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I'm trying to troubleshoot this for a friend and am not getting anywhere.  When I plug in, the FC lights up and I get the Windows chime and everything.  When I try to power it with 8V from my power supply, the green LED will blink once for a millisecond and that's all.  No pop, no smoke or anything like that.  Looking the board over, I noticed it looked like a resistor was missing (white arrow), so I e-mailed Fettec asking what the value of the resistor is so I could replace it.  Their customer service person got back to me super quickly and said that's how it came out of manufacturing and it's nothing to worry about. 

So now I'm stumped.  I believe I know where the 5V regulators are on the board (circled red) but not totally sure how to test them.  I saw Drone Mesh's video on testing schottky diodes but when I look up the "XJT" on that chip it comes up as a mosfet so now I'm not sure if that's the right chip. 

Any advice would be appreciated.  Thank you.

99% a 5v regulator issue if it works well connected to a computer via isb.

If you power the FC via Lipo voltage then all 5v pads should be reading 5v. Easy to check using a multimeter.

If no 5v then go for the regulator. Also inspect the board for any burnt traces or pads.

Agree it is likely the 5V regulator issue.

I blew the 5V reg of Fettec on my Impulserc Apex last year and it behaved similarly.
It's working fine again with an aftermarket 5V regulator.