SMA PLT HAD A TRACK DEV BECAUSE OF ERRONEOUS COMPASS READINGS.

Narrative

OP WAS A FLT WITH CLIENT AND SPOUSE AS PAX. DEPARTED I69; OH; ENRTE TO FINAL DEST OF SHD; VA; ON AN IFR FLT PLAN. (WX WAS CAVU THROUGHOUT THE PLANNED RTE OF FLT; BUT REQUIRE IFR OP.) RECEIVED MY IFR CLRNC IN THE AIR IMMEDIATELY AFTER DEP AND ATC ISSUED A VECTOR OF 150 DEGS TO INTERCEPT THE AIRWAY (V128 BTWN CVG AND YRK VORS). A FEW MINS ALTER; THE CTLR ASKED ME TO CHK HDG; AND RPTED THAT I HAD INITIALLY TURNED ON TO THE AIRWAY BUT WAS NOW DIVERGING TO THE R BY ABOUT 20 DEGS. I VERIFIED THAT THE DIRECTIONAL GYRO HDG MATCHED THE COMPASS HDG. HOWEVER; BOTH MY LORAN AND HANDHELD GPS CONCURRED WITH THE CTLR'S OBSERVATION. I CORRECTED MY HDG AS PER THE LORAN AND GPS AND REINTERCEPTED THE AIRWAY. A REBUILT DIRECTIONAL GYRO HAD BEEN INSTALLED IN THE ACFT JUST A FEW DAYS BEFORE. I THEREFORE FOCUSED MY TROUBLESHOOTING ASSESSMENT ON THAT INST. HOWEVER; WITHIN A FEW MINS I WAS BACK ON COURSE; AND ALL INSTS WERE IN AGREEMENT BEFORE I COULD ISOLATE THE PROB. ABOUT 10 MINS LATER I CHKED THE DIRECTIONAL GYRO AGAINST THE COMPASS AND FOUND THAT THEY WERE ABOUT 20 DEGS OUT OF AGREEMENT. AGAIN; I FOCUSED MY ATTN ON THE POSSIBILITY OF TROUBLE WITH THE DIRECTIONAL GYRO. BUT WITHIN A COUPLE OF MINS; I REALIZED THAT SINCE THE LORAN AND GPS INDICATED THAT I WAS ON COURSE; AND THAT THE COMPASS INDICATED A DIFFERENT COURSE; THE PROB MUST BE WITH THE COMPASS. WITH THIS IN MIND; I SOON REALIZED THAT I HAD PLACED MY HANDHELD GPS ON THE DASH AT CLOSER TO THE COMPASS THAN I EVER HAD IN THE PAST. I MOVED THE GPS OFF THE DASH; AND THE COMPASS IMMEDIATELY MOVED APPROX 25 DEGS; AND TO THE HDG IN AGREEMENT WITH THE LORAN AND GPS. I MOVED THE GPS BACK; AND THE FALSE READING RETURNED. SO THE GPS HAD BEEN INTERFERING WITH MY COMPASS. I FELT FOOLISH. I SHOULD HAVE FIGURED THIS OUT ON MY OWN! IT WAS ESPECIALLY INSIDIOUS BECAUSE THE INTERFERENCE WAS INTERMITTENT; AND THE RECENT REPLACEMENT AND THE DIRECTIONAL GYRO HAD CAUSED ME TO FOCUS MY ATTN ON THE DIRECTIONAL GYRO; THE LAST THING THAT CHANGED; INSTEAD OF OPENING MY MIND TO OTHER POTENTIAL ROOT CAUSES OF THE PROB.