Sharp LC-50LB261U - has audio, no picture

[ElectronicNovice]

Good Afternoon,

Working upon my neighbors TV that has audio but no picture. I have performed the reset functions of holding down Input and Vol - while plugging in etc. No success
I pulled the back off and checked upon the plug from the power supply to the main board - 715G6840-M0D-000-0041
Power board - 715G6335-P01-003-003M
I am getting over 12 volts - about 19 - 19.5 volts with the main board plugged in and 15.5 volts with the main board unplugged.
The LED back light does not come on but a faint picture is present. Replaced the T-Con board already since it is a generic $8 part.
The backlight came on after replacing the T-Con board and then went off about 15 seconds later.
I verified all the caps on the main board and getting good readings.
Per my user name, I am brand new to this. Any help is appreciated!

Thank you,

Aaron

[Doc38343]

Re: Sharp LC-50LB261U - has audio, no picture

That power supply looked so familiar.. I have one (scrap/donor) looks just like it from one i repaired in the past.
BUT it sounds to me like your having a backlight issue, is this a LED or CCFL type?
It would help everyone here help you if you took pictures of the full back and then each board (like you did the power supply)
Also on this power supply the voltages are clearly marked so it also would help
if you read each on pin for pin and recorded it for us. One set with TV off but plugged in. Another with it turned on and audio on...
Thanks

[budm]

Re: Sharp LC-50LB261U - has audio, no picture

Is the the exact match of your board?
Sharp PLTVDV751XXPR Power Supply / LED Board for LC-50LB261U.
Pictures provided by shopjimmy.
http://www.shopjimmy.com/sharp-pltvd...-50lb261u.htm#
You need to test the Voltages at those two connectors on the cold side of the circuit.
Make sure to put pin names and follow by pin Voltage readings.

[ElectronicNovice]

Re: Sharp LC-50LB261U - has audio, no picture

Doc - Trying to obtain more photos and information on the boards.

Bud - That is the exact power board I am working upon. Is there a way to jumper pins and force the LED back light to turn on? On the Pin out - all voltages should be 12V except the 3.3 volts I get to keep the main board alive. I am getting 15.5V with main board unplugged and about 19V with the main board plugged in. I am getting about 65V at the LED plug but no voltage information is printed on the board, just LED V+

[budm]

Re: Sharp LC-50LB261U - has audio, no picture

Run the power supply board by itself without the cable going to the main board connected, you will need to use 1K resistor between 5V and PS-ON to force on the power supply, 1K between O/F pin and 5V to force on the backlights, and may be 1K between DIM and 5V if the backlights still does not come on.
BTW, what is the Voltage rating of the cap C8110?

[ElectronicNovice]

Re: Sharp LC-50LB261U - has audio, no picture

Afternoon guys! Finally have had some time to tinker back with the television set. I put the boards back on the back side and obtained photos for you Doc.
Bud- I need to obtain a resistor to jump those pins you mentioned. Do I need to test that capacitor on the bench with the power supply plugged in?
The readings I obtained from the power board
VLED1 and 2 + 65 Volts - Power On - Standby light blue

To Main Board Plug - Power Off - Standby light Red
All 12V - 15.5V
No voltage on DIM, O/F, PS_ON, or NC
Power On - Standby Light Blue
All 12V - 19.6V
3.3V on DIM, O/F, PS_ON, and NC

[Doc38343]

Re: Sharp LC-50LB261U - has audio, no picture

Good pictures and great info. You need to do just like budm said to see if the backlights fire up. you could have a failing led... OR it could be one of the other boards that control the leds. Testing is essential....


added the service manual should help a bit

[ElectronicNovice]

Re: Sharp LC-50LB261U - has audio, no picture

Doc - Thank you for the service manual!
Bad news, my toddler cracked the LCD panel... my fault for leaving my work room unlocked!
Still, I have the front panel off. I am rigging up a power source to test the LED's just to confirm it is a bad back light.
If you would like the boards off the TV, PM me an address and I can ship them off to you pro bono.

[Dirtdenali05]

Re: Sharp LC-50LB261U - has audio, no picture

Newbie here. I have the same tv and it has sound but no picture. If i shine flashlight at it in dark room I can see a picture. I have checked the voltage on the led plug with power on an I get 0 volts. Do you guys think i need to replace the power supply board or do I need to try and get the leds to turn on by process mentioned above?

[budm]

Re: Sharp LC-50LB261U - has audio, no picture

Newbie here. I have the same tv and it has sound but no picture. If i shine flashlight at it in dark room I can see a picture. I have checked the voltage on the led plug with power on an I get 0 volts. Do you guys think i need to replace the power supply board or do I need to try and get the leds to turn on by process mentioned above?

What did you use for the Gnd for your meter when you test that LED+?
What DCV do you have as in post #6?

[fredearl]

Re: Sharp LC-50LB261U - has audio, no picture

Hello, I'm new to this forum and am trying to fix my tv that has no picture but has sound. It is a Sharp LC-50LB261U. I have done some basic troubleshooting by doing the flashlight test. With the flashlight test, I can see the images so it appears to be the power supply/inverter board or the CCFLs. I have 65 volts at the output of the power supply/inverter board on pins +vled1 and +vled2. Does that mean that my power supply/inverter board is good?

[budm]

Re: Sharp LC-50LB261U - has audio, no picture

Leave black probe of the meter on the chassis, red probe on +vled1 then turn the TV on and see how high the Voltage jumps up to before settling down to 65VDC then turn off the TV, do the same for +vled2; report the result.
Repeat the same test without the LED connector in place; report the result.

[fredearl]

Re: Sharp LC-50LB261U - has audio, no picture

With the connector in place, the voltage jumped up to 97 volts before settling down to 65 for both +vled1 and +vled2. Without the connector in place, I got the same results.

[budm]

Re: Sharp LC-50LB261U - has audio, no picture

With the connector in place, the voltage jumped up to 97 volts before settling down to 65 for both +vled1 and +vled2. Without the connector in place, I got the same results.

OK, so the boost converters are working, now you need to repeat the test on the -VLED1, -VLED2.

[fredearl]

Re: Sharp LC-50LB261U - has audio, no picture

with the connector in place and the power off the voltage is anywhere from .01 volts to .9 volts for both -vled1 and -vled2 (they are usually the same). When powered on they go to 0 volts for about 2 seconds then back to where they were before it was powered on. The behavior is the same without the connector in place. The voltage on -vled1 and -vled2 seems to be all over the place. Sometimes I see it as high as 3.5 volts with the power off. What should the voltage be on those pins?

[fredearl]

Re: Sharp LC-50LB261U - has audio, no picture

From what I can tell from my test results and from reading other websites, it appears to be the power supply module so I ordered one from Securecore Solutions. They have a large inventory, great prices, a warranty, and a very good return policy. I will install it next week and update you with my results. Thanks budm for your help.

[budm]

Re: Sharp LC-50LB261U - has audio, no picture

Well, it looks like you have open circuit in the LED strings.

[fredearl]

Re: Sharp LC-50LB261U - has audio, no picture

Thanks budm, is there any way to test for discontinuity in the led strings?

[budm]

Re: Sharp LC-50LB261U - has audio, no picture

You need something like this: http://www.shopjimmy.com/sid-gj2c-le...ght-tester.htm

[fredearl]

Re: Sharp LC-50LB261U - has audio, no picture

Well, I guess I'll need to get me one on those if I am going to work on tvs. I assumed it was the power supply because of the voltage that I was seeing on -vled1 and -vled2 and the fact that we had power going on an off the night before it failed. Hurricane Irma came through here last Monday night and the tv worked for about 3 seconds when I turned it on Tuesday morning. The video quit but the audio was still working.

Is there any other way to test the led strips without purchasing a backlight tester? Could I bypass the strips one at a time until I find the one that is bad? Just looking for another way to do it so I don't have to purchase something that I may only use a time or two. Is this TV worth fixing or should I trash it and get a new one?