LG CMS9540F Service Manual page 37

6. NO SOUND
There is no sound output in the USB FUNCTION, repair the set according to the following
guide.
6-1. IN THE USB FUNCTION
6-1-1. Solution
Please check and replace IC501 on MAIN board & IC300 on USB board.
6-1-2. How to troubleshoot (Countermeasure)
1) Check +5VU to USB board.
If +5.6 VA to pin2 of IC300(LDO) doesn't come out, check pin4, 5 of CN304.
If +5.6 VA is normal & +5VU of IC300 pin3 doesn't come out, then replace IC300.
2) Check if "Digital audio AMP block".
6-1-3. Service hint (Any picture / Remark)
< USB function signal fl ow >
ONE POINT REPAIR GUIDE
3-16
< Waveform of USB D± signal >
USB D+ / D- (CN502 Pin1, 2 & 9, 10)