LG CM9750 Service Manual page 36

NO OPERATION OF MD
When no sound output in the CD function, you can not listen to music reading data from a CD
disc if the pickup module in MD doesn't work. This step is for checking the laser tracking actuator.
9-4. LASER TRACKING ACTUATOR
9-4-1. Solution
Replace IC401 or IC400 on MAIN board.
9-4-2. How to troubleshoot (Countermeasure)
The tracking actuator makes the laser beam be positioned in the center of a track on CD disc.
1) Check the TRD signal from pin14 of IC401.
  If no signal, check 3.3 V (RF) and X400.
2) Check TR- & TR+ from IC400 to CN400 for driving the tracking actuator.
  If no signal, check +1.8 V and +5 V for IC400.
3) Check if the FFC cable is solidly connected between CN400 and MD.
4) Check the MD.
  If the pickup module has any trouble, it can not move the laser beam on the left or right side.
Please check the function after changing another MD.
9-4-3. Service hint (Any picture / Remark)
< Waveform of TR±
for driving TRACKING actuator >
ONE POINT REPAIR GUIDE
3-17
TR- TR-
pin17 of CN400 pin17 of CN400
TR+ TR+
pin16 of CN400 pin16 of CN400
< MAIN board top view >