Rigol DP712 Output Shorted; Fuse? / I Screwed Up

[macboy]

The Thyristor (SCR) is part of the over-voltage protection circuitry. It is designed to protect the device connected to the supply from excessive voltage. Various things could cause over-voltage including failure of the power supply's regulation, but the protection works the same in all cases: when excess voltage (over the user-programmed limit) is detected, the SCR is triggered, which essentially shorts the output. Usually, when the power supply itself is the only source of energy at the output terminals, this is OK. When a high energy battery is connected, this is not OK!

[Harry_22]

You are absolutely right, the SCR should short the output if the voltage exceeds a set threshold for any reason. This is confirmed by the cooling radiator designed for long-term operation. To discharge the energy stored in 470 uF capacitor a radiator is not needed.
Topic starter sent us the PS settings that he installed. We see that the OVP was 13 volts and the battery was 11.5 volts. In this case, the OVP should not have worked. Only the current through the shunt could exceed the OCP set value. But it flowed in the opposite direction and I'm not sure that the reverse current is measured here.

[zanfar]

Success!

I finally had time to get the gear back out and attempt the repair. The parts arrived last Thursday and unfortunately, I had a full weekend.

Most of this is a record in case someone else comes across this post and needs hints or details.

Desoldering was tricky but straightforward. Somewhere recently, I heard someone say that the secret to desoldering with a manual pump is to add solder, and that worked a treat. I melted the existing solder and then probably doubled the total solder. Going back with the iron and sucker, almost the entire joint got removed on the first suck. It's also much easier to simply clip the leads of the bad part. This reduces the heat sink, and makes it possible to pull each lead individually as they are released.

I finished with some braid until the holes appeared clean and round.

At this point, I reassembled the PSU and tested it without the SCR, as suggested. The open-circuit voltage was reported as 12.6V, as expected. I did not test with a load, but I wanted to see A) if something else was broken or B) if the replacement caused further issues.

Replacing the part was much harder. There is an insulating board between the PCB and the part itself, and the insulator holes don't align well with the holes in the PCB. In the end, inserting the part at an angle and then standing it up got the middle lead underneath the insulator and into the PCB hole.

I probably used too much heatsink compound, but I'd rather clean it up than add more later.

The heatsink held the part nicely in place for soldering, and the actual solder job was anticlimactic. It's not my best job, but it's nothing I'm embarrassed about. The photo is before I clipped the leads.

Reassembly and testing appear to report a fixed unit. It operates both in CV and CC mode.

Thanks everyone for your help.