Hi everyone, I've been flying for a little over a year now but this is my first time posting here. The last time I went flying (a few months ago) I was coming in to land but my thumb slipped on the throttle and sent my quad up about 10 feet before coming down flat after disarming. After that, I tried to rearm but could only hear the start up beeps and not the last two. I took it home and did a bit of research which suggested that it could have been a problem with the 5V regulator on the FC so, I foolishly ordered a new one (Mamba F722S) before doing much testing at all. After I installed it with a not so great but functional solder job, I got the same problem so I decided to take a break for a little while and do more testing after I came back. 

Now that I'm back, I can get the last two beeps when I'm in BF and BL Heli as well as make the motors spin which makes me think that the ESC (F50PRO) isn't the issue either. Now I'm not so confident that it was a hardware problem to begin with, but I can't be too sure. Most of the problems that I've encountered in the past were very simple and largely had to do with flashing firmware to my receivers lol so I'm not great at diagnosing these kinds of problems. There's been a lot of similar cases to mine that I read about but not quite the same. I feel like it has to be some silly little thing that I'm over looking so I'm coming here in the hopes that one of you knows what I'm missing. Thanks in advance

Just to get this clear, if you plug in a LiPo on it's own (with no USB cable plugged in) you only get 3 start-up tones from the motors (not the last 2), but if you plug in a USB cable first and then plug in a LiPo you get all 5 start-up tones from the motors. Is my understanding of that correct?

(11-Mar-2021, 12:31 AM)SnowLeopardFPV Wrote: Just to get this clear, if you plug in a LiPo on it's own (with no USB cable plugged in) you only get 3 start-up tones from the motors (not the last 2), but if you plug in a USB cable first and then plug in a LiPo you get all 5 start-up tones from the motors. Is my understanding of that correct?

Yes, in both BF and BL Heli which is also able to read the setup of the ESC and everything seems to be how it should.

Plug in a LiPo without plugging in a USB cable and use a multimeter to measure the voltage on each of the 5V pads on the FC. It you aren't getting a voltage then the 5V regulator and/or the Schottky diode on the FC has burned. I know you say you replaced the FC but it's not impossible for you to have been sent a faulty FC or the 5V regulator on that FC to have burned itself when you first plugged it in.

(11-Mar-2021, 12:42 AM)SnowLeopardFPV Wrote: Plug in a LiPo without plugging in a USB cable and use a multimeter to measure the voltage on each of the 5V pads on the FC. It you aren't getting a voltage then the 5V regulator and/or the Schottky diode on the FC has burned. I know you say you replaced the FC but it's not impossible for you to have been sent a faulty FC or the 5V regulator on that FC to have burned itself when you first plugged it in.

I'm not great with multimeters but when I set it to DCV 20 I get 0.7V on most of the pads but on one I get nothing

With the LiPo plugged in (no USB cable), put the black probe on a GND pad and the red probe on a 5V pad. If you get zero volts or 0.7V on that pad then it means your FC isn't getting power.

One other thing I just thought to check is the ESC-to-FC wiring harness. Remove both plugs out and look inside both sockets with a magnifying glass to see if any of the pins are bent or damaged.

(11-Mar-2021, 01:09 AM)SnowLeopardFPV Wrote: With the LiPo plugged in (no USB cable), put the black probe on a GND pad and the red probe on a 5V pad. If you get zero volts or 0.7V on that pad then it means your FC isn't getting power.

One other thing I just thought to check is the ESC-to-FC wiring harness. Remove both plugs out and look inside both sockets with a magnifying glass to see if any of the pins are bent or damaged.

Yeah, when I was testing the 5V pads I was confused so I also tested some of the 9V ones and I was getting an accurate reading. I just checked the wiring harness and none of the pins seem to be bent.

Unfortunately that means you do therefore have a burned 5V regulator and/or Schottky diode on the FC. The 9V output comes from a different regulator,

Darn. Not what I wanted to hear but thank you a lot for your help