New UNO r3 passes loop-back test but will not upload

I have two new UNO r3s from different suppliers. Ostensibly both genuine. One worked straight out of the box the other, I discovered, had an unprogrammed 16U2 and no bootloader in the 328. In Win 7 (64) I loaded drivers and hex file ...\atmega16u2-Uno-Rev3 into the 16U2, and replaced the 328 with one loaded with Optiboot\Blink. This board now passes the loop-back test but will not upload. I use Blink as a test upload.

When I upload to the working Uno, I see furious flashing of both TX and RX but with the non-working Uno the TX remains blank and after some seconds I get the error avrdude:stk500_getsync()... etc. In this test I simply connect the USB cable first to one board then to the other. The working Uno comes up on COM5 and non-working on COM3.

Seems to me my problem is with the 16U2 (or is it a fault on the board?) I'm new to this stuff. Have I missed something obvious? Thanks.

skiddaw: I have two new UNO r3s from different suppliers. Ostensibly both genuine. One worked straight out of the box the other, I discovered, had an unprogrammed 16U2 and no bootloader in the 328. In Win 7 (64) I loaded drivers and hex file ...\atmega16u2-Uno-Rev3 into the 16U2, and replaced the 328 with one loaded with Optiboot\Blink. This board now passes the loop-back test but will not upload. I use Blink as a test upload.

When I upload to the working Uno, I see furious flashing of both TX and RX but with the non-working Uno the TX remains blank and after some seconds I get the error avrdude:stk500_getsync()... etc. In this test I simply connect the USB cable first to one board then to the other. The working Uno comes up on COM5 and non-working on COM3.

Seems to me my problem is with the 16U2 (or is it a fault on the board?) I'm new to this stuff. Have I missed something obvious? Thanks.

The loopback testing OK and the PC seeing it as COM3 means the 16U2 is working fine, problem is in the 328P chip or the bootloader you tried to burn on to it. Try again reburning the bootloader.

Lefty