sharp LC-50LB601U No backlight

[Misterfixit]

I have a sharp LC-50LB601U that has no backlight. It starts up fine, and has sound, however, I can only see the screen using a flashlight. I took the tv back off and measured voltages at the power supply connector CN-8601. As you can see by the picture, the led connector has 2 sets of 2 red wires and 2 sets of black wires going to the led backlights. Between each set of black wires and corresponding red wires, I measure 30 volts (pins 1 and 4) and 30 volts from pins 8 and 12. From ground to each set of red wires ( pins 1 and 12 to ground), I measure 35 volts. This voltage seems very low compared to other led TV connections from, the power supply to the backlight leds on other sets. In looking at the back of the power supply circuit board led connector or cable to main circuit board, I cannot see any printed voltages of any mention of the typical 5volt pulse to turn on the power supply for the backlight. I would appreciate any help or even a service manuals that anyone has on this set.

[lotas]

Re: sharp LC-50LB601U No backlight

You need to measure the voltage at the output of the LEDs immediately at the moment the TV is turned on, it should be high, but if the LEDs are not working properly, the protection in the led driver will work and the voltage will immediately drop at the output.
You need to measure between +VL and -VLED1, also between +VL and -VLED2.

[Misterfixit]

Re: sharp LC-50LB601U No backlight

Here is a picture of the complete back of the power supply to the above post.

[Misterfixit]

Re: sharp LC-50LB601U No backlight

Thanks lotus, I am using a fluke vom, when I turn on the set using the remote control on your suggested led connector points, I only see 30 volts on both of the measurements. It could be that the protection circuit is way faster than what the digital meter can display. Before, I tear into the led backlights, is there anything else you can think of to verify it is the led backlights and not the power supply board. Thx

[lotas]

Re: sharp LC-50LB601U No backlight

You have only one output (led1 is connected to led2), four lines of 8 leds (3V), so 30v is normal at the output, these 30v are fed to each bar separately. What voltage are the electrolytic capacitors at the output (according to the photo, it seems to be 50v)?

https://aliexpress.com/item/1005003682808300.html

[Misterfixit]

Re: sharp LC-50LB601U No backlight

Yes Lotas, the voltage rating of capacitor c8605 is 50 volts. Also, C8634,c8635,c8629 and c8630 is also 50 volts. By the fact that you sent me the Ali express link for the backlight led strips, do you feel pretty confident that I have a serious of failed led backlights? Thank you

[Misterfixit]

Re: sharp LC-50LB601U No backlight

Thanks Lotas for all you help. I ordered the led strips from Ali express. They came in about a week ago and i just installed them. The set now works as though it is like new.
After placing the old led strips on the bench, I started testing them. It turns out that the leds were all working properly. It was the individual connector between the each strip that makes each row, was bad. As I wiggled the connector, the led strips all lit up. There are 4 individual rows of leds (each row has 8 leds, total 32 leds for this set)that are used in this set. One section of each row contains 5 leds is connected to another section of 3 leds through a connector. This connector in each row is what appeared to cause my backlight problem.

[Misterfixit]

Re: sharp LC-50LB601U No backlight

Unfortunately, to get to this connector, you have to disassemble the whole front end of the set. I could have simply soldered jumper wires across this connector once I had the whole lcd, diffusers, etc off, but I decided to just replace all of the led rows. Thanks again