Sharp LC-50LB481U Backlight problem

[Eaglecrest]

I have one of the stated TVs. It powers on fine, but has no viewable picture. Sound is there and if you shine a light on the screen you can see the video content. Thus there is a problem with the back light. I am assuming this could be because of the power to the backlight or because the light itself is the problem.

Below are the readings from the power supply:

Pin Label TV Off TV On Pin Label TV Off TV On
1 On/Off 69 mV 69 mV 9 Dim 3.3 V 3.3 V
2 NC 0 0 10 PS_On 3.3 V 3.3 V
3 Grd (-4.5 mV) (-4.5 mV) 11 Grd (-.8 mV) (-.44 mV)
4 Grd (-4.6 mV) (-4.6 mV) 12 Grd (-.8 mV) (-.844mV)
5 12V 12 V 12 V 13 12V 12 V 12 V
6 12V 12 V 12 V 14 12V 12 V 12 V
7 12V 12 V 12 V 15 12V 12 V 12 V
8 5V 0 0 16 5V 0 0

I don't understand the readings as I would expect different readings with power off (i.e., plugged in, but switch off) vs. power on (switched on). Though from above there is no 5V reading. Does this seem to make sense to anyone?

[nomoresonys]

Re: Sharp LC-50LB481U Backlight problem

Looks like the mainboard is not sending the backlight on (ON/OFF) signal to turn on the backlights.

[budm]

Re: Sharp LC-50LB481U Backlight problem

Lets see the pictures of the whole backside of the TV showing all the boards.
It is correct that the ON/FF (BL-ON) is not present to turn on the backlights circuit.

[R_J]

Re: Sharp LC-50LB481U Backlight problem

If you look at the bottom of the board you will see no connection for +5V. Check the voltage on CN8601, pin1,2,11 or 12. Then check the voltage on pins 3~6 and 7~10. Check these led voltages as the tv is turned on.
The voltage you checked on cn9102 can't be correct if the tv turns on. (audio and video with external light) I suspect open led in the panel.

[neilc6]

Re: Sharp LC-50LB481U Backlight problem

Those 50LB series have lots of backlight failures due to default settings running them at 100%. Lots of screws holding the outer frame and strips are both double sided taped and screwed down. I just did a full replacement on one and the replacement strips didn't match the screw hole locations for all screws. Ended up being a mm or two off with just two screws causing the reflector sheet to not sit perfectly. It pushed in on one edge causing a slight shadow on that side. Didn't want to take it apart a fourth time.

[nomoresonys]

Re: Sharp LC-50LB481U Backlight problem

Test it, unhook CN9102, use about a 1k resistor and jump ps-on pin to +5v and another resistor from on/0ff to +5v, plug power cord into powerboard, see if backlights come on and stay on.

[Eaglecrest]

Re: Sharp LC-50LB481U Backlight problem

Here are some pictures.

[Eaglecrest]

Re: Sharp LC-50LB481U Backlight problem

I assume that if one dead LED kills all video, then they are wired in series. What is the reason for this? Why not wire the LEDs so on the dead one is off?

[neilc6]

Re: Sharp LC-50LB481U Backlight problem

Samsungs don't shut down with shorted LEDs and you end up with a dim screen or a damaged power supply. LGs have shutdown and they can still burn up and start a fire inside the TV.

https://www.badcaps.net/forum/showthread.php?t=72425

[Eaglecrest]

Re: Sharp LC-50LB481U Backlight problem

Why would shorted LED cause a fire, why not put a fuse in the circuit?

[Eaglecrest]

Re: Sharp LC-50LB481U Backlight problem

I redid the CN9102 readings with a different meter, attached pdf shows the results. Pretty much the same except DIM goes from 3.3 to 2.5 V. As i understand it, PS On should change voltage when the power button is pressed, yet it doesn't. However, the TV seems to work fine except no visible picture.

I am also assuming that the "picture" I can see with a flashlight is what is seen on the screen when the backlight is lite. Is this correct?

To what does pin one "On/Off" refer? What does Pin 9 "DIM" do? Beyond this the 12 V seems active and the 5V isn't; however, I would have expected that the 12V wouldn't become active until the power button is pressed. What am i missing here?

On CN8601 there are only four connections; 1 & 12 are red (presumed positive) and 4 & 9 are black (presumed negative). Are these the power to the LEDs? There is no voltage shown between 9 & 12 and 1.5V between 1 & 3.

As far as jumpering PS On/ & On/Off with 5V, i haven't done this yet and will look to see if I have a 1K resistor. If I don't is there something else that will work? What is the purpose of this? Does the 5V run the LEDs and therefore I am trying to get 3.3 V into the 5V circuit to see if the lights power up?

[budm]

Re: Sharp LC-50LB481U Backlight problem

ON/OFF = BL-ON = Backlights ON/OFF control.
DIM = Dimming control, it can be Analog Dimming or PWM Dimming control.

[nomoresonys]

Re: Sharp LC-50LB481U Backlight problem

The test with the resistors will prove the powerboard and backlights if the backlights come on and stay on your issue is mainboard not sending bl-on/on/off signal, it's not best policy but you can just use 2 jumper wires instead of resistors. must be careful to get the jumpers correct, the resistors are used in case you make a mistake and jump or touch the wrong pins so nothing gets damaged.

[Eaglecrest]

Re: Sharp LC-50LB481U Backlight problem

These TVs are a little big to work on indoors and it is raining today so i should be able to get to it tomorrow. This is probably a stupid question, but i just want to make sure I understand correctly. I am looking to connect pin 1 (On/Off) to pin 8 (5V) and pin 9 (Dim) to pin 16 (5V) each through a 1K resistor, which will act kind of like a fuse if i make a wrong connection.

Further this will be with the plug removed from CN9102; therefore, those two will be the only connections. Since 1 & 9 have 3.3V that should be sufficient to drive the 5V circuit to the LEDs. Do I need to power on the unit or just keep it in standby?

Also, am I correct in assuming that CN8601 is the voltage connection to the LEDs? Should i be seeing 3.3 v at pins 1 & 12 there?

Also, i thought that the PS On reacts to the power being turned on and then sends a signal to the power supply to energize the various voltages. If this is correct, then why would i be getting 12 at standby? I am still trying to get the macro understanding of these systems.

[nomoresonys]

Re: Sharp LC-50LB481U Backlight problem

It jump starts the powerboard, YES you leave the cable from powerboard to mainboard OFF, no need to hit power button, just plug the power cord into powerboard and see what you get if working correctly you should have the backlights on and your other voltages.

[budm]

Re: Sharp LC-50LB481U Backlight problem

https://www.shopjimmy.com/sharp-pltv...-50lb481u.htm#
If you look closely at your board, you will see that the 5V pins are not connected to any circuit on your power supply/LED driver board, this is power supply is supplying only 12V to the main board, no 5V, so you should use 680K (Based on the schematic of the power supply) resistor between 12V & PS-ON, then you need to make Voltage divider to produce about 3V output based on 12V for ON/OFF pin. If the backlights do not come on then you may need Voltage divider to provide about 5VDC to the Dim pin, the test is done with no cable to the main board connected. It will be easier using 3-AA battery pack and 1K resistors to provide the needed control Voltages instead of using 12V as the source, if you look at the schematic you will understand why.
Using 2k resistor will limit shorted circuit current to only 6mA so you cannot damage the 12V power supply if accidentally short the lead to ground, resistor is not being use as fuse.
The above test is for testing the power supply board without main board connected, but based on your PDF, you do have all the correct control Voltages when using Meter 2. What is the difference between meter 1 and meter 2? Different brand?
But since per you readings which indicated that you do have correct control Voltages, then I would check the Voltages on the LED connector to see what you have, do this with all cables back in place.

[nomoresonys]

Re: Sharp LC-50LB481U Backlight problem

Ah I see, 5volt not connected, that is tricky.