Dsrjohn, AC voltage from one of the transformers taps produces positive voltage (D309,C309) for the status LED and the negative voltage (D308, C306, C307) to open transistor Q305. When all for transistors Q302, Q305, Q303, Q304 are open then voltage on C308 builds up (to +21V) turning on transistor Q306 that activates output relay.
It appears that Q305 detects presence of AC voltage, Q303 and Q304 detect presence of DC voltage on the output (average of both outputs), Q303 reacting to positive and Q304 to negative output voltage, with time constant 1.76s. Q302 appears to be controlled by Q301 that detects overcurrent in either channel. When voltage on either of 0.22ohm resistors R301, R302 is greater than 1.3V (two junctions) then Q301 activates Q302 turning off Q306 hence output relay. It happens at 1.3V/0.22ohm=5.9A current draw from either output positive rail.
I'm afraid I did not simplify anything for you, but wanted to show 4 possible reasons: no AC voltage, DC on either output or overcurrent on either output. It can be also faulty component - something is holding down voltage on C308 (something that activates one of four mentioned transistors). Since you have schematics you can start measuring voltages (be careful). Measure output DC voltage - it has to be more than +/-0.6V to trigger fault condition. Next voltage on C308 should be around 21V (voltages on schematic are in normal operating condition). If it is, then problem is around Q306 or even relay itself (or shorted diode across it - D307).
If it is low, then something triggers it. Check the voltage on the Base of Q302 and Q305 - should be less than 0.6V (most likely close to zero). If it is then you most likely have bad transistor. Measure what you can and post your findings. Again, be careful.
