Changhong LT32729 LCD backlight back to LED backlight practice - Appliances

Photocoupler

Model: LT32729.
Fault: When the LOGO is turned on, the black screen will be displayed, and the brightness will be dark, the sound will be normal, and the normal light will be seen by the flashlight, and the typical backlight will be faulty.
Check: The resistance values ​​of the high voltage windings of the two high voltage transformers are all around 1.27K and stable. Because the hand is in stock, the test replacement is invalid.
Internet access: Several protection and technical improvement measures have been found. After trial, the fault remains. But at the same time, IC302 [LX6512D] 7 feet for over-voltage protection, 8 feet for over-current protection, try to make 7 and 8 feet shorted to the ground and turn on the backlight, the backlight is long, but the blue screen has a horizontal shadow in the middle of the screen, conclude The lamp is aging.
Screen splitting: It was found that the machine uses 4 U-shaped tubes. When power is on, one of them is not bright and the ends are obviously black. The other ends of the tubes also have different degrees of blackheads.
On TB: Check the average selling price of each item for 20 yuan, plus the express delivery of nearly one hundred yuan, and then count the wages we deserve, the cost is not low.
Imagine: There is a direct-sale LED backlight strip and a matching constant current board on the Internet. The cost is low and the power consumption of the whole machine can be greatly reduced. After the interest, the customer agrees.
Practice: Remove the original LCD tube and attach the LED strip in the appropriate position. The constant current board is powered by 22V---42V. The unit has a 24V output on the 2-in-1 power board. It should be used for the power amplifier. It is fully qualified to indicate the output current of 1.5A. Remove the two high-voltage transformers for other purposes, fix the constant current plate in the appropriate position, connect the power supply and LED output line, and use a 390K resistor between the constant current board 24V and ON/OFF to turn on the backlight, because 24V Controlled by standby, 24V is turned off during standby, and the backlight is turned off. This simplifies the modification, installs the light guide plate test machine, and has no bright spots and dark areas. After the screen is installed, no signal blue screen brightness is uniform, the effect is satisfactory, and the work is completed. .


model


The pins 7 and 8 of IC302 are shorted to ground at the same time to protect


One tube is not lit


Remove the high voltage transformer and fix the constant current plate


Power on, led brightness is normal


Blue screen brightness is very uniform


Normal image