BULB ON THE RIGHT GEAR LIGHT WAS OUT (TWO BULBS FOR EACH GEAR GREEN LIGHT). THIS WAS NOT AN IMMEDIATE SAFETY ISSUE AND SINCE WE HAD LEFT THE GATE WE CONTINUED. ONCE AT CRUISE WE TRIED TO CHANGE THE BULB. SPARE BULBS ARE LOCATED IN THE COCKPIT AND TYPICALLY CREWS WOULD REPLACE FAULTY BULBS WITHOUT BEING REQUIRED TO NOTIFY MAINT. WE WERE UNABLE TO REPLACE THE BULB AND SHOW GREEN ON THE RIGHT MAIN. WE CONTINUED WITHOUT ANY FURTHER REGARD OR CONCERN. AS WE APCHED MEM WE SET UP FOR VIS APCH ON APPROX A 15 NM FINAL WE PUT THE GEAR DOWN. WE HAD A GREEN NOSE AND LEFT MAIN; NO RIGHT MAIN LIGHT AT ALL; AND A RED GEAR UNSAFE LIGHT. AT FIRST WE BOTH FOUND IT HARD TO JUSTIFY CHANGING A BULB HAD CAUSED THIS TO OCCUR. WE NOTIFIED OUR DISPATCH; MAINT CTLR; AND ATC. COMPLETED ALL CHKLSTS; BUT WERE UNABLE TO CHANGE THE GEAR STATUS. FLIGHT ATTENDANT CONFIRMED FOR US THAT THE GEAR APPEARED TO BE DOWN (CAN SEE IT FROM REAR OF CABIN). WE HELD FOR 45 MINS TO TROUBLESHOOT THE PROBLEM; COMPLETE CHKLSTS; PREPARE FOR AN EMER LNDG AND BURN FUEL TO A MINIMUM. DECLARED AN EMER AND HAD TRUCKS STANDING BY. WE LANDED UNEVENTFULLY; PINNED THE GEAR AND TAXIED TO THE GATE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED IT WAS HARD TO BELIEVE ATTEMPTING TO REPLACE A BULB COULD CAUSE AN EMER SITUATION. THE RPTR SAID MAINT RPTED THAT WHEN ATTEMPTING TO REPLACE THE DEFECTIVE BULB AN INPUT WAS SENT TO THE SWITCHING LOGIC BOX WHICH SENSED THE RIGHT MAIN GEAR WAS NOT DOWN AND LOCKED.