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I can understand how SCRs regulate voltage there. Could it be that changing position to the cursor of R8 potentiometer fixes things up ? U2D puzzles me more than the rest: emitter voltage of the buffer is 5.1V, but pin 14 is 1.6V - compared to 5V of pin 13 - because R10 and R13 make a voltage divider and output pin 12 is fully negative. The +/- 15V voltage stabilizers are working correctly. Outputs from these op amps is fully negative, making pin 5 of U2B fully negative as well (C2 electrolytic is working with the wrong polarity !) so that output pin 4 is as described. U2C and U2D seem to be part of the circuit that senses B+ and B- because the first is referenced to ground, and the second to a portion of a zener voltage, through a buffer transistor. This op amp has now a fully negative output, no pulses. U2B in particular, gets pulses from pin 6 and should have a DC voltage on pin 5, is supposed to deliver pulses to the four SCR's. I've made some checks last night to the control circuit on the upper left of the power supply schematic.