This is a tutorial that I got from the forum and looked to be very useful for troubleshooting. Feel free to read and learn :
(Bud Martin): Basic LCD Troubleshooting Guides

First, if you can, try the monitor with other PC first to make sure it is not the PC problem.
Notes: We are dealing with high voltage that can kill you! so if you are not sure what you are doing,
please have it fixed by the pro that has proper tools and safety equipment. Try to use GFI outlet and
isolation transformer when working around the TV set and DO NOT DEFEAT THE GROUND PIN OF THE

The monitor will have at least 2 circuits boards, one board will be the logic board where the video cable from PC is attached to, the second board used in most monitors today is the Power supply/backlight inverter board combinations. Some monitors will have separate power supply module and backlight
inverter board. 

The monitor should be connected to the running PC otherwise it will go into standby mode.
The operations of each board:

1) The Power supply board: 
It takes the 120vac 60Hz and converts it to high voltage DC (around 160~170vdc filtered by the 80~150uf
250~450vdc cap) by the bridge rectifiers to be used by the switching power supply circuits that converts
this DC voltage into a high frequency (around 50~100 KHz) AC for driving the step-down transformer. 

The outputs (usually 2 outputs) of the transformer will be rectified by the diodes to produce the regulated 5vdc
for the logic board, and regulated 12~20vdc (12vdc is usually for screen up to 17inch, 16~24vdc for
18~24 inch screen).

The power supply circuits are always on (unless the monitor uses the power switch that actually
disconnects the power from the outlet which is rarely used these days) which means that it is running
24/7 using the monitor or not, any spikes and surge will be fed into the monitor power supply.

Common problems: 
Blown fuses, bad caps (leaking/bulging tops or bottom seals, please note that bad cap may look normal but it can have high ESR (Equivalent Series Resistance). The DC filter cap should be low ESR type for using in the switching power supply; general purpose electrolytic types will not last very long in switching power supply circuits. Poor solder joints, overheated components.

Plug the monitor in but do not activate the power switch so the backlight inverter circuits will be off. Check the 5vdc and the 12~24vdc to make sure they are OK. They should be tested with the load, you can use
6V 1A (6watts) lamp for the 5vdc, and car lamps such as 1157 (12v 8watts lo/26watts high) turn signal
brake lamp using high filament connection for testing the 12~18vdc (or use two 1157 in series for
19~24vdc) for the backlight inverter circuits.  

If the power supplies are working, the output voltages should be steady at the rated voltages. The power supply will go into shut down if it detects too much current draw due to false in the power supply or short circuits in the backlight inverter or in the logic board.

2) The backlight inverter circuits:
It takes the 12~24vdc and converts it to high frequency AC to drive the inverter transformers CCFL (Cold
Cathode Fluorescent Lamp) assemblies. The transformers will drive the CCFL by applying the start up
voltage (around 1500~2000v), when the CCFL start conducting, the voltage will drop down to about

The inverter has detection circuits to detect open circuit if the lamp is not attached or does not fire up after
the start up voltage is applied, it will go into shut down. It will also shut down if the lamps draw too much
current due to ages (when lamp gets old it will draw more current). 

The inverter gets two signals from the logic board, one is the backlight ON/OFF signal, the other one is
the Dimming signal for the lamps. 

Common problems:
Bad filter caps, resonant caps (in the inverter output circuits), blown transistors/IC, shorted or open
transformer winding.
You should have spare lamps for testing the inverter circuits. You can get lamps from

3) Logic board:
The logic board get the signals from the VGA (ANALOG) or DVI (DIGITAL) and processes them and feed them to the LCD panel T-CON (Timing Controller) board on the back of the LCD panel.
It also sends out two signals (backlight ON/OFF and Dimming) to the inverter circuits when the monitor is
on and getting the signals from the PC. 

If the logic board does not get the signal from the PC, it will put the monitor into standby mode.
The 5vdc feeding it is converted to 3.3v, 1.8vdc by the switching buck converters to run the processor.
The logic board also sends the 5vd or 12vdc power for the T-CON board, if the voltage is not there, you
will see white/grey glowing screen only.  

Not much repair you can do on the logic board unless you have the full-service manual and surface mount repair station.

4) CCFL and Inverter circuits testing:
If you don not have the inverter and lamp tester boxes (see, you may be able to do a
simple test by using these steps: 

If the screen flashes on for a second, you can disconnect all the lamp connectors and connect it into one
of the transformer output connector and see if you will see the flash on the screen, if you do, then try it
with another transformer output connector to see if it also get the flash on the screen.  

Repeat the procedure with other 3 lamps. If all the lamps do flash on for seconds then more likely the
lamps are OK. If lamp only flash on one of the transformer output then you will know that the problem in
that transformer inverter circuits. 

If none of the lamps flash at all then the problem is in the inverter circuits, power supply, or not getting the
on signal from the logic board.

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