MD88 FLC HAS COMPASS DEFLECTION AFTER TKOF LEADING TO A HDG ERROR. SUSPECT PAX ELECTRONIC DEVICE PROB WITH PAX WHO TURNED ON HIS COMPUTER.

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DEPARTING ORD ON A HDG OF 180 DEGS AT 5000 FT AND 250 KTS; AUTOPLT ON; BOTH COMPASS SYS STARTED A R-HAND TURN INDICATING 180 DEG HDG TO BE FURTHER TO THE R FROM OUR EXISTING HDG AND DRIFTING R. AUTOPLT CONTINUED TO FOLLOW DRIFTING HDG TO THE R. A QUICK LOOK AT OUR VOR INDICATED PROPER COURSE TO ACTUALLY BE TO THE L. (SINCE BOTH HDG SYS GAVE SAME INDICATIONS AND COME FROM INDEPENDENT SOURCES; IT WAS VERY DIFFICULT TO ASSESS WHAT SYS WAS MALFUNCTIONING.) WE HAD NO ABNORMAL WARNING INDICATIONS TO VERIFY IF/WHICH SYS WAS GIVING BAD INFO. R TURN CONTINUED AT A VERY RAPID RATE TO ABOUT 60 DEGS TO THE R OF INTENDED HDG BEFORE WE INITIATED A TURN BACK TO THE L. ATC VERIFIED THAT WE WERE SIGNIFICANTLY OFF HDG. WE RE- ESTABLISHED ON COURSE USING RAW VOR DATA BUT HDG WAS STILL OFF BY 30-40 DEGS. HDG SLOWLY RECOVERED TO NORMAL OVER ABOUT 20 MINS. NO CAUSE FOR THE MALFUNCTION WAS FOUND IN THE COCKPIT. WE SPECULATED THAT WE MAY HAVE HAD SOME SORT OF EXTERNAL INTERFERENCE SINCE BOTH SYS WERE AFFECTED. AFTER QUESTIONING A PAX ADMITTED THAT HE TURNED ON HIS COMPUTER JUST AFTER TKOF AND PRIOR TO THE SEAT BELT SIGN BEING TURNED OFF. WE SUSPECT THAT THIS MAY HAVE BEEN THE CAUSE OF THE MALFUNCTION. THE SLOW RECOVERY OF THE HDG SYS IS A SYS LIMITATION.