Identify whether PCB 1/4" audio jacks are balanced or unbalanced
Looking at the block diagram, the input jacks are unbalanced, with the third terminal being a switch contact to the tip.  The switch contacts connect to RCA jacks on the rear panel, which will be disconnected when a plug is inserted in the front jack.



The tip contact is the "V" at the end of the top line of the jack, and the switch contact is the upward-pointing arrow under the line.