What was found
AFTER TAKEOFF, A STRONG ODOR OF BURNING. INSPECTED CABIN, & SMELL FOUND THROUGHOUT ACFT. AFTER CLEARING. PRESSURIZED W/CHERRY PICKER, BURNING ODOR RETURNED STRONGER. DIVERTED, & FIRE DEPART COULD NOT FIND SOURCE, AFTER F/D CLEARED ACFT, TURNED APU ON, & SELECTED APU BLEED, ACFT FILLED WITH A STRONG BURNING ODOR. C/W TC-C68-49-1001 RE100CS BORESCOPE OF APU COMPRESSOR AREA FOR OIL LEAKAGE/COKING & TC-68A-21-181006M ENVIRON CONTROL SYS BLEED DUCT BURN, NO DEFECTS. R & R CABIN HEPA FILTER IAW TC-68A-21-181006M. INSPECTED BLEED DUCT RESTRICTOR, NO DEFECTS. MEASURED DRAG TORQUE ON ACM BEARING. BEARING DRAG MEASURED AT 112 OZ-IN. APU ON MEL IAW MEL CH 49-20-01. NO DEFECTS & NO ABNORMAL ODORS.