Sharp LC-50LB370U no backlight

[73buick]

Got picture and sound, no backlight. Unplugged backlight connector and tested VLED1+/- and VLED2+/- using LED tester. VLED1 lit up, 71V steady. VLED2, no lights and tester never dropped voltage. Sound like VLED2 circuit is bad, correct? Looks like known issue on this model, too.

[budm]

Re: Sharp LC-50LB370U no backlight

You need to test each VLED+ pins and - pins with ref to chassis GND with LED connector in palce and every time the TV is turned on, do tah on each pin to see how high the Voltage jumps up to and then wait 10 seconds to see how low it goes down to on each pin, whcich means you need to leave the probes in place during the test.

[73buick]

Re: Sharp LC-50LB370U no backlight

Will do. And just to be clear, the test I did was with a LED test power supply, testing on the pigtail connector itself directly to the LED strips. I realized that wasn't clear in my post.

[R_J]

Re: Sharp LC-50LB370U no backlight

Seems like VLED2 string is open, it only takes one led to be bad. If you open the panel to repair that string, check the other string as well to make sure all led's are working.
one led can short allowing the others in the string to light with the tester, but with the tv power supply it would fail and shut down

[budm]

Re: Sharp LC-50LB370U no backlight

Will do. And just to be clear, the test I did was with a LED test power supply, testing on the pigtail connector itself directly to the LED strips. I realized that wasn't clear in my post.

OK, I see now, so yes it looks like you have open circuit in the LED 2 string some where.

[Kimbrelt]

Re: Sharp LC-50LB370U no backlight

Hi, I have a similar no LED backlight issue on the same model TV. The house experienced a lightning strike coincident with the failure of the backlight.

I used a multimeter to check DC voltage from chassis to VLED1 and VLED2. Relative to chassis ground both the VLED + pins were in the area of 65VDC after powering on the TV and letting it settle.

The voltage from ground to VLED - pins were somewhere less than a volt, but it was not 0. Then I measured the voltage between VLED + and - and it was less than a volt for both VLED1 and VLED2. All measurements were with the connector in place.

I'm not sure what the correct measurements should be, especially since the voltage between the + and - pins appears to be less than 1 Vdc. Does this seem like a power board issue, or should I be looking at the LEDs?

[nomoresonys]

Re: Sharp LC-50LB370U no backlight

Kimbrelt with tv powerr cord UNPLUGGED, unplug cable from powerboard to mainboard, plug power cord back in see if backlights come on.

[Kimbrelt]

Re: Sharp LC-50LB370U no backlight

Thanks, nomoresonys. I followed the directions and unplugged the multiconductor cable from the power board to the main board and plugged the power cord back in, but there was no backlight.

I also unplugged the LED cable from the power board and measured the voltage between VLED + and VLED - and got less than a volt.

Not sure if the power board is supplying the necessary voltage to the LEDs.

[nomoresonys]

Re: Sharp LC-50LB370U no backlight

Shopjimmy says they are known for backlight failure much more often than powerboard failure, see note in red: https://www.shopjimmy.com/sharp-pltv...c-50lb370u.htm

[Kimbrelt]

Re: Sharp LC-50LB370U no backlight

Copy, that. I also saw that note on shopjimmy. It's just odd that a probable lightning strike at the house would take out the LEDs. I understand if one is shorted the rest would not light, but the VLED - voltage out of the power board seems to be ungrounded, which gives the indication that the powerboard is at fault. I just don't have much experience with the backlights.

[budm]

Re: Sharp LC-50LB370U no backlight

Hi, I have a similar no LED backlight issue on the same model TV. The house experienced a lightning strike coincident with the failure of the backlight.

I used a multimeter to check DC voltage from chassis to VLED1 and VLED2. Relative to chassis ground both the VLED + pins were in the area of 65VDC after powering on the TV and letting it settle.

The voltage from ground to VLED - pins were somewhere less than a volt, but it was not 0. Then I measured the voltage between VLED + and - and it was less than a volt for both VLED1 and VLED2. All measurements were with the connector in place.

I'm not sure what the correct measurements should be, especially since the voltage between the + and - pins appears to be less than 1 Vdc. Does this seem like a power board issue, or should I be looking at the LEDs?

Try taking the measurement between GND and V+, V- by leaving the probes in place and then turn the TV on to see how high the Voltage jumps up to and then wait 10 seconds to see how low it goes down to, do that on each pin of the LED connector with connector in place. We also need the Voltage readings of the power supply connector that goes to the main board.

[Kimbrelt]

Re: Sharp LC-50LB370U no backlight

ok. I'll give that a try tonight. from what I recall on an earlier test, I didn't see the VLED + voltage spike and then settle. It was just 65vdc during and after the power on test. No voltage on VLED - relative to chassis GND.

I'll try to check the power supply connector to main board. I don't recall off hand, but hope there are markings with the various pins to determine which are for power. thx.

[budm]

Re: Sharp LC-50LB370U no backlight

ok. I'll give that a try tonight. from what I recall on an earlier test, I didn't see the VLED + voltage spike and then settle. It was just 65vdc during and after the power on test. No voltage on VLED - relative to chassis GND.

I'll try to check the power supply connector to main board. I don't recall off hand, but hope there are markings with the various pins to determine which are for power. thx.

The VLED - which the the Cathode return pins for the LED string should show lots more than 1V if there is no open circuit in the LED string, the LED + is the Anode pin of the LED string.

[Kimbrelt]

Re: Sharp LC-50LB370U no backlight

Couldn't get to check it last night, but here's what I found tonight.

The VLED + pins would jump up to ~97vdc and then settle down to 65vdc relative to GND.
The VLED - pins did not jump, but stayed around .05vdc relative to GND.

I checked the pins on the cable to the main board. All the voltages seemed to be okay based on the pin description, except for the 2 pins that were supposed to be +15.5vdc. Both of them were around .8vdc.

[budm]

Re: Sharp LC-50LB370U no backlight

Couldn't get to check it last night, but here's what I found tonight.

The VLED + pins would jump up to ~97vdc and then settle down to 65vdc relative to GND.
The VLED - pins did not jump, but stayed around .05vdc relative to GND.

I checked the pins on the cable to the main board. All the voltages seemed to be okay based on the pin description, except for the 2 pins that were supposed to be +15.5vdc. Both of them were around .8vdc.

1)You have open circuit on the LOAD (LED string, wire, or linking connectors) that is why the boosted Voltage went up to 97V so it goes into shutdown and goes back down to 65V, basically you have Voltage feeding the Anodes but nothing comes out on the Cathode sides.
2) The 15V circuit is not used so the parts are not installed, that is why you do not have the 15V.

[Kimbrelt]

Re: Sharp LC-50LB370U no backlight

Thanks, budm. Sounds like both LED strings must have an open. It looks like I can either buy the LED mounted strips, or just the LED pack. I don't have any of the tools to replace the LEDs, so I'll do a cost analysis to determine if it's worth pursuing.

If I do tackle the repair, I'll report back how it goes. Thanks for all the help on this thread.