XPONDER OP PROC. PLT HAD CODE 7700 IN HIS XPONDER FOR THE WHOLE RETURN FLT FROM A MAINT FACILITY.

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I HAD DEPARTED PNE ENRTE TO 19N THEN ONTO 17N FOR A XPONDER CHK. I LANDED AT 17N; HAD THE XPONDER CHK DONE AND EVERYTHING WAS OK. THEN I STARTED UP THE ACFT; MADE SURE THE XPONDER WAS ON; BUT OVERLOOKED THE SQUAWK CODE THAT THE TECHNICIAN USED TO PERFORM THE CHK (7700). I PROCEEDED TO THE RWY; PERFORMED MY TKOF CHKLIST AND FAILED TO SQUAWK 1200. THEN TOOK OFF TO RETURN TO PNE; 'THE SAME RTE;' THEN 7 MI S OF PNE I CONTACTED THE TWR; STATED MY INTENTIONS AND THAT'S WHEN THE TWR TOLD ME TO SQUAWK 1200 AND IDENT. AT THAT MOMENT I REALIZED THAT I HAD BEEN FLYING ALONG SQUAWKING 7700. I THEN SQUAWKED 1200 AND IDENTED. THE CTLR ASKED ME IF THERE WAS A PROB AND I TOLD HIM THAT EVERYTHING WAS OK. I WAS THEN CLRED TO LAND AND DID SO THEN THE CTLR CLRED ME TO THE RAMP. I EXPECTED; BUT WAS NOT ASKED; TO CONTACT THE TWR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS PLT SAID THAT HE NORMALLY CHKS THE XPONDER AS PART OF HIS PREFLT CHK ON THE C-150. HE ASSUMED THAT THE MAINT FACILITY TECHNICIAN WOULD LEAVE THE XPONDER ON THE PROPER SETTING WHEN HIS CHK WAS COMPLETE. THE PLT DID A PREFLT FOR THE RETURN FLT; BUT SIMPLY TURNED ON THE XPONDER WITHOUT CHKING THE SETTING.