Which board did I fry?

I messed up (yes I know), so now just wanting some confirmation of how much damage I might have done to my son's prusa. With the system powered ON (stupid) I touched the soldering iron to the +12V connector of the heatbed (I was trying to fix the connection). I saw a spark which instantly reminded me I was stupid for not unplugging things. So any educated guesses of whether I fried the Arduino, the RAMPS, both, or even more components? I get power and L LEDs lit when I connect the arduino (by itself) to my USB, but any download eventually times out. I assume and hope it was only the arduino that got fried (and not the RAMPS, etc.).

At a minimum I would say that you fried the mosfet on the RAMPS. Also check your power supply. Funny thing is that if you don't figure out what is gone, you can ruin any new boards by swapping out one board at a time. Folgertech has the complete set of boards (RAMPS, 5 A4988 drivers, and Arduino mega) for $42.00. Your stepper motors should have managed to survive, as well as the extruder and heatbed, but I would check them with an ohm meter to be sure.

[www.reprapsource.com]
for the mosfet on the ramps board.
Replace that and see what happens is, I think, the best bet.
It isn't clear to me what you mean by "download". The heatbed won't heat so prints won't start if that is what you are getting.

Quote
tjbonzo
I get power and L LEDs lit when I connect the arduino (by itself) to my USB, but any download eventually times out. I assume and hope it was only the arduino that got fried (and not the RAMPS, etc.).

I would start by removing the RAMPS board from the Arduino and working toward getting it to connect to your computer. If you cannot connect (not found on the USB, timeouts, etc.) then there's no reason to troubleshoot further until you have a new Arduino... though you may have some luck after re-flashing the boot loader on the Arduino.

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-David

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Replaced the Arduino - which I knew wasn't working, and the RAMPS just to be on the safe side. The stepper drivers and everything are working just fine. Thanks for the advice.