Be sure to never connect a power source to this Vcc pin of the serial connector (which is unnecessary anyway). I did by accident with the USB 5V supply. This broke my MR33. The LED stayed dark, and in fact I got a short circuit between pin 1 (Vcc, normally 3.3V) and pin 4 (ground) of the serial connector on the board.
I investigated the issue and found that the short circuit was made by U26. U26 is tagged AEUH which means that this is a MP2233DJ DC-DC converter made by Monolithic Power Systems. With some SMD reworking skills and some luck to find a supplier for a single piece, you can replace the circuit. I got the MR33 back up and running this way.