From the diagram and your readings it looks like the A06 is not being turned on by the optocupler. You have two in the circuit and both are connected to the remote turn on circuit. I’d troubleshoot the remote turn on circuit. You have multiple diodes and resistors in that circuit that could fail or you may have a defective optocoupler, or an open trace.
If the other channel works normally, it could be one of several things. Shorted or leaky outputs or drivers, defective rectifiers, shorted transformer, defect in the second power supply...