Arduino uno r3 not powering up i bought two arduino uno r3 card (not original)
one is working perfectly:

the other is not powering up (no led is lit), i did some measuring and i found that two usb connector pins on the board are shorted, how can i proceed to repair it taking in account that it works from time to time !

thank you
More images:
It looks as though there may be material between at least one USB pin and the ground layer. This may be flux of solder 'splash'.

If visual inspection shows that this may be the case I would try carefully scratching with a sharp poibt (eg scriber) along at least the green arc on the image below. Be careful not to break intended conduction paths (eg as shown in black).