Problem
This is an LG edge LED lit LCD Display. The LEDs are on the bottom edge of the display. There are 2 bars (left and right) Each bar has many LEDs and a 6 pin connector. Each pin drive several LEDs thus is the highest current flow / heat junction. The weak lead free solder gradually fails with thermal expansion/contraction cycling and increases resistance. PSU will compensate up to a point, then when the current is too high, PSU just shut down the backlight causing a dark display. I have even seen one which the connector just fell off as solder points became completely detached.
Usually this problem starts manifesting itself with brighter settings only. Can work around by reduce brightness. Eventually, even the lowest brightness won't work anymore.
Skill Level
This repair requires quite high skill level. Furthermore, much of the LCD frame is sticky taped on and fragile LCD and frame construction makes it quite difficult. Gets easier with experience but most people reading this are probably first timers. Anyway, you must be fairly skilled and go slow… multiple days if necessary
Additional Docs
Previously made a PDF for repair directions. This posting is more update to date. Including it for reference
https://drive.google.com/file/d/13Gq...usp=share_link
Tools
2 drives are Torx-10 (T10) and Phillips.
Orient the iMac/TB Display
Screen assembly is heavy so when laying it on its back puts significant pressure on the stand hinge (it can break internally) Roll up a bath towel and use as support
Remove Protective Glass
Use 2 suction cups and pull up.
Remove 8 screws holding the display to the chassis
Use a decent size T10 handle (you will need to tighten it securely on reassembly to avoid thermal cycling popping noises)
Put 2 fingers around the tip of the T10 driver whenever removing or installing these screws. LCD glass surface is VERY soft (The removed protective glass is the real protection) and scratches easily. One slip and you will have a visible scratch on the LCD. I've actually accidentally dropped a small light weight T-10 onto the LCD and instantly got a scratch. Be really careful here
Disconnect the LCD
Pull up on the 2 black tabs and place a ~4” box to keep screen tilted while reaching in to disconnect the cables. Sometimes the tabs are broken or breaks easily. A L shaped hook tool / Allen key can help lift up by the corner.
4 connections to remove. First 3 with LCD supported by the box
Power cable connection to the main board is just under right edge of the board. Cable is also taped down to the Aluminum chassis. First remove the tape to release the cable. Then press the tab (blind since under the board) and pull gently to remove.
Next remove the 6 pin connector. Just pull straight out
Next unlatch the LCD connector (latch keep the connector secured against the board and need to be unlatched (rotated 90-180 degrees CCW) latch has a tab so its easy to grab. After unlatched, pull straight out
After these 3 connections are off, tilt the LCD screen up about 60-70 degrees and T-10 to remove the ground screw.
Now the LCD is free, remove and place somewhere safe if not working on it right away. Again, I tilt it and lean the LCD facing downwards to avoid dust build-up.
Opening the LCD
This is where all the complexity is and need to completely strip it down to get to the LED light bar connectors. This including removing 3 magnetic bars that secures the protective glass, layers of framing. Fragile LCD and its fragile edge buffer board. Diffuser sheet and light guide before finally getting to the inner most metal tub with LCD bars.
3 magnetic brackets
Each is secured by 4 T10 bolts. The 2 L shape ones have a really sticky surface and is quite difficult to remove the first time. Wedge tool is helpful. However, there is another couple of layers framing internally so probably want to review the steps ahead to make sure don't wedge into the wrong place.
Outer Metal Frame
This frame clams over LCD and internal diffuser/light guide/LED bar assembly and is one of the most difficult steps. Looks like this once removed
Foil Tape Removal
First remove the aluminum foil tape. Quite laborious. Can cut this IF know where to cut but maybe too dangerous for first timer. On reassembly need to be refoiled to avoid thermal cycling metal popping noises. Can just tape over cut foil but maybe best to remove all the old foil tape for first timers. Take a lot of time but is safer process to remove the frame without so many sticky parts and not sure where to pull / wedge etc.
Open T-Con LCD buffer board metal cover
After the side + corner foil tape removed. On to opening the LCD buffer board + T-Con flip metal cover.
Remove T-Con
Flip open T-Con to buffer board flex connection lock. I do the connection on buffer board side (less stuff to dangle later) Use both hands, 1 finger from each hand on both side of this long flex lock.
T-Con is taped down in 2 spots, just need to pull to separate. Once T-Con is freed from sticky tape, wiggle side to side to free the flex from buffer board.
Free Buffer board
Buffer board is also stuck on with double sided tape. Gently pull until they separate. Note buffer board has super fragile flex bonded to it and the LCD. Don't want to yank on this. So go slow and gradual until buffer board separates. A tool maybe helpful knowing with the double sided tape is. But the buffer board is thin and fragile so be gentle
Removing the outer frame
Peel back the 2x black lifting tabs off of the outer frame.
Finally able to remove this frame. But note all the fragile LCD buffer board is exposed now AND LCD screen can easily fall out once this frame is removed. Need to be ultra careful.
NOTE it is MUCH safer to wedge up frame from the opposite end to the buffer board for removal. This way you never squeeze the frame against buffer board to LCD flex. During reassembly, its the reverse, you want to insert the buffer board end first to keep it safe while clipping on the opposite end.
Dangle T-Con / buffer board cover and buffer board over the edge of the table so these don't get damaged. However, it is then harder to use a wedge to pry loose the metal frame clipped into the plastic frame underneath. But this is what I do. Tilt up and wedge open from the opposite (to the buffer board) end. 7 Clip points total (see removed outer metal frame pic above). First 2 corners are the hardest.
Maybe good to have a 2nd person to help handle LCD/tilt while the other find wedge points and free the 2 challenging corners.
Removing the LCD
Now maybe good time to wear gloves to handle LCD glass, diffusers and light guide.
Only 1 or 2 brown fabric tape on edge of the LCD blocks the LCD from coming out. Just peel these back, 2x suction cup to lift LCD onto a clean surface (I open a new trash bad (want to mark sure no oily surface) and line it on a table) Dangle the buffer boards over the edge so they hang free. Note the underside of the LCD is critical to not get dirty. The protective glass side can be easily cleaned. Any dirt getting to the underside require full LCD disassembly to remove.
Remove suction cup after moving LCD glass. On LCD TVs, I have seen suction cup leave a distorted mark on the LCD for a few hours if not removed for long periods.
Remove the plastic frame
This plastic frame holds the diffuser sheets and light guide and the LED bar and the tub they all sit in. 6 Phillips screws on one side of the plastic frame and wedge off its clips to the tub (fairly easy. Finally plastic haha)
NOTE there are other holes near the 6x screws for the magnetic bar. Probably good to mark which screws are for the plastic frame to avoid mixing it up (If use the wrong hole, will find out in the last steps assembling the magnetic bar and have to disassemble the outer frame again)
NOTE diffuser sheets will fall out as soon as plastic frame is removed so best to remove this while LCD is sitting on the metal tub with diffusers facing up. Rotate the tub over edge of the table to gain space to remove screws and unclip.
Remove diffuser, light guide, and white reflector sheet
There are 3 diffuser layers, a 1/4” thick plexiglass like light guide and a white reflector sheet on the Botton of the tub. Can all be removed together and rest it on a table on the back of the reflector sheet that was facing the tub. Use a clean trash bag to cover the top diffuser sheet.
NOTE if diffuser sheet is bent or crimped, it will be visible on the LCD with uneven light. Be very careful.
Finally the failed LED connector
No need to remove the LED bar (will destroy the heat transfer tape underneath) , 90 degree reflector from the tub (brittle and will break) Just clean off old solder on the connector and reflow nice fresh leaded solder. Protect the general area with Aluminum foil from soldering iron heat.
Note will need fairly good soldering iron. Area is small as are the traces.
Reassembly
Largely reverse. But there are few key points
Diffuser Reassembly
The tub has guide posts that diffuser sheet must align to. They are slippery so carefully align these
Snap and screw in plastic diffuser frame
Note to use the correct holes for this frame rather than magnetic bar hole.
Make sure screw is on tight. Use something bigger than a tiny Phillips driver. It is plastic hole so don't want to put too much torque.
Install LCD
Before installing LCD, it is critical to make sure no debris on top of the diffuser to sandwich under the LCD glass. Best not to wipe with lint cloth etc. Maybe gentle sprayed air is best. Again, avoid strong force (including air pressure) any damage on the diffuse will be visible as uneven backlight. Out of 10-15 LCDs I have refurbed, only 1 had a tiny hair that was trapped here and visible when finished and required a 2nd disassembly to remove.
Reinstall buffer board + T-Con
Its really important to reseal this cover properly or likely will produce popping noises between metal cover and metal tub and the rotating seam.
This step is really really important. The T-Con/buffer board cover is thin sheet metal folded against the sheet metal tub with a long seam between them. Cover is also a long piece only held on by 4 screws. If the foil and black retape is skipped, display will have random pop/click noises as it thermal cycles.
Magnetic bracket installation
Lay LCD face down on a flat surface (and make sure nothing scratches it) Use the flat surface as a guide to level the magnets on the magnetic bar. Note the LCD will rest on the foam around 4 sides of the LCD. This foam is easily damaged so be gentle rotating and sliding the LCD AND when working over the table edge (for better screw access) where it can damage the foam.
Make sure the bars are as straight as possible. The L bracket likely have slight bend due to difficult disassembly with its sticky inner surface.
Additional bonus problems
Light guide tape distorted
The 1/4” light guide has a tape near the LEDs. This tape is below the LCD viewing region. However, I have seen one where tape has bent towards the viewing region and appears as dark spots on the bottom of the screen. Cleaning off this tape with ISO fixed it.
Bleeding left vertical edge on LCD
Have seen quite a few TB Display with this. Appear when left unplugged in and gradually goes away in a couple of hours when plugged in and used.
PSU resistor drift
https://www.instructables.com/Fixing...le-27-Display/
This is an LG edge LED lit LCD Display. The LEDs are on the bottom edge of the display. There are 2 bars (left and right) Each bar has many LEDs and a 6 pin connector. Each pin drive several LEDs thus is the highest current flow / heat junction. The weak lead free solder gradually fails with thermal expansion/contraction cycling and increases resistance. PSU will compensate up to a point, then when the current is too high, PSU just shut down the backlight causing a dark display. I have even seen one which the connector just fell off as solder points became completely detached.
Usually this problem starts manifesting itself with brighter settings only. Can work around by reduce brightness. Eventually, even the lowest brightness won't work anymore.
Skill Level
This repair requires quite high skill level. Furthermore, much of the LCD frame is sticky taped on and fragile LCD and frame construction makes it quite difficult. Gets easier with experience but most people reading this are probably first timers. Anyway, you must be fairly skilled and go slow… multiple days if necessary
Additional Docs
Previously made a PDF for repair directions. This posting is more update to date. Including it for reference
https://drive.google.com/file/d/13Gq...usp=share_link
Tools
2 drives are Torx-10 (T10) and Phillips.
Orient the iMac/TB Display
Screen assembly is heavy so when laying it on its back puts significant pressure on the stand hinge (it can break internally) Roll up a bath towel and use as support
Remove Protective Glass
Use 2 suction cups and pull up.
- Thunderbolt Display lift from bottom corner
- iMac lift from upper corner
Remove 8 screws holding the display to the chassis
Use a decent size T10 handle (you will need to tighten it securely on reassembly to avoid thermal cycling popping noises)
Put 2 fingers around the tip of the T10 driver whenever removing or installing these screws. LCD glass surface is VERY soft (The removed protective glass is the real protection) and scratches easily. One slip and you will have a visible scratch on the LCD. I've actually accidentally dropped a small light weight T-10 onto the LCD and instantly got a scratch. Be really careful here
Disconnect the LCD
Pull up on the 2 black tabs and place a ~4” box to keep screen tilted while reaching in to disconnect the cables. Sometimes the tabs are broken or breaks easily. A L shaped hook tool / Allen key can help lift up by the corner.
4 connections to remove. First 3 with LCD supported by the box
Power cable connection to the main board is just under right edge of the board. Cable is also taped down to the Aluminum chassis. First remove the tape to release the cable. Then press the tab (blind since under the board) and pull gently to remove.
Next remove the 6 pin connector. Just pull straight out
Next unlatch the LCD connector (latch keep the connector secured against the board and need to be unlatched (rotated 90-180 degrees CCW) latch has a tab so its easy to grab. After unlatched, pull straight out
After these 3 connections are off, tilt the LCD screen up about 60-70 degrees and T-10 to remove the ground screw.
Now the LCD is free, remove and place somewhere safe if not working on it right away. Again, I tilt it and lean the LCD facing downwards to avoid dust build-up.
Opening the LCD
This is where all the complexity is and need to completely strip it down to get to the LED light bar connectors. This including removing 3 magnetic bars that secures the protective glass, layers of framing. Fragile LCD and its fragile edge buffer board. Diffuser sheet and light guide before finally getting to the inner most metal tub with LCD bars.
3 magnetic brackets
Each is secured by 4 T10 bolts. The 2 L shape ones have a really sticky surface and is quite difficult to remove the first time. Wedge tool is helpful. However, there is another couple of layers framing internally so probably want to review the steps ahead to make sure don't wedge into the wrong place.
Outer Metal Frame
This frame clams over LCD and internal diffuser/light guide/LED bar assembly and is one of the most difficult steps. Looks like this once removed
- Once this frame is removed, LCD can fall out so need to be quite careful handling and orienting.
- Frame is aluminum duct taped on and then need to be wedged off of few clips. Again, more internal framing so need to be familiar where to wedge.
- Frame folds over the T-Con and LCD buffer boards which are also double sided taped down.
Foil Tape Removal
First remove the aluminum foil tape. Quite laborious. Can cut this IF know where to cut but maybe too dangerous for first timer. On reassembly need to be refoiled to avoid thermal cycling metal popping noises. Can just tape over cut foil but maybe best to remove all the old foil tape for first timers. Take a lot of time but is safer process to remove the frame without so many sticky parts and not sure where to pull / wedge etc.
- Foil is along the 2 sides AND all 4 corners. Foil also over the flip metal cover for the LCD buffer boards and t-con.
- Side foil also cover a brown fabric tape that covers wiring so be careful and not cut that.
- To lift corner of tape off of metal chassis, a razor blade is helpful and much easier than fingernails.
- No need to remove the 3 foil tape that cover the LCD buffer board + t-con cover seam.
Open T-Con LCD buffer board metal cover
After the side + corner foil tape removed. On to opening the LCD buffer board + T-Con flip metal cover.
- 4 screws + numerous tape secures the cover.
- Remove ground strap and screw
- Peel off all black tape (can carefully save it and reuse but I used electrical tape instead to minimize chance of thermal cycling noises)
- After black tape removed, removed the remaining 3 screws
- Unlatch the LCD cable and remove it
- Disconnect 6 pin connector
- Disconnect LCD temp sensor connector
- Lift up the cover
- Carefully clear the temp sensor cable
Remove T-Con
Flip open T-Con to buffer board flex connection lock. I do the connection on buffer board side (less stuff to dangle later) Use both hands, 1 finger from each hand on both side of this long flex lock.
T-Con is taped down in 2 spots, just need to pull to separate. Once T-Con is freed from sticky tape, wiggle side to side to free the flex from buffer board.
Free Buffer board
Buffer board is also stuck on with double sided tape. Gently pull until they separate. Note buffer board has super fragile flex bonded to it and the LCD. Don't want to yank on this. So go slow and gradual until buffer board separates. A tool maybe helpful knowing with the double sided tape is. But the buffer board is thin and fragile so be gentle
Removing the outer frame
Peel back the 2x black lifting tabs off of the outer frame.
Finally able to remove this frame. But note all the fragile LCD buffer board is exposed now AND LCD screen can easily fall out once this frame is removed. Need to be ultra careful.
NOTE it is MUCH safer to wedge up frame from the opposite end to the buffer board for removal. This way you never squeeze the frame against buffer board to LCD flex. During reassembly, its the reverse, you want to insert the buffer board end first to keep it safe while clipping on the opposite end.
Dangle T-Con / buffer board cover and buffer board over the edge of the table so these don't get damaged. However, it is then harder to use a wedge to pry loose the metal frame clipped into the plastic frame underneath. But this is what I do. Tilt up and wedge open from the opposite (to the buffer board) end. 7 Clip points total (see removed outer metal frame pic above). First 2 corners are the hardest.
Maybe good to have a 2nd person to help handle LCD/tilt while the other find wedge points and free the 2 challenging corners.
Removing the LCD
Now maybe good time to wear gloves to handle LCD glass, diffusers and light guide.
Only 1 or 2 brown fabric tape on edge of the LCD blocks the LCD from coming out. Just peel these back, 2x suction cup to lift LCD onto a clean surface (I open a new trash bad (want to mark sure no oily surface) and line it on a table) Dangle the buffer boards over the edge so they hang free. Note the underside of the LCD is critical to not get dirty. The protective glass side can be easily cleaned. Any dirt getting to the underside require full LCD disassembly to remove.
Remove suction cup after moving LCD glass. On LCD TVs, I have seen suction cup leave a distorted mark on the LCD for a few hours if not removed for long periods.
Remove the plastic frame
This plastic frame holds the diffuser sheets and light guide and the LED bar and the tub they all sit in. 6 Phillips screws on one side of the plastic frame and wedge off its clips to the tub (fairly easy. Finally plastic haha)
NOTE there are other holes near the 6x screws for the magnetic bar. Probably good to mark which screws are for the plastic frame to avoid mixing it up (If use the wrong hole, will find out in the last steps assembling the magnetic bar and have to disassemble the outer frame again)
NOTE diffuser sheets will fall out as soon as plastic frame is removed so best to remove this while LCD is sitting on the metal tub with diffusers facing up. Rotate the tub over edge of the table to gain space to remove screws and unclip.
Remove diffuser, light guide, and white reflector sheet
There are 3 diffuser layers, a 1/4” thick plexiglass like light guide and a white reflector sheet on the Botton of the tub. Can all be removed together and rest it on a table on the back of the reflector sheet that was facing the tub. Use a clean trash bag to cover the top diffuser sheet.
NOTE if diffuser sheet is bent or crimped, it will be visible on the LCD with uneven light. Be very careful.
Finally the failed LED connector
No need to remove the LED bar (will destroy the heat transfer tape underneath) , 90 degree reflector from the tub (brittle and will break) Just clean off old solder on the connector and reflow nice fresh leaded solder. Protect the general area with Aluminum foil from soldering iron heat.
Note will need fairly good soldering iron. Area is small as are the traces.
Reassembly
Largely reverse. But there are few key points
Diffuser Reassembly
The tub has guide posts that diffuser sheet must align to. They are slippery so carefully align these
Snap and screw in plastic diffuser frame
Note to use the correct holes for this frame rather than magnetic bar hole.
Make sure screw is on tight. Use something bigger than a tiny Phillips driver. It is plastic hole so don't want to put too much torque.
Install LCD
Before installing LCD, it is critical to make sure no debris on top of the diffuser to sandwich under the LCD glass. Best not to wipe with lint cloth etc. Maybe gentle sprayed air is best. Again, avoid strong force (including air pressure) any damage on the diffuse will be visible as uneven backlight. Out of 10-15 LCDs I have refurbed, only 1 had a tiny hair that was trapped here and visible when finished and required a 2nd disassembly to remove.
- Extend the tub over the edge of the work table so LCD buffer boards dangle in air when LCD is installed.
- User suction cup to handle the LCD for reinstall
- Insert the 2 corner opposite of the buffer board into the slotted corner on the plastic frame
- Lay down the buffer board end, note the buffer board flex cables will lay into cut open channels in the plastic frame.
- Reattached the 1-2x brown cloth tape on the edge of the LCD.
- Extend the assembly over the edge of work table so buffer board + tcon cover can dangle over the edge
- Insert/install the buffer board end first
- Gently press and clip in the opposite end
Reinstall buffer board + T-Con
- Buffer boards and T-Con screw holes are also alignment posts for board screw holes. Line these up
- Insert the tcon to buffer board flex cable and latch it, lock the wide latch with 2 fingers from both hands.
- Use a little pressure to secure board against their double sided tape.
- Attach the LCD cable, 6pin cable, temp sensor cable
Its really important to reseal this cover properly or likely will produce popping noises between metal cover and metal tub and the rotating seam.
- Close the metal cover. Becareful not to smash the temp sensor cable
- Install 3 cover screws and ground cable screw.
- I use electrical tape to secure the cover seams against the tub
This step is really really important. The T-Con/buffer board cover is thin sheet metal folded against the sheet metal tub with a long seam between them. Cover is also a long piece only held on by 4 screws. If the foil and black retape is skipped, display will have random pop/click noises as it thermal cycles.
- Peel off the 3x foil along the seam and install new ones. This is a long metal to metal seam. Important to secure this area without movement as display thermal cycles.
- Cut strips of foil tape and seal the left and right side similar to original.
- Foil tape all 4 corners
- Foil tape both edge of the buffer board cover firmly against the tub.
Magnetic bracket installation
Lay LCD face down on a flat surface (and make sure nothing scratches it) Use the flat surface as a guide to level the magnets on the magnetic bar. Note the LCD will rest on the foam around 4 sides of the LCD. This foam is easily damaged so be gentle rotating and sliding the LCD AND when working over the table edge (for better screw access) where it can damage the foam.
Make sure the bars are as straight as possible. The L bracket likely have slight bend due to difficult disassembly with its sticky inner surface.
- Install bottom straight bar. Make sure the 2 black lifting tabs are under the bar (between the bar and outer metal frame) Press down with magnet flush on the table and therefore flush against the foam around the LCD on the outer metal frame.
- Tighten the screws reasonably tight but not over torque.
- Install L brackets. Press down and alignment magnet to table surface. Top maybe more difficult to get it completely flush to table surface.
- Tighten all the screws on the board, fan, speaker, etc on the chassis. Note some are plastic and maybe rubber bumpers so avoid over torquing.
- Insert LCD top (camera side) first, tilt 60 degrees and attach ground cable (good to have 2 people : 1 hold LCD, 1 attach ground cable)
- Lay on 4” tall box, attach and latch lock LCD cable, 6 pin cable, power connector
- Drop LCD onto chassis, 8x T10 screws. Again, hold 2 finger over driver head to avoid slipping and scratch LCD. I usually leave the screws just a little loose and tighten them when the screws is vertical on the stand. Tighten 8x T10
- Clean the LCD before closing up. Use microfiber cloth. LCD surface very soft.
- Line up guides top near camera. Make sure top is flush the close the bottom. 2 people helpful as magnet is strong and makes it more difficult with 1 person.
Additional bonus problems
Light guide tape distorted
The 1/4” light guide has a tape near the LEDs. This tape is below the LCD viewing region. However, I have seen one where tape has bent towards the viewing region and appears as dark spots on the bottom of the screen. Cleaning off this tape with ISO fixed it.
Bleeding left vertical edge on LCD
Have seen quite a few TB Display with this. Appear when left unplugged in and gradually goes away in a couple of hours when plugged in and used.
PSU resistor drift
https://www.instructables.com/Fixing...le-27-Display/
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