LG 50LB6500 No backlight, image on screen, right side bad picture

[garry7263]

Have a 50LB6500

Backlights do not come on. I checked the backlights and they are all working. Plugged in, the light is red , when the joystick/button is pushed, the backlights do not come on. I hear something like a relay buzzing in the lower part of the board. Cant pinpoint it. The LED output reads 80 something even when not on, when turned on it seems like it jumps to 90, but goes back. Need to fix the powersupply before troubleshooting the main board.

[neilc6]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

Need the voltages on P201 with tv in standby and on state with 1 decimal point in DCV. We are looking for the main voltages and if DRV_ON (turns on backlights) and PWR-ON (turns on power supply) signals are coming from the main board.

Pin/Label/Stby DCV/On DCV

[garry7263]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

the voltages from top down on that connector are:

first number is on standby, second is when set is turned on

1 - 0.000 v 0.000 v
2 - 0.001 v 12.01 v
3 - 0.001 v 12.01 v
4 - 0.000 v 0.000 v
5 - 0.000 v 24.29 v
6 - 0.000 v 0.000 v
7 - 3.595 v 3.590 v
8 - 3.595 v 3.590 v
9 - 0.000 v 3.474 v

also, the red indicator light on the front stays on till it is turned on. Then it flashes the following sequence:

short - long - short - long - short - short - short (then goes out completely)

[budm]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

Pin counts are wrong and the result does not mean anything without the pin labels to tell what each pin is for.
P201 has 18 pins, one column is for ODD pins (exposed pins you can see from the top side of the board), another is for EVEN number pins.

[garry7263]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

Measurements from the back side , see pic

the 3.685 is the same voltage as the other two at 3.586

[nomoresonys]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

Kind of hard to tell what is what, looks like the mainboard isn't sending the DRV-ON signal so no backlights.

[budm]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

https://www.badcaps.net/forum/attach...1&d=1571873015

You need to recheck.
The left column, from the top pin down:
The top pin is labeled as GND.
The second pin down is labeled as NC (no connection), you show 24.29V!
I cannot clearly read the pin chart as shown in your picture.
Why can't you list the pin name and the readings?

[garry7263]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

the DRV-ON is getting .076. So that sends power to the relay to send current to the backlights? Which would make sense that the relay is not getting enough power so it just buzzes a few times.

I know the main board is bad, it sends bunch of vertical lines to the display, which I can vaguely see shining a flashlight into the front of the panel. Disconnecting the ribbon cables does not affect the power readings in anyway, so I am thinking there is not a problem in the panel.

What baffles me is that the backlights have 80 vdc going to them even in standby. I have pulled off the screen and checked the backlight LEDs. Didn't have enough voltage to test the full strips so I tested the long ones with 30v at 50 ma, and the short ones with 24v at 50 ma. Maybe I should focus more on the mainboard.

Replacing the main board is out of the question. I only found only ONE for sale for almost $400, so I really need to fix this one. I am trying to find a schematic for it.

[budm]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

DRV-ON = BL-ON for turning on the backlights circuit. No BL-ON = No backlights, <1V = off, >2V = ON.

What relay?
https://www.badcaps.net/forum/attach...6&d=1571860581

PWR-ON (PS-ON) is for turning on the rest of the power supplies on.
Look at the Voltage chart as printed on the board to see what the LED Voltages should be with loads connected.

[garry7263]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

the pin out is on the board, reflecting the one on top

they read

GND | GND
NC | 12v
12v | 12v
GND | GND
24v | 24v
P-D1M2 | GND
3.5v | 3.5v
P-D1M1 | 3.5v
DRV-ON |PWR-ON

[nomoresonys]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

.

[garry7263]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

the buzzing noise sounds like it is coming from around C601, the rectifier, LF102, and CX102

Dont want to put by ear that close to a live hot side of the board. Cant find my stethoscope from college, that would sure help find out what that is. Seems like I need to look at the backlight circuit on the main board?

[garry7263]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

there are two banks of LED strips. One has two strips and the other has three. Voltages are supposed to be 113 for the low and 170 for the high.

[nomoresonys]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

A hollow tube can sub for a stethoscope, that looks like a match for your mainboard as far as I can tell, 71 bucks, $400 is way out of line: https://www.ebay.com/itm/LG-50LB6500...UAAOSw2ipdZuan

[budm]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

I will not worry about the buzzing noise at this pont.
1) Unplug the TV from the AC outlet.
2) Unplug the LED connector from the power supply board.
3) Set meter to 200 VDC scale and then keep the red probe and Black probe on each leg of the 33 uF 200 VDC cap next to the LED connector.
4) Plug the TV into the AC outlet while watching the meter to see how high it jumps up to, then wait 5 seconds to see where it settle to.
5) Hit the TV power switch while keeping an eye on the meter to see how high the Voltage jumps up to, then unplug the TV from outlet and wait 30 seconds.
6) Move the meter probes to the two legs of the 33 uF 350 VDC cap next to the LED connector.
7) Repeat step 4 and 5.
8) Report the results.

[garry7263]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

Probes on the 33u 200v cap:

When plugged in, it jumps to about 84.8v, then drops to about 77v and settles at about 80.9v
When turned on, it jumps to about 89.9v, then drops to around 87.8

Probes on the 33u 350v cap:

When plugged in, it jumps to 85v, then drops to 81.1v
When turned on, it jumps to 90.05, then drops to 87.9v

[budm]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

LED Boost converters (2) are not turned on due to no BL-ON (DRV-ON) control signal.

[garry7263]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

Yes, DRV-ON is only getting 0.076v, so the relay that it is supposed to turn it on is sounding like trying to start a car with a half dead battery.

That pin that is supposed to be no connection, is not connected, there is no solder on that pin, but the pin sticks though the board (even though it is not connected)

Wish I had a microscope. I am going to have to trace that DRV-ON back to the main board and look for a bad voltage regulator or chip? Something has to send it more voltage.

Found the service manual, but no schematic, just flow charts and pin outs of chips.

[budm]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

What relay? There is no relay on the power supply board.

[garry7263]

Re: LG 50LB6500 No backlight, image on screen, right side bad picture

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