USB input is not working on my PSAudio Digital Link DAC


I bought this DAC last year and it worked perfectly, but since march the USB input is not working. Some days USB input works perfect, most of time not. I changed the USB cable, try another source, change the power chord, but nothing. Last owner told me that the DAC works perfect for him. It has Cullen circuits (DL3 Stage IV) modifications.

Both coaxial and optical inputs works without problems.

Any suggestion?