FC powers RX only if USB is connected?

Question

this is my first UAV build, so please don't bare with me :)

I use basically a S500 frame with 4S setup and a flightstack using a Holybro Kakute F7 1.5 FC, connected with a Holybro Tekko32 4in1 ESC board. Connected with the default 8pin bus cable and like the FrSky R-XSR RX wired up like the FC manual recommend (RX,TX and GND, 5V). Everything is working fine, if I connect LiPo and FC with USB.

Unfortunatly, the RX isn't powered, if I use only the LiPo. It's LEDs are just off and also the FC has only the green LED on, but not the blue one flashing.

I double checked the manuals and the web but have no idea whats the problem. There is a B+ cable within the bus to share full Bat power from ESC to FC but why is the 5V down? And why is there a different beep sequence?

Answer 1

A quick look at the kakute shows that it definitely has a regulator on board, so it should be able to make 5V from the provided vbat. I would suspect a bad cable from ESC to FC, which can be confirmed with a continuity check on each line.

Regulator issue

Compare the 5v pad where the rx is connected (and try some other 5v pads, to rule out one particular bad trace). Use a multimeter and compare against the battery negative terminal.

Vbat issue

Does betaflight show a battery voltage when connected to the PC and also plugged into battery? This confirms your supply to the fc is good.

Ground issue

Find a ground on the flight controller, and temporarily jumper it to the main negative battery terminal. Note, it is not recommended that you fly like this (ground loops), but it rules out a potential grounding issue.

Answer 2

Well thank you all for pointing me to the right direction. While googling, I found a Mateksys(!) blog entry which exactly describe this behavior for FCs which have issues on the (5V) power lane.

So I tested for shorts between pins, on +5V line and GND, but nothing strange. Then I used an external lab PSU to power ESC and FC externally by 14.8V and separately both are fine. But combined the mentioned strange behavior and the voltage on the PSU dropped drastically.

After some time I switched cables but no progress. But then I noticed this issue within the socket:. So the most right pin was shorted against the one next. And this is the VBAT supply. It took me some time to fix the pin properly and it fits right in the socket again. Thank you folks!