Only arms on USB

I built and flew (successfully) an APM 2.6 based quad… for about a week.

The other day it would appear to be ready to arm (blue GPS and solid red flashing), but wouldn’t arm. I hadn’t done anything with it from the previous flight.

To make a long story short, I found that it arms when connected to a USB, but not on a lipo only.

I found this thread: … %3A1234237

Running the recommended tests, I can’t get a reading on the top of the diode (“A”), and the bottom (“B”) is ~5.3v. The fuse (“C”) is also ~5.3v. Also the diode isn’t burned, melted etc.

It sounds like the voltage is running through it, or is the lack of a reading at “A” indicative of something?

Thoughts on a solution?