LGT WAS FOUND OFF COURSE BY ATC.

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PROBLEM: ELECTROMAGNETIC INTERFERENCE WITH ONBOARD NAV (VHF) CAUSED BY PAX USE OF PORTABLE BATTERY PWRED AM-FM RECORDERS. DATES: 4/WED/88--DEPARTING DFW; CLB PHASE OF FLT. 4/SAT/88--INITIAL CRUISE AFTER DEPARTING DFW. ON THE DATES LISTED ABOVE; WE EXPERIENCED AN ANOMALY IN OUR VOR CDI INDICATIONS IN THE MAGNITUDE OF 3-4 DEGS. ON THE LATEST OCCURRENCE; ATC (FORT WORTH CENTER) INFORMED US THAT HE SHOWED US 4 MI RIGHT OF COURSE. AT THE TIME OF THIS CALL; OUR VOR CDI INDICATIONS WERE SPLIT BY 4 DEGS; WITH THE CAPTS CDI NEEDLE 2 DEGS LEFT AND COPLTS SHOWING 2 DEGS RIGHT OF COURSE (DFW VOR 117.0; 078 DEGS OBS). IN BOTH INSTANCES OF SUSPECTED INTERFERENCE; A CHK OF THE CABIN BY THE F/A'S REVEALED THAT THERE WERE SEVERAL PORTABLE AM-FM RECORDERS OR CD PLAYERS IN USE. WHEN THEIR USAGE WAS TERMINATED; BOTH VOR CDI INDICATIONS SHIFTED AND AGREED; WITH NO DEVIATIONS/ANOMALY NOTED BTWN THE 2 SEPARATE NAV INDICATORS NOTED FOR THE REMAINDER OF THE FLTS. RECOMMENDATIONS: IN LIGHT OF THE FACT THAT THE F/A'S INFLT MANUALS DO NOT MENTION THAT THE USE OF THESE DEVICES ARE PROHIBITED BY OUR COMPANY AND FAR 91.19; AND THE GENERAL LACK OF UNDERSTANDING OF THEIR AFFECT UPON ACFT NAV; IT IS RECOMMENDED THAT 1) F/A TRNING EMPHASIZE THIS RESTRICTION; 2) F/A INFLT REF MANUALS INCLUDE THIS VITAL INFO; AND 3) PRE-DEP 'WELCOME ABOARD/EMER EQUIP' PAX ANNOUNCEMENTS INCLUDE A WARNING TO THE PAX THAT USE OF PORTABLE RADIOS OR OTHER ELECTRONIC DEVICES NOT PREAPPROVED FOR INFLT USE IS PROHIBITED. IT SHOULD BE NOTED THAT IN ALL CASES; PAX WERE USING THE CASSETTE RECORDER PLAYBACK PORTION OF THEIR RADIO/RECORDERS. THE SPECIFIC LOCATION WITHIN THE ACFT/MAKE/MODEL AND USAGE WAS OBTAINED AND IS AVAILABLE IF NEEDED.