I have a Model B Raspi and - shame on me - overpowered it with about 9V. And to make it worse I powered it using the test points TP1 and TP2, avoiding the polyfuse. See Raspberry Pi Model B Schematics.
It booted up fine. I think my power supply wasn't that powerful enough to blow the SoC while it was doing work and the rest of the power was going through the suppressor diode D17 but this all is speculation.
When I was done for the day I was too lazy to pull the power cord and only shut down the Pi via command line.
The next morning the diode D17 was so hot that the case above deformed. After letting it cool off the Pi didn't boot any more. The red PWR LED is on while the green ACT LED is glowing dimly.
I removed the diode to be sure that it didn't short the power flow. It was broken anyway. A replacement is ordered but if the Pi is broken there is no need to solder a new diode on the board.
I already tested the polyfuse F3 as described here: Troubleshooting power problems.The voltage drop is less than 0.1V. But as the power isn't supplied through the MicroUSB that has nothing to say.
I tested my SD card with a friends Pi to exclude it from the list of possible errors. I also put a fresh image on it (Arch but that doesn't matter).
Now the question: Is there a way to
a) find out if the SoC is definitively broken?
b) load the GPU firmware code onto the GPU again (as it seems to me that the first stage bootloader is not executed as described here: How does Raspberry Pi boot?)
c) revive the Pi in any other way? Perhaps using a JTAG device?
EDIT: I checked all power regulators: RG1 is providing 1.80V, RG2 is providing 3.29V, RG3 is providing 2.50V. So it seems that it is not a power issue.