Can anyone explain in lame terms how this circuit operates?
Q1114 and Q1124 are a differential pair. When in-regulation, the voltage at the bases should be near-identical. D1114 provides the reference of -9.1V, and R1121, R1122 and R1123 provide a scaled sample of the output for the diff pair.
Q1133 and Q1137 are the darlington pass element. They get base current from R1117, which is robbed by Q1114 to keep it in regulation.
R1129 is a current sense resistor, and Q1129 is the over-current "comparator". When the voltage drop across R1129 exceeds its VBE, it'll also rob base current from the pass element. Looks like the overcurrent protection should hit around 1.2-1.4A.
Note that due to R1137, you won't achieve regulation unless there's a sufficient load on the output to soak up the current going through it. I(R1137) = (input voltage - 12)V / 50Ohm. The schematic is fuzzy, but it looks like the input to the pass element is tagged at 5.9V, which would mean you need a minimum load of ~120mA.
I sketched this in LTSpice, and it regulates just fine once the minimum load is satisfied.
By reading the theory of operation and by measuring voltages I suspect that Q1124 is faulty. I grabbed my cheap Chinese tester and it shows that Q1114 and Q1124 are ok but with a beta of 50 and 62. I ordered the transistors and I'll replace them on the circuit to see what happens. I do not suspect that Q1133 and Q1137 are defective but I have not been able to test them as they are mounted on a platform under the power supply board.
It should be fine to replace Q1114 and Q1124 with just about any jellybean small-signal NPN transistor you have at hand for testing at least.
Also note that you should be able to test Q1133 and Q1137 in circuit, as you have access to all their leads through the interconnects. You can also see whether they're grossly bad by measuring the voltages at "I" and "L" while operating. Point "L" should be one VBE above ground, and point "I" should be two.