Repair Yihua PS-3010D Power Supply Stuck at 50 Volts

I have a Yihua PS-3010D 30V 10A power supply and one day it would only output 50V. After doing some measuring with my volt meter it looked like the top supply rail was shorted to the output. I could see a bunch of power transistors on a heat sink and assumed they were the output devices. I removed 4 screws at the base, 2 at the top that the control board was mounted to, cut a few zip ties, and used some tin snips to make the hole at the bottom the wires were running through into a slot. Then I was able to carefully work the output stage out enough to work on it.

I could see that the transistors were being used as emitter-followers with the emitter of one of them being used to drive the bases of the other 3 wired in parallel with sharing resistors on the output.

I removed the orange wire you see running around the bases and did a back to back diode test on each of the 3 transistors wired in parallel. I could see one was shorted and the others looked OK. I went to my junk box, found a 2N3055 transistor (with a date code from the 1970s!), put it in, soldered the orange wire back into place, and…the power supply was back on-line. My best guess as to what happened is I connected the power supply to a circuit that had a charge on it and it back feed the power supply. So took a TO-220 diode (FERD40H100STS) and connected it so it will dump back fed power to the big filter caps.

Not sure if this will work or not, and I am not going to try and blow up my power supply to test it. But the diode has not caused any problems so I will leave it in and hope it saves me next time.

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