Panasonic LED TV TC-40A420U - Replaced Backlight LEDs but now no image/display or garbled

[Diah]

yours Point 1: you have the lines distribution at the middle because the LVDS was in place.. so we cant say right now which exactly caused this.lines.
points 5 , when i wrote to take the 12V from LVDS socket... it mean we disconnect the LVDS totally, and jumpers thin wire between this 2 points as in photo.

you disassembled the screen... and start to repair LED, but before that did you saw picture on the screen with touch light ???

[signal.quest]

Since, I blocked all pins using tape but the 12V source on the LVDS cable on the panel side of the connection isn't that the same as jumpering trom the points you mentioned? If not, what am I not understanding about this?

Before disassembling the TV I did not test using a flashlight (torch) to see an LCD image. I was not aware of that method until after I had already taken it apart.

I have watched youtube videos and I am concerned about the panel side tabs or a shorted line. I fear that I was not aware enough when reassembling the panel and might have accidentally damaged something. I also am not confident I placed the diffuser sheet layers in the correct order. I still think I should see some image even if I have an error?

[Diah]

Since, I blocked all pins using tape but the 12V source on the LVDS cable on the panel side of the connection isn't that the same as jumpering trom the points you mentioned? If not, what am I not understanding about this?

Before disassembling the TV I did not test using a flashlight (torch) to see an LCD image. I was not aware of that method until after I had already taken it apart.

I have watched youtube videos and I am concerned about the panel side tabs or a shorted line. I fear that I was not aware enough when reassembling the panel and might have accidentally damaged something. I also am not confident I placed the diffuser sheet layers in the correct order. I still think I should see some image even if I have an error?

diffuser sheet order never being problem as you can correct them if they are wrong.
about your masking the LVDS cable... the end of the cable had 2 faces contact. when you mask one face the other will be in touch.... and if you masking the 2 faces you had add thickness to the LVDS which make the contact pin at socket went inside to catch the new thinness....
disconnecting LVDS the only one things we can work with out adding any another damage or issue which make you lost.

this detail explanation not only for you.... but who didn't follow the adviser.. then all of us will be in lost., no need to spend time any more... stay check Youtube and your mind please.

[signal.quest]

My sincere apologies. I have now disconnected the LVDS cable and jumpered the 12V connection points that you suggested. The results are now a complete black screen when both yellow connection cables are connected. When the cables are disconnected the screen is white where the cables are disconnected and black on the opposite side. You are correct that there are no colored lines down the center now. Please tell what is next to test. Thank you very much.

[Diah]

My sincere apologies. I have now disconnected the LVDS cable and jumpered the 12V connection points that you suggested. The results are now a complete black screen when both yellow connection cables are connected. When the cables are disconnected the screen is white where the cables are disconnected and black on the opposite side. You are correct that there are no colored lines down the center now. Please tell what is next to test. Thank you very much.

its okay... no more test.. don't play with panel any more. its fine.i suspect the LVDS cable or the LVDS socket at mainboards side..... i don't know how you stored the TV till you worked on it again...
clean reset at mainboards side... play pressure on the MB at several area and on the sockets see if there any change

[signal.quest]

I tried cleaning and reseating the LVDS cable on both mainboard and panel sides. I also tried applying pressure to the cable connector. But there is still black screen. I noticed this time the mainboard makes a faint (hard to hear) ticking sound when the power is initially connected. This sound repeats each time I disconnect and reconnect the power. I also have never heard audio coming from the mainboard when I had laptop connected to the HDMI to play video file.

[signal.quest]

I just connected laptop to HDMI and randomly pressed the side buttons until I heard audio coming from the laptop. Sadly, still nothing but black screen on LCD panel. I tried again to tap the connector on both board with no success.

[EazyBone]

Tedious but to test lvds cable you can turn multimeter to beep beep mode and go pin by pin and test if each one has continuity from one side to next. This helped me once and I found cable to be bad. It only had 9 pins though so it wasn't so bad. Interesting journey here so far.

[signal.quest]

EazyBone, thanks for advice. I will try to do that soon. I reseated the yellow connection cables and now the faint clicking sound on the main board has gone away. But still black screen on display.

[Diah]

if you put every things back and removed the jumpers you did before.. and screen remain black....check the LVDS cable and be sure too you have 12V on Fuse FP1 from both side.
good luck

[signal.quest]

Yes, I have 12V after removing everything. Can you please tell me what other test points on the panel I need to check for specific voltages? Is there a way I can test for shorted lines on the panel? Thank you.

[Diah]

Yes, I have 12V after removing everything. Can you please tell me what other test points on the panel I need to check for specific voltages? Is there a way I can test for shorted lines on the panel? Thank you.

why you are still considering on panel... you did the 12V test and removed the half screen and you got clean half white and black.. do you wish to scrap the panel ?

so my questions... did you removed the masking you did before on LVDS cable at Panel side ? and if used before scotch Tape... be sure you clean the pins as it left glue behind. which ,make bad contact.....
after you clear the issue above,.. try mask at MB LVDS sockets pins 49-50-51 then turn on .. see the screen what it will show. ( this test to stop first p+ p- which drive panel with interrogated T-CON to start test pattern ) ... hopefully it will work on yours Panel.

[signal.quest]

I did try to clean all connections on the ribbon cable and board receptacles. This did not make a difference. Next, I will attempt to mask the pins you suggested. Thanks again.

[signal.quest]

I masked the last 3 pins as in your diagram with red box. However, the display is still black.
Is it possible that the main board is working for audio and HDMI identification by laptop (Panasonic) but the video output is failed? Perhaps the video processor is bad or something similar? Thanks.

[Diah]

I masked the last 3 pins as in your diagram with red box. However, the display is still black.
Is it possible that the main board is working for audio and HDMI identification by laptop (Panasonic) but the video output is failed? Perhaps the video processor is bad or something similar? Thanks.

this is sign of issue look like no LVDS cable connected or no Data from Mainboards.... this why you need to find sign of life on Mainboards like the Optic Audio output if it lit red or not. no mean MB didn't work., yes need to check first LVDS cable pin to pin from both side of connectivity

[signal.quest]

I tested the cable and it tested good. I then replaced the logic (A) board with the same model from ebay (it was used). The replacement board briefly powers on the backlight then goes into standby mode. I suspect the replacement board is bad. Maybe it has a power issue that is easily repairable? I check caps for shorts and did not find any. Do you have any advice on logic board troubleshooting/repair? Thanks.

[signal.quest]

I tried another used main logic (A) board and got slightly better results. I could see some text. This makes me think all of the logic boards I am trying have different issues. Or could this mean the LCD panel or integrated TCON has failed? Please see the attached pictures and let me know your thoughts. I greatly appreciate your help.