08-22-2018, 05:03 PM
It sounds like the one board is definitely bad, I believe I've seen the same thing once, it baffled me and I never got a reason why/how it happened. With the board disconnected from power use the continuity meter and I bet the 5v and 3.3v pin on display board are shorted with no Arduino plugged in. When I saw that I slowly removed all the components on the board and the short remained so it was internal, I believe I even removed all the headers. Because of the pullup resistors on the 3.3v line the main 5v is being pulled down that's why the odd readings, it's not the regulator.
When you power the board with no Arduino you should get 5.0v and 0v but with Arduino mounted you should have 5.0v and 3.3v.
When you power the board with no Arduino you should get 5.0v and 0v but with Arduino mounted you should have 5.0v and 3.3v.
