LG CMS4750F Service Manual page 27

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 servo motors in MD don't work. This step is for checking the
TRAY OPEN / CLOSE MOTOR among them.
4-3. TRAY OPEN / CLOSE MOTOR
4-3-1. Solution
Please check and replace IC407, IC408 on MAIN board.
4-3-2. How to troubleshoot (Countermeasure)
1) Check MOT_OPEN & MOT_CLOSE signals from Pin K4, L4 of IC501 to IC408.
  If no signal, check +1.8 V & + 5 V to IC408.
2) Check LOAD± from IC408 to CN405 for driving the tray open / close motor. It is about 3.85 Vp-p.
  If no signal, check +5 V to IC408. If it has any trouble, replace it with a new one.
3) Check if the FFC cable is solidly connected between CN405 and MD.
4) Check the MD.
  If the tray motor is sort-circuit or has any trouble, it can not open or close the tray.
Please check the function after changing another MD.
4-3-3. Service hint (Any picture / Remark)
MOT_OPEN
CLOSE
LO- TO CN405
LO+ TO CN405
< Waveform
for driving TRAY open / close motor >
ONE POINT REPAIR GUIDE
LOAD+
Pin9 to CN405
LOAD-
Pin10 to CN405
< Signal check point >
3-8
MOT_OPEN
Pin7 to CN405
MOT_CLOSE
Pin6 to CN405