LG 42LG70 Series, Training Manual

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LCD DV 42LG70
29
March 2009
Power Supply (SMPS) Board Operational Troubleshooting Power Supply (SMPS) Board Operational Troubleshooting
Power Supply 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.
Remember to observe the Front Power Indicating LED this may save
some time a lit LED indicates the Stand-By 5V voltage is present!
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 Ballast and the 16 Volt and 12 Volt lines to the Main board. The 2
nd
command is INV ON. It passes through the Power Supply to the Ballast Board as a Lamp Lighting
Command Signal.
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 Pins 1,2,3,4 or 5 for the presence of
the 24 Volt Supply. Confirm Pin 12 of P204 went to 3.3V. This is the INV ON signal needed to light the
Lamps.
Problems with either voltage can be easily solved by following the simple steps on the next page.