LG 37LH55 POWER SUPPLY TROUBLE SHOOTING

    AC should not be applied at any time while adding resistors or unplugging connectors as damage to circuit boards might occur.
[A] The SMPS PWB 'Must' be producing STBY 5V on either pin 7,8,9 or 10 [5V].  If this voltage is not produced SMPS PWB is defective.  There is no need to continue with next test.
[B] Unplug  P700 on the main PWB and P203 to the ballast.

Add a 19K resistors between [5V Standby] pin 7,8,9 or 10 and pin-2 [PWR].  Apply AC.  This will turn On the power supply.

• Check that the 24V and 12V power supplies are turned ON, P201 (24V pins 17 & 18) (12V Pins 13 & 14) and  24V to the Ballast P203 Pins 1-5.
• Remove AC Power.  Reinsert the plug P203 to the ballast.

Continue if the above test is OK.  Leave original 10K resistor in place.

• Add an other 10K resistor between 5V pin 7,8,9 or 10, and pin 20 [INV On]

Unplug the small ribbon connector  CN-1.  [This floats P-DIM].

• Apply AC power.  This simulates a Power ON and Back light command.
• If back light is normal:  If all is normal the back light should turn ON
• If back light is abnormal:  Recheck connector P203 and confirm  24V on pins 105.  If low, or missing , use diode mode to check the 24V line on Ballast connector CN 1.  If low, this indicates a shorted output FET.
• If Open, check Ballast fuse, possible open.  
• Confirm the INV On/Off line P203 pin-10 is going to 3V
• Confirm Pin-3 of CN-1 has >2V.  If not, need to add 3V by adding a 10K resistor from 5V_ST to pin -3 of CN-1.  Back light should light.

Remove AC power

• Check the connections to the back lights.  'Do Not' check with these when AC is applied as they can carry [1.2Kv each].