I finally figured out the issue with the tx/rx and was almost ready to fly... and then the battery started dying, so I pulled it out, replaced it with a newly-charged 4s, and ZAP!

The soldering of the battery cable to the + pad loosened and I saw a nice little spark. After which, from what I can gather from a bit of multimetering:

- The 12v voltage regulator no longer works properly, as I get 14+ v from the 4s on the "REG" pads.
- the FC is no longer getting power. With battery, there is no blue light, and while it's getting a measured 5v from the PDB to an the ESC rail, all the other rail pins on the board are showing less than 1v. What gives me some minor bit of hope is that it does work when powered via the USB port (blue lights on, accessible via Cleanflight).
- The receiver no longer works as it's not getting 5v from the FC
- The Aomway 5ghz tx is working very sporadically. Usually with battery power it's not showing anything on the channel display. Once in a while (I think when it cools down) it will work again. It's not voltage dependent like the camera, and it's getting a measured 12.5v off the 3s battery for power, so I'm guessing something went boom in the transmitter itself.

I'm worried that I'm looking now at replacing the FC, the 12v regulator, AND the video transmitter. And that's if I'm lucky. Any ideas? Stuff I could try? I'd be much obliged. So close to flying, and now it feels like I'm back at square one...

Doesn't sound promising. Sounds like you shortednthe lipo to ground, which probably shot 16v and 100amps through the wrong pipes. You'll just have to get another pdb and see what happens. If you have a 5v source you could troubleshoot the individual components. Some vtxs come with a balance lead pigtail. If you have one you could hook the vtx to camera and test it not connected to the quad. Good luck though!

Yeah, that's what I was afraid of. I'm sure there's a lesson here somewhere! The PDB seems to be routing power okay, so I'm hoping it's just the FC, voltage regulator, and vtx that need replacing...