 FACT: The guitar cord Marty plugs into the amplifier in 1985 is a  TRS cable (tip, ring, sleeve), it has 2 bands going around the plug.  The plug Marty plugs into his guitar is a right angled instrument  cable (1 band around the plug). If Marty had been using a TRS cable  he would have had another single banded cable hanging from his  guitar (y