Maintenance log: 42-inch Panasonic LCD boot black screen after 10 seconds into the protection state - android - Phones Developers

LC03-3.3 SOP8 TVS Static Protection 3.3V

Electronic scale crystal oscillator 3.2*2.5mm 3225 16M (16.000MHZ) 12PF 10PPM 20PPM 30PPM

Panasonic 42-inch LCD TV:

The fault phenomenon is that the black screen is turned on, there is sound, and the protection state is entered after 10 seconds.
Open the back cover, check the power test, and measure the various voltages output by the power board during the time before entering the protection.

During the period from the buckle of the iron back plate, the backlight is turned on three times (about half a second each time), and then enters the protection state.
First of all, I suspect that there is a problem with the LED constant current board. I carefully disassembled it and found no faults. I suspected the LED backlight, but the risk of disassembling the screen is very difficult. How can I detect whether there is a problem with the backlight LED without disassembling the screen? What? I remember seeing the method of using the shaker to detect the backlight LED lamp on the Internet. The shaker is a megohmmeter. It can output a voltage of several thousand volts, but its internal resistance is very large, and it cannot output a large current. It is suitable to detect LEDs in series.
Try it out now: Backlight LED lights are divided into two groups to find the positive and negative poles of each group (analyze the output of the LED constant current plate can be found), and then find the positive and negative poles of the shaker output (you can do it with a multimeter). First measure the first group, the positive and negative poles of the shaking table are respectively connected with the positive and negative groups of the first group of LED lights, and connected with a multimeter to measure the input voltage of the two ends of the first group of LED lamps, shaking the table (100 revolutions per minute) Left and right), the multimeter shows about 60 volts.

Using the same method to measure the second set of LED lights, the multimeter shows about 40 volts.

Obviously, the two sets of LED lights are seriously unbalanced, and there must be a lamp bead burned to make the circuit enter the protection state. Only remove the screen and change the light bar. I am not professionally doing home appliance repair. I haven't dismantled the screen before. The confidence of the first screen splitting is not very good. After explaining the situation to the owner, the owner strongly supports it. Let me rest assured that I have no worries and I will start immediately.
Remove the surrounding screws and remove the frame:

Carefully open the many buckles on the four sides and pick up the LCD screen. This work must be handled with care by two people. Be careful not to damage the four film cables connected to the two boards. Otherwise, this screen Just finished playing.
After removing the LCD screen, you can see the white polarizing film.


Then it is easy to remove the close three-layer polarizing film and a scattering film.

The LED lamp beads finally came out.

Then open the reflective film (applied to the bottom plate with double-sided tape), the light bar is in front of you.


At this time, it is very convenient to use a multimeter to check the quality of the lamp bead one by one. If you find a burnt lamp bead, you can see the burnt LED lamp bead by opening the lens. This lamp bead is a leadless chip package and cannot be soldered with a soldering iron. So the most convenient is the replacement of the entire light bar.

However, Taobao could not find this type of light bar. Only the scalp was replaced with a lamp bead. In Taobao, the packaged LED lamp bead has 6v160mA and 3v350mA. Which one is used for this light bar? ? I used a 9v battery in series with a 100 ohm resistor to light one of the beads, and then use a multimeter to measure the voltage of the lamp bead to determine that the lamp bead is 3v350mA.
I was afraid that the welding process was not successful at home. I bought four lamp beads at once and was prepared. I found a colleague to help me.


Directly solder the lens with a soldering iron, restore the LCD screen, connect the circuit board, power on the test machine, and the TV returns to normal.