I recently upgraded to arducopter 3.0.x on my APM2.0. I have run through all the calibration, including radio, compass and accelerometer. I can arm my quad fine when connected via USB, disconnect USB and fly with no issues. However if I cold start and attempt to arm without the USB, the red LED continues to flash. The blue LED is solid, indicating GPS lock.
I have bypassed the pre-arm safety check in EEPROM, and I can arm without issues. I cannot use APM planner to look at the pre-arm safety check reason, as doing so prevents the problem manifesting.
To me, it smells like a voltage problem. The 5v from the USB is enough to get through the pre-arm checks. Unfortunately I don’t have a multimeter with me to check right now.
TCIII: Powered from ESCs on the output pins. I had J1 installed.
I’ve think I’ve identified the problem after a bit of poking around with a multimeter. A while ago I blew the schottky diode attached to J1, I replaced it with a “Schottky Diode 30mA 30V SSMini2-F5-B” from RS, and a bit of modwire (+1 to open source schematics).
Unfortunately for some reason this diode is showing a 1.5v forward voltage drop, instead of the 0.4ish it should have. This brought the input 5v rail down to 3.5v, causing it to fail the voltage part of the pre-arm check. Plugging in USB boosted the voltage up enough to pass the check. I’m surprised anything worked at that voltage.
I suspect I’m seeing the higher drop because the replacement diode is in a different package, it’s probably got a lower current rating, and it’s dropping more volts than it should. For now I have removed J1 and put a jumper lead between the input 5v rail at the output 5v rail. I’ll just have to remember to remove it when powering from ESC and USB.