hi, my arduino is burnt out. currently there is a mega 328p U-KR 35473D 2032DT4 chip on the arduino, but I can only replace the chip if I can, how should I disconnect it and the new chip must be exactly the same as the previous one?
How did the chip get "fried" ?
There is a chance that other components are damaged
Is the chip in a socket or is is soldered to the board ?
What size and shape package is the chip in ?
A photograph would help
I don't have any idea how did it get fried. I uploaded a code and pc and arduino shut down.. i managed to start pc again but on arduino just leds on, rx and tx are turned on and chip gets very hot.. does that help?
The picture helps a lot
What are your soldering skills and equipment like ?
What was connected to the board when the incident occurred ?
Is a COM port created when you plug in the board to USB ?
Soldering skils are pretty good, I have a soldering iron, and hot air fan, arduino was connected on lcd, relay with external power, on uln 2003 chip for stepper motor with external power, dht 22 temp sensor. And com port is created when I connect it.
Replacing those QFN packages is not too difficult using a hot air station, plenty of flux and solder braid. I have done it on a couple of Nanos but, of course, it may not fix the problem and you don't know why it happened in the first place
A good guess would be that you had something connected incorrectly or that something in one of the external components caused the problem
Did anything release the "magic" smoke when the problem occurred ?
Arduino was running all night it was connected to external power and usb serial monitor, few days ago it was working good for 6 hours, but yesterday I left it on for all night and after 2 or three hours it got stuck and it was repeating same part of code over and over again. This morning I reseted it and it was working ok, I uploaded a bit changed code and than I wanted to upload new code one more thime and i was getting error stk 500... so i disconected it and when i conneced usb again this L (pin 13 was blinking all the time) i have it connected to relay so i heard clicks, so i disconected it couple of times, then uploaded code and when i connected 12v external power to it it just crashed.. I think there was no smoke
+1.
Component level repair of this board, without either knowing the cause of the fry-ation nor its full extent as far as what components are no longer functioning correctly has a very low chance of success.
If it is the kind of fun you like, or you want to practice removing and replacing surface mount components, go for it.
Before you do anything, however, it would make sense to hunt down and perform every kind of test there that can be done with instruments you own. Or acquire under the excuse that now you need them. 
com port is created when I connect it.
So you might be able to use a PC terminal program to do a loop-back test and confirm the functioning of the USB interface to a further point than simple creating the port.
then uploaded code and when i connected 12v external power to it it just crashed
No no no.
With just the UNO, running only off the USB cable supplying communication and power, you should be writing small programs that can test the i/o pins… jumping ahead to connecting stuff or running off an external power supply only muddies the water.
If simple programs work, it might mean all you've managed to destroy, somehow, has nothing to do with the microprocessor or most of the circuitry that supports it directly.
TBH again, fun if it is fun, good practice, but what you should really try to figure out is what happened, you've mentioned a relay, for example, so you don't do the same thing to your new UNO.
a7
I will try to find the cause of this, but i really don't know what could be the cause? Maybe pin 13 on relay (because of L blinking when arduino starts), or this that i connected both usb and jack at same time? Could be this related to tmp 36 which wasn't working properly and it got fried? But could be solenoid on relay..?
Please say that
with the Arduino only, connected by the USB cable only, you can
upload and run small programs that
verify the functioning of some (or all!) the i/o pins.
Divide and conquer. Leave off all the external power supply, relays, solenoids and use only
A logic probe or voltmeterand a jumper wire, or LEDs with series resistors and pushbuttons (INPUT_PULLUP configuration).
I am not clear at all what works and doesn't. You speak of uploading, running and crashing, these are not words I associate with a freid board.
a7
Sorry for my bad english. These things were happening before it stopped working. I just doesn't want to make same mistake with new arduino.
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