Full repair guide of LCD screen failure

 

 

Definition

Vin: PWB input voltage (12V)
VDD: ASIC, source IC, gate IC driving power (3.3v)
VGH: TFT component switching voltage (~30V)
VGL: TFT component turn-off voltage (~ -6v)
VAA: step control voltage (~17V)
VCOM: liquid crystal reversal reference voltage (~7V)

 

 

 

1. #Confirm whether the COF on side X is hot compared with the normal temperature, whether there is fracture or wear crack, and whether the COF is burnt.

2. #Confirm whether the VAA is normal (normally about 17V). If abnormal, disconnect the RP32 to confirm whether it is caused by DC/DC loop or X-side COF: disconnect RP32, if the VAA is normal, the COF is bad, CO must be changed; COF can be Disconnect one by one to determine which NG disconnects RP32, VAA NG, try to change UP1; at the same time, confirm whether the continuity of the surrounding triode is OK.

3. #Confirm whether EIO is OK (normal is 0.03v) and try to change ASIC if there is any abnormality.

4. #Determine whether the gate IC is OK. There is a signal test point on the back of COG-IC, and the green paint can be scraped for measurement confirmation; If there is a gate IC problem, which IC fault can be confirmed. The confirmation of gate IC fault is only for analysis when you are interested, and this method is not recommended.

 

Phenomenon: gradient fault occurs in the tone gradient screen or some gradient NG happens

 

1. #Measure GM1~GM14, the values are arranged from large to small. In general, a certain gamma value will be abnormal in the case of NG, then try to replace gamma-IC;

2. #See if there is any damage to the gamma line on the back of the PWB;

3. #If all of the above are OK, you should try to change the COF. You can disconnect the COF one by one to determine which IC is NG.

 

Phenomenon: The left and right screens are different

 

Because the RSDS transmission basically adopts the double-channel mode, the analysis only needs to be carried out on the RSDS loop.

1. #Confirm whether the conductive foam is short-circuited RSDS

2. #Confirm whether X-COF has poor bonding

3. #Confirm whether the RSDS value is correct, normal RSDS is about 1.2v, and the signal difference is about 200mV; At the same time, we can confirm the resistance between RSDS signal (normal 100 or 50 ohms) and RSDS resistance to ground. If the voltage is NG, check if the ASIC and X-COF are hot.

Note: Confirm S1 RSDS if the left half screen is abnormal; confirm S8 RSDS if the right half screen is abnormal.

4. #In general, ASIC is mostly invalid, please try to replace it first;

5. #If it is still NG, remove the COF from the middle or last COF, and the RSDS voltage returns to normal.

 

 

1. #Adjust the VR knob to see whether it can be adjusted and whether the screen performance changes. At the same time, confirm the VCOM value (about 7v), if NG, replace the VR knob.

2. #Confirm VGH/VGL voltage (about 30V VGH and -6v VGL), and confirm whether it is DC/DC loop NG or COF IC NG; The corresponding resistance of disconnected VGH and VGL can determine whether it is a DC/DC problem or a COF-IC problem. If it is DC/DC NG, try to replace UP1 or confirm whether the corresponding transistor is OK.

3. #If the whiteness changes significantly with the view Angle, and above 1&2 analysis is all OK, polarizer NG or CELL NG can be basically determined.

Possible Causes	How to Repair	Note
WOA line is broken	Replace PCB	PCB scrapped
Gate-IC NG	Replace Gate-IC
X1 Poor Looseness	Replace X1

Possible Causes		How to Repair	Note
LVDS connector NG	Wrong pin	Replace conn.
	Poor connection
F1 fuse open		Replace FP1
Conductive foam short circuit element		Reattach or remove the conductive foam	Suitable for LCD module models with conductive foam
FFC Goldfinger short circuit		Replace FFC	Suitable for LCD models with L/R boards
ASIC NG	3.3V short to ground	Replace ASIC
	No control signal
DC/DC NG （无VAA）	PWM-IC NG	Replace PWM-IC
	X-COF NG	COF replacement （LCD bonding&pressing equipment required)
	QP1 burned	Replace QP1
	G-COF NG	COF replacement （LCD bonding&pressing equipment required)
	Other components NG	Replace other components

Possible Causes	How to Repair	Note
COF poor bonding (PWB side)	COF replacement （LCD bonding&pressing equipment required)
Poor COF

 

 

Backlight failure

Possible Causes	How to Repair	Note
Lamp line is broken	Replace the lamp tubing	Depending on the backlight structure, there will be different results. The failure of the performance may be a point-off, or it may be a backlight with a dark band.
Broken high and low voltage plugs	Replace plugs
Lamp holder	Replace lamp/lamp holder
Poor engagement of lamp holder	Replace lamp/lamp holder
Lamp is broken	Replace the lamp
Lamp welding failure	Resolder

Possible Causes	How to Repair	Note
Lamp line is broken	Reconnect / replace lamp tubing	Depending on the protection status of the power board, it may be a backlight with a dark band or it may be a point-off.
Plug disconnected / burned	Replace plugs
Lamp is broken	Replace the lamp
Lamp welding failure	Replace the lamp

Possible Causes	How to Repair	Note
The light bar is short-circuited to ground	Insulate or change the light bar
Lightbar plug pin off	Change the light bar
The light bar cable plug is not fully inserted into the conn.	Reinsert the light bar
Lightbar conn. Pin is skewed	Change conn. Or re-insert the pin
Light strip conn. Off	Reweld Conn.	If the PCB copper foil comes off, the light bar needs to be replaced
Bad light bar	Replace the light bar

 

1. #Change the power board first to determine whether the LCD panel itself is defective.

2. #look at the appearance of the light bar to determine whether there is a problem with the light bar.

3. #Disassemble the backlight, confirm whether there is a short circuit with broken skin on the lamp strip, whether the plug of the lamp strip is fully integrated with the socket, whether the pin is aslant/off, whether the connector is off, and whether the LED bead is black and injured.

4. #The fault of the product is basically caused by the above reasons. If the appearance is fault-free, the lamp bar can be crossed to confirm whether the phenomenon follows the lamp bar, or the voltage of the lamp bar and the conduction condition between the lamp beads can be measured.