Share an easy way to repair SD connect c4 multiplexer if your multiplexer is dead, not powering up via any adapter cable.
TRY AT YOUR OEWN RISK!
First open the MB SD C4 main unit. Slide PCB out a little bit, from the connector side, so you can see the units buttons wiring. Carefully pry the lock out & up, after that you can pull the wiring and PCB out.
Then Open 4 Main PCB screws, Carefully wiggle the PCB off from another, it might be pretty tight, so carefully, side from side!
Disconnect the LAN cable, carefully, pretty easy to break the small clip.
Then you can disconnect the Connector PCB, very carefully, might be really thight! Again slowly from both sides. You now have access to the small Fuses.
You can use multi-meter to see if they work, usually the LAN connector side 5A is burnt. So desolder the broken one, install new & enjoy your powering up mux again!
This repairing above requires experienced and professional techniques; or you are suggested to buy a cheap clone MB SD Connect C4 of high quality to replace it. Google to get the good one. I helped to put the first site here for you (no affiliation to the vendor)
（special thanks to JarsaR)