SUPER MD80 ON BONHAM 1 STAR DFW NOTED DISCREPANCY OF 8 DEG BTWN CAPT AND FO HSI; INCREASING TO 15 DEG. CTLR QUESTIONED HDG AT ABOUT THE SAME TIME THE ERROR CEASED.

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BONHAM 1 ARR. AFTER PASSING BYP WE TURNED TO INTERCEPT THE DFW 037 DEG RADIAL AND WERE GIVEN A CLRNC TO TURN TO A HDG OF 250 DEGS AT LEMYN. I HAD DIFFICULTY IN CTRING THE HSI EVEN WITH A GENEROUS INTERCEPT ANGLE. I CHKED MY INSTS AGAINST THE FO'S AND NOTICED AN 8 DEG NEEDLE AND HSI SPLIT. AS WE WATCHED THE SPLIT INCREASE TO ABOUT 15 DEGS FAIRLY RAPIDLY; I BALLPARKED THE DIFFERENCE BTWN THE 2 NEEDLES AND CALLED THE #1 FLT ATTENDANT AND TOLD HIM TO CHK THE CABIN FOR SOMEONE USING ELECTRONIC EQUIP. ABOUT THAT TIME CTR ASKED IF WE HAD TURNED TO A 250 DEG HDG A LITTLE EARLY. WE SAID NO; AND I HEDGED BACK A LITTLE BIT MORE TO THE L. THEN THE DISCREPANCIES DISAPPEARED AND THE FLT ATTENDANT CALLED TO SAY THAT HE FOUND SOMEONE USING ELECTRONIC EQUIP AND THAT IT WAS NOW SECURED. RADIAL WAS INTERCEPTED AND REMAINDER OF APCH WAS UNEVENTFUL. DEV OFF OF THE RADIAL WAS NOT SIGNIFICANT BUT WAS NOTICEABLE BY APCH CTL. I DID NOT FIND OUT WHAT TYPE OF ELECTRONIC EQUIP WAS BEING USED. IT WAS BY FAR THE LARGEST AND MOST NOTICEABLE INTERFERENCE I HAVE SEEN TO DATE.