Well,
the repair is done now, so here is the final story...
It turns out there was a manufacturing defect in my unit. The main pass transistor (a MOSFET) for the "Red" channel was not tightened against the heat sink properly so it got too hot. It then failed with a short across its two main terminals causing a dead short to GND. As I found out, these types of supplies often have a fuse somewhere before the bridge rectifier as a safety precaution against a transistor failure. So typically, when a MOSFET transistor fails, you end up replacing both a transistor and a fuse.
I missed the fuse the first time around confusing it for another one next to it which tested okay. Then for the repair, I tried a fast blow fuse but it didn't work. It kept blowing due to the inrush current which is present when the unit is first switched on.
I then used a slow blow PCB style of fuse. I ended up deciding on a Littelfuse TR5 Series, No. 374. At 2.0 times the rated current value this fuse blows in about 45 seconds. At 3.0 times, it will blow in only 2.5 seconds. Therefore, it seemed like a good choice.
Rigol had used an axial fuse for F2. I looked around online for one but I didn't have much success. I could only find the more common radial style of PCB fuse. When I tried to test fit the new fuse I found out that J1 was placed too close to F2 and the PCB fuse wouldn't fit. (See the attached photo.) To make more room, I moved the bleeder resistor (R16) over a bit. I then had to desolder and remove J1 so that I could file a rounded notch on the edge of the connector. This allowed a standard PCB fuse to fit in the space reasonably well. I soldered the new fuse in, fired her up and lo and behold, it worked. Yes!
The whole thing was a bit frustrating as I "fixed" the unit four times altogether.
Once for the transistor, twice with fast blow fuses, and then finally by changing to the slow blow type of fuse. My perseverance paid off though and I got my power supplying working again.
It was a good learning experience overall, and I feel good about being able to repair a piece of commercial test equipment without a schematic, parts list or service manual!