LG CM9750 Service Manual page 37

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 focusing actuator.
9-5. LASER FOCUSING ACTUATOR
9-5-1. Solution
Replace IC401 or IC400 on MAIN board.
9-5-2. How to troubleshoot (Countermeasure)
The focusing actuator makes the laser beam keep a regular interval with the surface of a CD disc.
1) Check the FDO signal from pin13 of IC401.
  If no signal, check 3.3 V (RF) and X400.
2) Check F- & F+ from IC400 to CN400 for driving the focusing 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 top or bottom side.
Please check the function after changing another MD.
9-5-3. Service hint (Any picture / Remark)
< Waveform of F±
for driving FOCUSING actuator >
ONE POINT REPAIR GUIDE
3-18
F- F-
pin18 of CN400 pin18 of CN400
F+ F+
pin15 of CN400 pin15 of CN400
< MAIN board top view >