A B737-300 ON APCH AT 7000 FT HAD ALL LNDG GEAR POS INDICATION LIGHTS FAIL. MADE TWR FLY-BY AND LANDED UNEVENTFULLY. CAUSE UNKNOWN.

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ON APCH TO RWY 17R AT DEN AFTER LNDG GEAR EXTENSION WE NOTICED ALL (GREEN AND RED) LNDG GEAR INDICATOR LIGHTS INOP (INCLUDING NO 'PRESS-TO-TEST' AND TEST POS OF LIGHT SWITCH); THUS WE HAD NO INDICATION THAT THE LNDG GEAR WAS DOWN AND LOCKED. WE DID GAR AND ADVISED ATC OF SIT AND REQUESTED VECTORS TO PROVIDE TIME TO ASSESS OUR PROB. I MADE CABIN ANNOUNCEMENT OF PROB AND INITIAL PLAN. WE CHKED CIRCUIT BREAKERS AND FOUND NONE POPPED; BUT DID NOTICE THAT IN TEST POS OF LIGHT SWITCH OTHER LIGHTS WERE INOP: 2 WINDOW HEAT; L SIDE OF PITOT HEAT/STATIC; AND 3 'B' SYS HYD LOW PRESSURE/OVERHEAT (GUESSED WE HAD AN ELECTRIC BUSS FAILURE). WE DID INITIAL PART OF IRREGULAR CHKLIST 'LNDG GEAR UNSAFE INDICATION DURING EXTENSION' XYZ WHICH DIRECTED CHKING VIEW-PORT WINDOWS TO CONFIRM GEAR -- NOSE WAS DOWN AND LOCKED; R SIDE MAIN WAS ESPECIALLY TOO DIRTY TO CONFIRM ANYTHING; AND I WAS NOT CERTAIN OF L MAIN GEAR (ALTHOUGH WHAT I COULD SEE LOOKED MECHANICALLY OKAY). PROC XYZ NEXT DIRECTED A TWR FLY-BY. I CALLED FLT ATTENDANT PURSER TO COCKPIT AND ADVISED OF SIT AND MADE ANNOUNCEMENT TO PAX. FO ATTEMPTED RADIO CALL TO COMPANY DISPATCH; BUT WAS UNABLE. I THEN DIRECTED FO TO CALL MAINT TO GET FURTHER INFO ON ELECTRIC BUSS/CIRCUIT BREAKERS RELATED TO LNDG GEAR LIGHTS. AFTER DISCUSSION WE WERE GIVEN RADIO PATCH WITH MAINT WITH WHOM I FURTHER DISCUSSED SIT AND WE AGREED THAT WITH TWR OBSERVATION OF LNDG GEAR WAS DOWN; WE WOULD PLAN NORMAL LNDG. WE DID FLY BY AND THEY CONFIRMED OUR GEAR WAS DOWN. I ADVISED FLT ATTENDANTS (CABIN ADVISORY -- PREPARATION FOR EVAC NOT NECESSARY; BUT BE ON ALERT) AND PAX OF PLANNED NORMAL LNDG. ATC INFORMED US THAT COMPANY DISPATCHER HAD REQUESTED EMER VEHICLES FOR OUR LNDG. WE COMPLETED PROC XYZ AND THE OTHER REGULAR CHKLISTS AND LANDED NORMALLY ON RWY 35R AT DEN. AFTER SMOOTH TOUCHDOWN AND ROLLOUT I STOPPED AIRPLANE NEAR EMER VEHICLE; THEY NOTICED NO OBVIOUS PROBS SO WE TAXIED TO OUR GATE FOR DEPLANING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE EXACT COMPONENT FAILURE IS UNKNOWN AS MAINT MADE NO RPT TO THE CREW. THE RPTR SAID NO PREVIOUS HISTORY OF LNDG GEAR INDICATION PROBS WERE NOTED IN A LOGBOOK REVIEW.