So, I tried connecting a 5v supply to IN5V and it just doesn't work.
I bridged the jumper from VIN, also doesn't work...
Followed the pinout instructions, tested on two boards, no success.
Voltage is very stable right at 5.02v.
Powering from USB-C works fine!
Am I missing something???
I would appreciate any help on this basic issue!
did you solder headers?
the doc seems to indicate you need to solder the pads only if you power from Vin
Hello J-M-L.
Thanks for the reply!
On the first arduino no, on the second yes.
So damage from overheating soldering is excluded.
Same with the VIN-Jumper, on one yes, on the other not.
Electrically, when the VIN-Jumper is bridged, it connects „VIN“ and „IN5V“ together.
OK.
I don't have that board so don't can't test but a typical issue we see on the forum is just a crude connection made assuming all will work right. Just wanted to check but you seem to know what you are doing 
and probably to the USB C port, so I was wondering if a conflict could arise from that if you also connect to your computer for code upload or whatever Serial debug. But again, if you have a board without the bridge and it does not work then it's probably not a reason.
enough current from your 5V power supply?
Good call though, sometimes it is exactly these things that one forgets!
I never connected both at the same time.
PSU is stable and rated up to 20 A so it should be enough, but I will check with a different one to make sure it is not a PSU issue, you never know.
hope so !!! 
Mine does have a current limiting settings, may be it's turned way down from previous use?
I suppose that if you power it from the 3V3 pin it works, as it works from the USB.
It's strange. Looking at the datasheet:
from the symptoms the only thing I could imagine is that the Q3 mosfet is damaged. But it's strange that it's damaged in both boards.
Q3 here works like a diode, more or less.
If you can identify in the board the point between Q3 and U3, for example the C3 capacitor top lead, you can check the voltage there with a multimeter, when only IN5V is powered. It should be 5V, and zero and at VBUS.
Check also the voltage at 3V3.
It's a hard voltage switcher with no limiter. 
Thank you for the response! I will try it and see what readings I get.
To be clear, should I measure here? Top Lead of C3 and U3 VIN?
I didn't fully understand what you meant by "and zero and VBUS", could you perhaps briefly point out where I should measure on the schematic?
Yes. If the mosfet is working you should see 5V there.
Find the top lead of C5 (the red arrow in Jim's post #9), or Vin of U3, or the "2" pin of the mosfet... whatever is easier to identify and test.
And if you see 5V there, you should see 3.3V at the output of U3, that is also the 3V3 pin, I think.
If you don't see 5V there, then you have to check the mosfet. Check the 3 leads. Without the USB cable the lead 1, the gate, should be zero. But anyway it should conduct between 3 and 2 (source and drain), so leads 2 and 3 should be 5V.
Sorry, was a typo, should be: "and zero at VBUS" I mean, when you don't have the USB cable connected VBUS should be zero.
