RAMPS fried Mega, was I doing something wrong?

Hi!

I'm building my first 3d-printer and tried to connect RAMPS + Sainsmart Mega 2560 to ATX power supply. Almost immediately the magic smoke appeared, presumably from the Mega board. I had nothing connected to the RAMPS board (wrong?!?) and the Mega board wasn't connected via USB.
It burned at least the voltage regulator (showing strange voltages now when i connect a 12V adapter straight to Mega board) and probably something else too. For some time it seemed to work via USB but wasn't recognized in Windows any more. Now after testing it with 12V it seems to be completely dead.

I bought a new Mega board and a new RAMPS board, just in case. Just wanted to ask what precautions should I take for this not to happen again? Is it ok to plug 12v into RAMPS when there's nothing connected to it but the Arduino board? Or was this just bad luck, not sure if the voltage regulator was working properly as it was an old board which had never been used with 12V.

First Rule of diy 3d printers: Never plug anything in or out while the power supply is on. All powering on and off of the printer should be handled on the mains supply to the power supply.Most of us including myself have found this out the hard way and have lost electronics in the process.
It is possible that you had a faulty regulator, shorts caused by poor soldering, or other broken components all of which are common on cheap Chinese boards.
I will say whenever you get your new boards plug in a stepper driver, turn power on, check everything I'd working fine, turn power off repeat. Take the time to test individual components before plugging it in together.

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Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller

Forgive me my bad english; what do you mean with : "All powering on and off of the printer should be handled on the mains supply to the power supply"? I should use PSU on/off switch to power it?

I didn't make any connections when the power supply was on.

Just to recap: I guess I didn't do anything that should fry the board? Bad China luck? I also had marlin firmware on Mega board.

Yes, the psu power switch or a power strip with power plugged in. There is a good chance it was a bad board. Good luck.

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Prusa i3 Rework - Ramps 1.4 - E3d Lite6 - Full Graphic LCD Controller

There were rumors about certain Ramps clones , where a diode ( D1 or D2 ? ) on the Ramps board was mounted the wrong way round. Would that kill the arduino?

If that´s the case, you can ask your dealer for a warranty replacement.
Can someone confirm my theory and guide us to the thread?
-Olaf

Diodes are correctly mounted and seem to be ok. Thx for the info, both of you.