title_body: 4 in 1 ESC and FC won't turn on when connected together After crashing my drone recently, my ESC and flight controller won't turn on (the power led lights up on the FC, but nothing else works either it doesn't anymore, or I was wrong before) any more when they are connected together. When I disconnect the ribbon cable between the FC and the ESC, the ESC turns on as it should (Beeps when the battery is connected), and the FC turns on when I connect it to USB, and it seems to function correctly.
  
  I have measured the connections going between the two, and all of them appears to be okay, except that 5 V from the ESC drops to about 0.5 V when the flight controller is connected. If I connect both the ribbon cable and a USB cable, the FC turns on and the video feed goes from showing black (no OSD) to show the actual video. The ESC still won't turn on though.
  
  I noticed that the capacitor had come loose from one terminal, but it seems unlikely that that caused these problems. Unless it caused voltage spikes that damaged something?
  
  My FC is a HobbyWing FC F4 G2 and the ESC is a HobbyWing XRotor Micro 60A 4in1.
  
  Any help is appreciated, I can't even figure out which board causes the problems.
upvoted_answer: Your description of the dramatic voltage drop (from 5V to 0.5V) upon the connection between the flight controller (FC) and 4-in-1 ESC makes me think that the issue you're seeing is most likely due to a short or low resistance path to ground. There are other possible failure modes that could cause this, but a ground fault sounds most probable here.
  Why?
  When a ground fault is created in one of the components attached to the 5V power rail generated by the ESC's BEC (Battery Eliminator Circuit), a substantial current greater than the BEC's rating for continuous power delivery flows. This is due to Ohm's law, $I = \frac{V}{R}$ which states that a large current (I) will flow if the circuit resistance (R) is tiny.
  The usual mode of operation for power supplies when they experience surges in power demand is to drop their output voltage in an attempt to prevent catastrophic failures of themselves or downstream components by decreasing the power output. (Power $P = \frac{V}{I}$) This is a near-ubiquitous safety feature.
  Confirming a Diagnosis of a Ground Fault
  Since ground faults manifest themselves as low-resistance paths between the power rails, the way to test for the existence of one is to (use a multimeter) measure the resistance between the +5V and GND lines on the FC and the 4-in-1 ESC independently.
  It would be best if you could fully disconnect all peripherals from the FC before measuring for a ground fault, as it is possible that the issue is with one of them and not the FC, but this can also be done once a problem with the ESC is eliminated to possibly avoid desoldering and resoldering the components back together.
  NOTE: remember to take resistance measurements with no power attached to the device being measured!
  If the measured resistance for the FC isn't high (the exact threshold differs between circuits, but the difference between a ground fault and a safe circuit is significant and should be noticeable), then there is a ground fault somewhere inside the FC. Likewise, if the measured resistance for the ESC isn't high, then there is a ground fault somewhere inside the FC. If both components have low resistance paths between the power rails, then they both have ground faults.
  Resolution
  You could either attempt to correct the ground fault in the affected component(s) or replace them. Sometimes the cause of the ground fault is some SMD (Surface Mount Device) component that is too small for you to comfortably work on or broken solder mask in the PCB, in which case fixing it may be more trouble than it's worth.
  However, the only cost of trying to fix and not buy new is your time. And hey, you never know when a troubleshooting venture like this will teach you something new about electronics!