LG - 47LG90 _ POWER SUPPLY TROUBLESHOOTING_ DIRECT VIEW LED-LCD TV

POWER SUPPLY [SMPS] TROUBLESHOOTING

     Ac voltage is supplied to the Power Supply at Connector SC100. AC Detect is generated and should be present at connector P201 pin 8 (5V). 

     The AC input also generates a Hot Ground primary power supply that runs in two states, Stand-By (156V) and Run (386V) measured at Fuse F101. This primary voltage develops all other voltages that are output from the power supply. During Stand-By, the 5 Volt Standby should be present at connector P201, Pins 9,10,11 or 12.

     If Missing remove AC Power and unplug Connector P201, apply AC Power and recheck for presence of both 5 Volt Standby and AC Detect. Loss of either 5 Volt Standby or AC Detect would be a Power Supply Failure. Presence of 5 Volt Standby and AC Detect would be an indication of a failure on the Main Board. Suspect a possible short circuit loading the supply. Observe the Front Power Indicating LED this may save some time. A lit LED most likely indicates the Stand-By 5V voltage is present.

     A loss of the AC detect line will prevent the front power LED from lighting, because it is driven by IIC bus communication.

    The Main Board sends two commands to the Power Supply Board one being PWR the other is INV ON.  These two voltages are used to control the power on turn on sequence. First via PWR (Pin 19) also known as RL ON activates the 24Volt to the Inverters and the 16 Volt and 12 Volt lines to the Main board. 

     The second command is INV ON. It passes through the Power Supply to the Inverter Boards to turn them on.  If either command voltage (PWR or INV ON) is missing it will result in a no picture symptom. These voltages can easily be checked with the volt meter. 

     Remove AC Power, unplug Connector P204, reapply AC Power and press the ON-OFF Button on either the Remote Control or Power Button on the Unit. Watch for the Power ON LED to change color from red to blue. This is an indication the PWR Signal was created on the the Main board. Check P800 or P201 pin 19 for the PWR command (2.8V) to the Power Supply.  Check P201 for 16V (Pins 1 or 2) and 12V (Pins 5 or 6). Check P204/P205 Pins 1,2,3,4 or 5 for the presence of the 24 Volt Supply. Confirm Pin 12 of P204/P205 went to 3.3V. This is the INV ON signal needed to turn on the Inverters and light the backlight LEDs.

CHECK-1 [POWER SUPPLY PWB LOW VOLTAGE CHECK]

• Add a 10K resistor between (5V STBY) pin 9, 10, 11 or 12 and Pin 19 (PWR). Apply AC. This will turn on the power supply.
• Check that the 16V and 12V power supplies are turned on, P201 (16V pins 1 and 2) (12V pins 5 and 6).
• Check that the 24V (Inverter B+) is turned on on P205 and P204 pins 1~5.

Remove AC power.

Reinsert the plugs P204/P205 to the inverters.

CHECK-2 [POWER SUPPLY PWB BACKLIGHT TEST]

Continue if the first  test was successful. Leave original 10K resistor in place.

Add a jumper wire between Pin 20 (INV On/Off) to Pin 19 (PWR).

Apply AC Power. This simulates a Power On and Backlight On command.

Observe the Back light.

• If normal, the backlights should turn on. (They will be dim)
• If no backlight activity, reconfirm the 24V is being generated and output on P204/P205 connector Pins ~5. If not, unplug P204 or P205 and recheck. If not SMPS is defective. If yes, Inverter is loading down the 24V line.

• Confirm the INV On/Off line Pin 20 is going to 3V.
• Confirm the Inverter On command is leaving P204/P205 Pin 12.

If all the above is normal,  check voltages on the Inverters.

POWER SUPPLY BACKLIGHT TEST

Using the Power supply SMPS Voltage Label [Silk Screen]

Using the (Silk Screen) Label can assist when dealing with an over current situation. Normal loads will keep the current reading under the Label’s maximum currents output reading.

• The AC input Fuse draws a maximum current 5 Amps.
• The 5.2V output (Stand by 5V) maximum output current 6.5 Amps.
• The 12V output (Video Processing and T-CON board B+) maximum output current 2.25 Amps.
• The 16.5V output (Audio) maximum output current 1.3 Amps.
• The 24V output (Inverters) maximum output current 11 Amps.