When I plug my arduino to USB the led Power, Tx, Lx and LED reimains on and is not possible to connect. I've connected to the 5v output a 20v input voltage and some smoke apears. I know it's horrible but was it a mistake. When I take out At Mega and plug usb or 5 Volt only the power led and the Tx and Lx remains on. I suppose I need a new USB interface chip FT232RL.
Some idea while I wait my new chip.
Thanks a lot
You can check out the FTDI chip independently of the AVR processor chip by doing the following.
Remove the AVR chip from it's socket.
Wire a jumper from pin 0 to pin 1.
Plug in the USB cable to the board and PC.
The PC should recognize the connection of the USB device, if not the FTDI chip is toast.
If the PC recognizes the USB connection then launch the Arduino IDE and then open the serial monitor.
Any character you type into the serial monitors output window should echo back to the monitors receive window. If no data is echoed back then the FTDI is toast.
Note: Arduino boards don't like 20vdc applied to any of their pins as well as to it's 5vdc power pin. There are capacitors on the board that also wire to +5vdc and they may also be damaged. It's hard to recommend trying to repair such a board by just chip ( AVR & FTDI)replacements unless you can also make sure none of the other passive or active components on the board didn't also suffer from overvoltage failure.
Lefty
I suppose the pins so connest are 0 and 1 also named TX RX. Tested and I think FTDI chip is burned. Is it possible to take out FTDI and test the board?
Thanks a lot
SMD mounted devices like the FTDI are pretty hard to remove without special tools and experiance, I wouldn't attempt it, but I'm old 
Have you checked out the prices on some of the many clone Arduino boards, they are pretty cheap these days. Like:
http://store.fundamentallogic.com/ecom/index.php?main_page=product_info&cPath=29&products_id=484
Lefty
...Or a FTDI board (for example from Sparkfun) : cheaper but bigger than having a new Arduino. Plus, you could use it for Arduino board without USB if you need it.
Thanks to all. I've repared the board with two new chips (ft232 and atmega328). And now it's working.
Good! With which tools did you do? Simple soldering iron or a rework station?
To take out rt232 I've used a 13 watt solder with 1 mm tip and a tweezers. To re-sold I've used the same solder 1 ton of patience. It isn't easy but is possible. You must to take attention don't brake any track, leave some tin in the track and don't add tin with new chip.
Ok, thanks!