I have now two non-working Arduino Uno (one R3 and one R2 - the version without extra I2C pins).
All worked fine some hours of testing. I connected the Arduino via Power Jack 12V and parallel per USB. After a dozen of successful uploads the Arduino is working with the last uploaded sketch but tx/rx LED is no more working/lightning up. Also uploading is no more possible. Error message
Binary sketch size: 5,586 bytes (of a 32,256 byte maximum) avrdude: stk500_getsync(): not in sync: resp=0x00
The Arduino is working after exchanging the ATmega328 . And this is now the second Arduino / MC that I destroyed with this setting; On the Arduino side only two test LEDs with resistors are working and a radar module http://www.produktinfo.conrad.com/datenblaetter/500000-524999/502667-da-01-en-RADAR_MODUL_RAD_MOD.pdf (power via Vin) with open collector and 10K pullup. All is still working. So I do not think some wiring mistakes are the reason. But uploading is not working. Also after restarting Windows and resetting / unplugging the Arduino uploading is still not working.
Do I have to unplug the 12V Power Jack always before uploading a sketch? Could this be the reason for the failure? How can I find out what part of the ATmega328 is destroyed? How can I “reset” the ATmega328–if this is possible without a special programmer hardware?