LG CM9750 Service Manual page 38

10. NO SOUND
There is no sound output in the USB function, repair the set according to the following guide.
10-1. USB FUNCTION
10-1-1. Solution
Replace IC501 on MAIN board, IC312 on FRONT board.
10-1-2. How to troubleshoot (Countermeasure)
1) Check +5 VU to FRONT board.
  if +5.6 VA to pin2 of IC312(LDO) doesn't come out, check pin4 & 5 of CN314.
  if +5.6 VA is normal & +5 VU of IC312 pin3 doesn't come out, then replace IC312.
2) Check "Digital audio AMP block".
10-1-3. Service hint (Any picture / Remark)
< USB function signal fl ow >
ONE POINT REPAIR GUIDE
3-19
D D +
D- D-
USB D+/D (CN502 pin5, 6, 8, 9)
< Waveform of USB D± signal >