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How to detect cracked LCD panel without backlight or power on
Somebody asked me to repair this Samsung LCD TV Model: LA32R71BAX which is unable to switch on. When I switched it on there is no backlight or sound. The power supply standby voltage is normal at 5.2V and other voltages measured to be low including the backlight inverter voltage.
It is quite obvious that the smps power supply board is having problem and in protection mode .The power supply board is tested standalone i.e without the T-con, main and inverter board but the voltages still remain low.
There is no visible components burn or bulged capacitors, so I proceed to measure all the diodes and FET driver transistors(QM801/802). Unable to detect any faulty components , I searched the internet and managed to get a copy of the power supply board schematic with part no: BN44-00192B .
Please click on the schematic to enlarge it
Figure A showed a part of the smps schematic that is giving the problem. ICM801 is the MOSFET driver IC with pin 10 function as fault input detection which the voltage derive from the smps primary transformer. Capacitor CM808 were found to be very low capacitance when measured and the low voltage detected at pin 10 activate the IC
to switched to protection mode. Since I do not have the actual capacitor I replace an equivalent with the capacitance and voltages close to the original. When I tested the power supply board again,he voltages are back to normal. I re-connect all the boards and power on the tv worked as normal with the sound & backlight on. Unfortunately, the LCD panel screen was cracked & feel frustrated with the time and energy wasted to repair the power board.
I did some research to find a quick way to detect cracked LCD panel when the backlight or tv cannot turn on. After failed to find a solution from the internet, I was hoping that high intensity light will be able to show the cracked. I tried with high intensity LED torchlight and bright table lamp spot light but unable to view the cracked partly due the mirror reflection of the bright light on the surface of the screen or not it is not bright enough to penetrate the polarizer screens. (see Photo 1).
Since strong light is need to penetrate into the backlight screen layer to be reflected back to the front, I try putting it under strong sun light. I am delighted to see the cracked area of the LCD panel appeared as a faint greyish/white colored patch (Photo 2). You have to move around in different view angle and height in order to have a clear view of the cracked area. To prove that the greyish/white colored patch is indeed the cracked area, the tv was power on again (see Photo 3). This colored patch location coincide with the cracked area shown when the backlight is on.
It is easier to view the cracked screen with the LCD panel lying flat and facing the open sky without any shelter (see Photo 4)
Photo 1 (Attempted to detect lcd panel cracked by using a table lamp spot light)
Photo 2 (Location of the visible cracked screen under the sun)
Photo 3 ( Location of cracked LCD screen when power on)
Photo 4 ( LCD screen lying flat and facing the sky for easier viewing)
I hope my article is helpful to those people in the repair business to diagnose cracked lcd panel screen before proceeding to repair in order to save time and money. Most customer will not pay for a new screen which is expensive and not worthwhile to repair.
This article was prepared for you by SK Wong from Malaysia and he graduated from London with a degree in Electronics and Electrical Engineering. He has more than 40 years of experience in the consumer product manufacturing. With the passion to repair electronic products from young , he has been doing it part time until now.
Currently, he can provide turnkey solutions for consumer products including services from idea or concept to design , prototyping , finished products (including electronic,mechanical and plastic parts) and manufacturing.
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