I tried everything. But I found it. Pin 28 had bad contact in conector. I slightly bended pin 28 and it works!

Hi, I have problem with powering core from base15. 5V is present, but core doesn't work. I figured out that when 5V is present on pin 28, no 3,3V is on pin 12. When is core powered form USB. Core works (5V and 3,3V are pressent). But i can't find why 5V from base15 doesn't step down to 3,3V in core.