INSTRUCTOR PLT; WITH STUDENT PLT; DECLARED AN EMER TO THE TWR CTLR WHEN SMOKE CAME FROM THE INST PANEL DURING THE DOWNWIND LEG FOR LNDG. POSTFLT INSPECTION BY MAINT FOUND A FAULTY XPONDER.

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ON A XCOUNTRY TRAINING FLT; WE PROCEEDED VFR FROM PRC TO SDL. SDL UNDERLIES THE PHX CLASS B AIRSPACE AND IS WITHIN 30 NM. WE DSNDED BELOW THE CLASS B AND CONTACTED SDL TWR. PROCEEDED TO SDL AND MADE A TOUCH-AND-GO AND DEPARTED TO THE NW. IN CRUISE FLT; I CONTACTED ZAB FOR VFR FLT FOLLOWING. THE CTR CTLR ADVISED THAT HE WAS GETTING NO XPONDER AT ALL; AND WE CYCLED THE XPONDER SEVERAL TIMES WITH NO RESULTS. I BELIEVE WE WERE WITHIN THE 30 NM MODE C VEIL AND AT AN UNDERLYING ARPT WITH AN INOP XPONDER. THERE WERE NO RPTS OF ANY XPONDER MALFUNCTIONS IN THIS ACFT IN THE NEAR PAST. UPON ENTERING R DOWNWIND FOR RWY 3R AT PRC; I NOTICED A SMALL AMOUNT OF SMOKE COMING FROM UNDER THE PANEL. I ALERTED MY STUDENT; AND THE SMOKE BECAME THICKER. I THEN TOLD TWR THAT WE HAD SMOKE IN THE COCKPIT AND WERE GOING LOST COM. I SHUT DOWN THE ELECTRICAL SYS; THE SMOKE STOPPED. SO; I TURNED ON THE MASTER SWITCH AND 1 RADIO. I RPTED BACK WITH TWR; SAYING THE SMOKE HAD STOPPED. THEY INFORMED US THAT WE WERE #1 CLRED TO LAND AND THAT THE EMER EQUIP WAS RESPONDING. WE LANDED WITHOUT INCIDENT AND WERE ESCORTED TO THE RAMP BY ARFF PERSONNEL. MAINT FOUND THE XPONDER WAS RESPONSIBLE FOR THE SMOKE AND IT WAS REPLACED.