Troubleshooting a LCD

Discussion in 'General Electronics Chat' started by Jotto, Oct 14, 2011.

  1. Jotto

    Thread Starter Member

    Apr 1, 2011
    Troubleshooting LCD's is very easy.

    1. Dead unit, will most likely be something in the P/S. PWM chips have a capacitor, the value is usually 47m@50vdc. On some power supplies there is more then one PWM. The PWM circuit will be a IC chip usually a 8 pin, and also a FET. Some units will use a TO220 case with the PWM and FET built into one unit. But they will always have a capacitor the control the start up of the PMPS. The capacitor value can vary on this, but not by a lot. The highest I have seen is 100m@50vdc.

    2. No picture could be a power supply, faulty inverter or CCFL going bad. If its possible you can shine a light through the TFT screen, if there is a picture your A/D board is operating properly and you must now look at the inverter and CCFL installed. The unit could have a picture for a second and then go black, that is a good indication that the inverter or the CCFL's installed are bad. Most units will use 4 or more lamps depending on the size of the screen. Most of the time if the inverter is bad you will lose a transformer or a IC that controls the transformer. Sometimes it can be a capacitor, but those failures are not common and I have had units with capacitor is swelled out the bottom, but the funny thing is that they very seldom affect operation of the unit. I do replace them as part of preventive maintenance that I do any time I open the unit up.

    a. Troubleshooting the inverter is not as complicated as some may think. You have voltage going to the inverter which can vary depending on the unit, you also have the switched voltage that turns the inverter on. The inverter is the only high voltage on a LCD, it can be between 200-1000 VRMS, sometimes it might be higher. The A/D board will send the signal to the inverter for enable and blacklight adjustment for brightness. You can look at the output voltage for the lamp with an Oscope, not all scopes have this ability. You must first power down the unit and then hook up the scope, doing this with power applied with shut down your inverter. I have never seen it damaged doing this with power applied but there is that possibility. 20k hours is the life of a lamp before it is 50% of the initial output of the lamp. All inverters have a switched input to turn the inverter on/off, the value of of the switch varies depending on the unit being tested. You can use 2 power supplies or 1 power supply and a resistor to the switch transistor for turning the unit on. I have done it both ways, but prefer using two power supplies.

    b. Using a dummy load for testing the inverter is a simple part to make if your troubleshooting a lot of LCD units. You can use 50k ohm resistors at 10 watts, I use 7 in series to simulate different loads, an tap off where I need them. I can simulate 4 lamps on each board and have 2 boards configured that way. I have a power supply that tells me what the current draw is as the unit runs, so getting the load right is very easy. If you load is not balanced your unit will come on for a few seconds and then shut down the inverter. I match the load, and then I remove the load an add one lamp, if it stays on that lamp is good, an of course if it goes out that lamp is bad. The hardest part of replacing a lamp is trying to keep the rubber caps on the ends of the lamp in one piece. If you break them apart which is most likely to happen you can use silicon on the ends, but do let it dry 24 hours before powering the unit back up. Also make sure you have it all around the lamp metal end, any exposed metal will arc to the metal of the unit housing the lamps.

    3. A/D boards are not a common problem, but if there is it could be a capacitor or voltage regulator. I have seen the LVDS go bad that drives the TFT panel. A/D boards for the most part are very reliable since its really only a low voltage unit. There is varying voltages produced on the A/D board, some are very critical to the operation. I have seen some microprossors go bad. Some of these parts will be hard for the hobby type person because you need very good soldering and desoldering equipment.

    Maybe someone who works on units with a tuner could add to this, since mine are monitors that require input from other devices I can't say much about those.
  2. tom66

    Senior Member

    May 9, 2009
    Very good guide.

    A few other checks:
    - Faint ticking sound a few times a second? That means a short/overload on the secondary side. Check diodes. Run without load.
    - The startup capacitor is charged through a 220k-2.2Meg resistor. If this resistor goes bad, then the unit will not start.
    - If Vcc is very high or very low, but resistor is okay, suspect the PWM IC.
    - Check main filter capacitor for either ~160V or ~320V dc.

    The small signal board / A/D board / main board can go bad. As well as LDOs, the switcher diodes are a common fault.

    Rainbow coloured lines, weird patterns, distorted picture? Usually a bad T-con or bad main board.

    Screen only lights up for 1-2 seconds but the picture is still faintly there? Bad inverter or bad CCFL. If the inverter is at fault, it is usually the transformers. Test each transformer's secondary while off. They should all measure the same.

    On some BenQ LCDs (also rebranded by Dell, HP etc.) the transfomer driving transistors are very stressed and usually fail with low Hfe or go open. This will cause the same 2 seconds to black problem.