Everything seems to work except serial communication.
- Arduino MEGA 2560 R2
- Serial communication is broken (tried different USB cables and computers, and things were working fine previously)
- RX and TX lights stay off no matter what I do. Loopback test predictably doesn't work either.
- Resetting makes the L led blink twice, and it loads the last program just fine
I had an external 5v regulator feeding 5v to the Vin pin (below recommended voltage but seemed to work fine). This 5v regulator was also powering a few servos, which meant that current could flow from USB -> Vin -> servos, if the external 5v regulator was not powered (my mistake for not putting in protection diodes). The current coming out was probably > 0.7 amp peak.
This page: http://www.ruggedcircuits.com/10-ways-to-destroy-an-arduino/ , method #8, says that that drawing too much from Vin when powered via 5v can destroy the onboard regulator by making current flow backwards. Pin #1 on the 1117 regulator is at 0 volts, which I think is wrong (compared it to a redboard).
One chip is getting REALLY hot, the one with 8 pins which I think is an op amp, in the picture below. I had also been sloppy with thermal paste (10% silver) so there are some flakes sitting around, but those didn't cause any problems for a long time and this only happened when I was fooling around with the power supply.
I just want to confirm my mistake before I possibly destroy another board. Thanks in advance.