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Does the standby LED light? If so is it flickering or steady?
Does the standby LED change color when you press the power button? If so does it change back if you press the button again (after say 30 seconds, some sets take ages to "boot")?
A typical LCD TV power supply consists of a power factor correction stage (in sets over 26" or so, not usually found in small sets drawing less than 60W), the standby supply (which supplies the always on 5V @ 0.5A or so), the main supply itself (which usually supplies an additional 5V rail, 12V rail and a 24V rail for the backlight inverter/driver) . In some sets the backlight inverter/driver is integrated into the power supply.
Long list of possibilities (all of which I have experienced in various sets) as I may not be around to answer replies for a few days
If the mains fuse is open you will most likely find shorted FETs and/or diodes in the PFC stage. If so check the resistors surrounding them and hope they haven't taken the IC with them - often they blow apart leaving you to guess the guess part number.
If the PFC stage is working you will have rectified mains on the big reservoir capacitor (around 360VDC for us with 240V mains, 180VDC for 120V mains).
The standby power supply will usually be based around an IC with an integrated switching FET. Most of these are DIP-8 (sometimes with a missing pin). Vcc for these ICs is usually 12V-18V derived from a winding on the primary side of the transformer via a diode and a capacitor (often 47uF).
When these smaller capacitors fail they are less likely to visibly bulge. I generally wave the ESR meter over all of them and replace any that are suspect. Without an ESR meter replace it if you're not getting the voltage there.
If the standby power supply is running, but you are getting less than 5V (or whatever it should be, it is usually marked on the board near the connector) this could be the capacitor on the secondary side (sometimes they fail without bulging) or it could be a failure on the mainboard loading it down. The 5V will be regulated down to 3.3V on the mainboard. On the most common 3 terminal regulators the tab is the output, the rightmost pin is the input.
If you have standby 5V and 3.3V at the regulator, but still no standby LED it is probably an EEPROM or Microcontroller fault which often means a board swap.
If the standby LED does light, but does not change color when you press the button check the power on signal. This is a pin on the power supply that is either pulled to gnd or to 5V to turn on the main supply. It can be labelled switch, stby, pon, etc. If it doesn't change state when the power button pressed it is most likely a EEPROM or Microcontroller fault on the mainboard.
A shorted FET on the backlight inverter/driver (common) will stop the 24V from coming up. A shorted audio IC (less common, but happens) will stop the 12V from coming up. If the 12V and 24V share a transformer either will take out the other. These are often on separate connectors so it is easy to isolate them.
If they are not on separate connectors it may be easier to disconnect the mainboard entirely and tie power on line to gnd (or 5V as the case maybe) via a ~220 ohm resistor to force the power supply on for testing.
If the standby LED changes color when the button is pressed, but does not change back when it is pressed a second time it is either not starting up completely or freezing. Check the voltages at the rest of the regulators on the mainboard. Some will likely be 3.3V, 2.5V, 1.8V. If all the voltages are correct this is likely an EEPROM problem.
If the standby LED changes color when the button is pressed and changes back when it is pressed a second time and all voltages are correct then it is likely the set is "working", but you can't see anything because the backlight is off - either because the inverter/driver is bad or because the CCFL/LEDs inside the panel are bad. In some panels they are wired in series - if one fails open none will light.
If this is the case you may be able to see "something" on screen under a bright enough light - easier with some panels than others.
Something that has caught me out before has been child lock. If it on (and often it seems to get put on unintentionally) and the remote dies (say from leaking batteries) it becomes impossible to turn the set on even though there is nothing actually wrong with it. On most cameras the IR from the LED of a working remote shows up as white.
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