While Behringer doesn’t host the QX1202USB service manual on their main site (they prefer you go through authorized service centers), the document is widely available as a free PDF from:
The first thing you’ll notice is the . This single page reveals exactly how the audio signal travels: behringer xenyx qx1202usb service manual
Before you crack open the chassis, try these common fixes sourced from the user community: Behringer Xenyx QX1202USB Service Manual - Manuals.plus While Behringer doesn’t host the QX1202USB service manual