I've got a brand new B404x that doesn't seem to ever actually turn on fully. The yellow charge indicator flashes continuously very fast but no status light ever comes on. The 3.3V pin is 0V as well regardless of if I ground the EN pin or not.
I initially sent this through the email support, but was told to post here instead.
I had the same thing happen to a Boron404x recentl. Was testing some code involving sleep mode and it would not reset, etc. tried CLI and not luck the led was off mostly and would briefly flash white when I tried rest or tried to enter DFU mode. The charging light was flashing very quickly. That was all I got. Thought it was bricked. Hit on the idea of powering it with the battery only using my LiPo battery on the normal charge port. Once I did that and then plugged in the usb cable it stopped as the battery was already fully charged. Used the buttons to get it in DFU mode with CLI then updated and loaded Tinker. All now working. Don’t know if it will work for you, but worked for me. Figured something to do with charging firmaware had gone wrong, but don’t really know. I had entered some dodgy sleep code for testing……
If you have code that immediately is going into sleep on wake you can also hold down the MODE button while plugging in USB power. This will allow you to go into safe mode or DFU mode when you can't press the normal button sequence fast enough at boot.
As for the original problem, if there is no 3.3V output on 3V3, it's probably a hardware problem, most likely with the 3.3V regulator.
If you power by USB or battery, the pull-up on EN should be 3.6V to 5V depending on your power source. If that's high and 3V3 is off, then it's almost certainly a regulator issue.
Sounds like this one has a regulator issue then. Is there a warranty that we can work through? I'm not sure what its device id is since it has never been powered up and communicating over USB.
VUSB: 4.96V
EN: 4.26V
3V3: 0.0V