What was found
AIR TURN BACK RETURN, VERY STRONG PLASTIC BURNING ODOR THROUGHOUT COCKPIT AND CABIN; ACCOMPLISHED INSPECTION ON FWD AND LATERAL AVIONICS COMPARTMENTS AND APU AREA AND PACK NR 1 WAS FOUND THE ODOR CAME FROM FROM PACK NR 1. TROUBLESHOT IAW TSM 05-50-00-810-831-A, RAN NR 1 PACK WITH APU BLEED AND WITH NR 1, NR 2 ENGINE BLEED. NO ODOR NOTED.