one of my APM 2.5 - it never flew, fails to init accel. - no red+blue blinking.
I found that the 3v3 line is 4.7v. , when powered with 5v over USB.
-Disconnected 3v3 regulator output . the 3v3 line is still 4.7 volt.
- something must be feeding it.
-So I used thermal camera on the APM, when loading (shorting) the 3v3 line to GND. the current is 47mA and HMC 5883L (magnetometer) lights up as a Christmas tree on thermal when I load the 3v3 line (disconnected from 3v3 regulator.)
HMC 5883 is supposed to get 3v3 only , no VCC, - so the only logical part - is that it's fed 5v via DRDY, SDA or SCL pins. - maybe it's internal protection diodes forward that to VDD ? - possible only if the SDA/SCL *requires* 3v3 level.
both SDA1 and SCL1 and DRDY, are 4,7 v too ! but what's feeding 4v7 to them ? - or , are they expected to be 4v7 ? - but the HMC 5883 is broken and somehow supplies that to it's VDD ?
What do you think ?