Anybody ever repaired a behringer eurorack ub2442fx-pro?

So for my day job, I repair automotive computers. Many different types. Many of which I've never seen before and that nobody else in the company really knows anything about. Pretty much the boss hands me a computer and says fix it. Then I spend several days starting from the pins and tracing back to the processor(s) to figure out where everything goes and what chips control what functions.

My experience with dj gear has been doing a few repairs and mods to my technics and replacing the usb port on my hercules controller after it started loosing connection during gigs when my subs were bumping and girls were twerking into the table.

I've never worked on an analogue mixer until recently when a friend's behringer quite working. I offered to take a look at it for the opportunity to learn. I've got it completely disassembled and the boards layed out on my coffee table. There are three main boards with a smaller fourth board for the power supply. One board fits on the back and has all the outputs. One sits on top and has all the inputs. Both are connected by ribbon cable to a "main" board which has all the knobs, faders, and buttons.

He told me that he lost his main xlr output channels and that's all he really knows about it. I traced back from the xlr's on the output board to the line trim knobs for each channel and everything in between seemed ok. So I don't believe the problem is the outputs. Nothing looks burnt and I don't see any capacitor leakage. What are the common problem areas on these mixers and what should I be looking for?