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Sharp Roku LC-55LBU591U Back light is on but no picture. - RESOLVED

Hello. I have a Sharp Roku LC-55LBU591U TV that has no picture but seems to have all other functions. When using the remote I can hear the normal sounds as I’m making selections but the screen is simply black with a back light. I unplugged the ribbon cable to the t-con and half of the screen went white.

I tried to test voltages at the t-con board but I’m not really sure what most of the test points should be reading. I get a decreasing range of voltages at the various ribbon test points.

I will post pictures momentarily.

Any help is appreciated.

Update (07/14/2020)

Pictures:

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Update (07/15/2020)

Main board:

Block Image

Update (07/24/2020)

I am proud to report that the replacement main board solves the problem. Full functionality and picture has been restored to the tv.

thanks so much for the help

Beantwoord! Bekijk het antwoord Dit probleem heb ik ook

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Hi @chillin014 ,

Just verifying the following:

No video at all when using different inputs, input types e.g. coax, HDMI, Component video, input cables, signal sources e.g dvd player and also no TV menu screen?

Audio works though?

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@chillin014 ... please test the 5a 32v fuse on the tcon board. (power on state with backlight lit) test both side and list voltage here. also, test vdd4 and vdd12 and list the voltage here. if no voltage is present, the likely culprit is the main board. post a pic of your main board so we can see whats going on there as well.

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@jostewcrew Fuse seems to be good. 12 V coming through it. I could not find VDD4 or VDD12 for the life of me. Searched all over the board. Can you point me in the correct vicinity? I've added a picture of the main board to my original post.

@jayeff Audio works fine. Admittedly I haven't tested the inputs because I have no idea how to cycle through them. It's a roku remote and no designated input button. I can hear the sounds when I use the remote like its searching through the menu.

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@chillin014 .. if looking at the main ic on the tcon, to the right youll see a smaller ic. vdd4 is directly above this as a test point. vdd12 is the ic immediately left and the test point is above this.

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@jostewcrew I found vdd12 but I couldn't find vdd4, do you mean VCOM?

Vdd12 - 6.36 V

VCOM - 1.18 V

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At the very top, the grey wire Ribbon that connects your PCB to your Main Control Board should have a pin out diagram on one of the boards by the connector. by the looks of the picture, this pinout diagram is located on the power board side of the connection (right above the connector). Throw your meter on DC and Test your 5v Standby, 12v and 24v rails (if there are any) BE CAREFUL NOT TO SHORT ANY PINS THIS CAN KILL THE PCB IN SOME CASES

If the voltages don’t line up with the Pinout Diagram, unplug the main control side and see if the voltages jump up where they are supposed to be. (remember nothing will kick on until the 5v standby is pulled low) if the voltages aren’t there at all, you have a bad PCB. if the voltages are there only when the Main control board is unplugged, you have a drain somewhere in your main control board that’s creating a voltage drop from the Power Board causing it not to run the GPU or other components properly. at this point unless you are good at component level repair, its time for a new Main control board.

Here is where to get parts for your Model SHARP tv

If going for a main board MAKE SURE it lines up with the model code as there are multiple versions of this board for this model.

https://www.shopjimmy.com/sharp/lc-55lbu...

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Thanks, I've done what you said and recorded the readings with the main board both connected and disconnected. Hope this helps.

Main board connected:

(1. GND) - Good

(2. GND) - Good

(3. PWM) - 2.47 V

(4. SW) - 2.35 V

(5. 12V) - 12.14 V

(6. 12V) - 12.14 V

(7. STB) - 2.54 V

(8. GND) - Good

(9. 18V) - 20.1 V

(10. 18V ) - 20.1 V

Main board DISCONNECTED:

(1. GND) - Good

(2. GND) - Good

(3. PWM) - 0 V

(4. SW) - 0 V

(5. 12V) - 12.2 V

(6. 12V) - 12.2 V

(7. STB) - 0 V

(8. GND) - Good

(9. 18V) - 21.2 V

(10. 18V ) - 21.2 V

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with your testing…. your power supply is fine, as well as your backlights. based upon the readings of your tcon board, it is receiving the proper voltage to operate, but its not getting the proper signal from the main boards processor. In your case, the main board is the cause of the signal error. this can be caused by old, cracked solder connection under the processor itself or a failed chip. you could try heating the processor with a hair dryer to see if you can get a signal (essentially reflowing the solder balls) if it comes back, it wont last but will confirm the issue. if it doesnt, you can try to heat for a longer period of time…( with a hair dryer, anywhere from 5 to 10 mins..careful not to move the chip.)

of course you can find a reputable vendor that can take your order, help with symptom vs solution and accept any parts back with no fee if it doesnt correct. i recommend electropartsonline for this, shop jimmy is another good source. i checked for an exact match for your tv, both vendors do not have in stock. i have one other vendor i use, but they have a restocking fee if board is sent back. your troubleshooting confirms the main board, but there are always chances. here is a link to your exact board needed for the repair:

https://www.tvpartstoday.com/216995-Best...

if you have any other questions, please post back here for assistance. please also update the post with success for others to use as reference.

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I sure appreciate the help! Do you mind telling me which voltages would have been acceptable on the T-con board?

I'll order a main board replacement and try the hair drier in the mean-time.

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12v is what typically powers the tcon. you have this present. vcom should be around 6v..its only getting 1.3v . vref is going through after a booster so being higher that the 12v from the main board indicates the rest of the board is working.

in addition, another test was to disconnect one side of the tcon at a time and look for a change. you already indicated this test and that the tcon is affecting the picture with a white display on the side disconnected. i took this into consideration.

your tcon isnt receiving proper signal to convert to an image. (there still is a possibility that the processing chip on the tcon is an issue, but other faults would have shown themselves) if youd like to test this processor as well to cover all bases, you can heat up the same way as the main board or freeze with a can of freeze spray.

pro tip:

do one board first, look for change, repeat on other. if heating, remove board from tv and lay on flat surface and let cool off without movement after for a few mins.

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@chillin014 ... glad to hear the replacement worked. good job on the troubleshooting and thanks for the update. keep up the good work.

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