i find solution problem came from my USB-C hub i disconnect it and 7740 working normally 
hello guys I added a new SSD hard on my 7740 (1TB+1TB Raid on) after added new SSD, during power up and in BIOS, Screen continuously NO/OFF!!! when going to windows screen going to normal! can you help me?
Dell Precision 7740 Bios/Startup Continuous Screen NO/OFF