LG 49LB5550-UY with backlights that flicker on and off

[SdlewisSr]

I have an LG 49LB5550-UY with backlights that flicker on and off. Other than that everything works.
I checked the voltages on P201 of the PSU (leading to mainboard) and all are as supposed to be.
P801 (leading to the LED strips) has fluctuating voltages with LED strips connected and disconnected.
LED+ on one side fluctuates from 123v to 126v when connected and from 174v to 194v when disconnected.
LED+ on other side fluctuates from 128v to 145v when connected and from 135v to 148v when disconnected.
Since the voltage fluctuate with the LED's disconnect I suspect a problem with the LED driver section.
I do not see any visual signs of failing components. I was wondering if anyone has seen this behavior before or has the service manual.
I am including an animated GIF of the flickering and image of the PSB/LED Driver.
Thanks in advance for any help/support.

[neilc6]

Re: LG 49LB5550-UY with backlights that flicker on and off

Did you try turning down your backlight level?

[attainteddragon]

Re: LG 49LB5550-UY with backlights that flicker on and off

are you measuring each pin in reference to chassis ground.

led+ =....v
led- =....v
led+ =....v
led- =....v

each pin with all cables attached.
on power up, measure spike and then what the voltage settles to?

[SdlewisSr]

Re: LG 49LB5550-UY with backlights that flicker on and off

(I used underscores _ to help with alignment of the text below)
I do not have the remote to lower the backlight level.

I measured from chassis ground
____________________LED+________LED-________LED+________LED-
Power up high voltage__:155v ________31v_________145v_________83v
Fluctuating low _______:123v-126v____19v-21v _____128v-145v____70v-80v

I then measured the fluctuating volts on each while the TV ran with flickering.
_______LED+______LED-_____LED+______LED-
HIGH__136.9v_____34.6v_____146.2v_____81.5v
LOW __121v_______18.9v_____130.8v_____65.5v

I reversed the LED strings and still get the same flickering. I disconnected only one string and still got flickering. I disconnected the other and still got flickering. I put reversed the strings, with only one string at a time connected, and still got flickering.

[attainteddragon]

Re: LG 49LB5550-UY with backlights that flicker on and off

your led- voltages look way too high. i believe that you have shorted led's in the panel.
this is where an led tester comes in handy.

let's see if others come to the same conclusion.

[SdlewisSr]

Re: LG 49LB5550-UY with backlights that flicker on and off

My next step was going to take the panel apart and check the LED's. I did not even consider a backlight tester. Thanks attainteddragon. I just ordered one and will have it Thursday.

[dick_barton]

Re: LG 49LB5550-UY with backlights that flicker on and off

Is the DC voltage across the two legs of C610 holding steady or is this fluctuating and causing your backlight problem.

[SdlewisSr]

Re: LG 49LB5550-UY with backlights that flicker on and off

C610 is holding steady at 400 V.

[dick_barton]

Re: LG 49LB5550-UY with backlights that flicker on and off

I would have expected the voltage at the led connector to be steady at some value when the led's are disconnected. I see there are 4 electrolytic capacitors around the connector. Have you checked them for value and ESR readings.

[Andrew F. Ali]

Re: LG 49LB5550-UY with backlights that flicker on and off

This is atypical case of bad LED backlights. Perform the checks dick barton recommends. Be prepared to tear down the Panel to get to the LEDs. Being a 49 incher be careful when handling as the break very easy. If you want along tem fix, change all the strips even those that appear to be OK. They will fail in the short term and you will have to rip her open again.

[budm]

Re: LG 49LB5550-UY with backlights that flicker on and off

https://www.badcaps.net/forum/attach...4&d=1530592637
If you look at the Voltage chart as printed on the board, you will see the TWO output Voltages for the two LED strings which are not the same, one LED string has more LED than the other string. The loaded Voltage for the smaller string is 113V 205mA, the larger LED string is 170V 205mA.
How steady are the Dimming control Voltages?
The two 47uF 160vdc caps (C223?, 224) are for the filter caps power supply that feeds the two Boost converter circuits, so you need to check the Voltage between the two legs of those two caps to see if they are steady or not.

[SdlewisSr]

Re: LG 49LB5550-UY with backlights that flicker on and off

I appreciate all the input. I have a new development in this mystery. When I removed the PSB to check the caps, I discovered corrosion (see pics). Also the screws that secure the PSB in place were not tight. I am going to attempt to clean the corrosion and determine if any of the traces, solder joints, or components are damaged before proceeding. Probably will half to just replace the PSB.
Thanks again for all the support

[budm]

Re: LG 49LB5550-UY with backlights that flicker on and off

All the through holes solder rivets should be inspected closely for cracked/poor solder joints.

[Diah]

Re: LG 49LB5550-UY with backlights that flicker on and off

this is atypical case of bad led backlights. Perform the checks dick barton recommends. Be prepared to tear down the panel to get to the leds. Being a 49 incher be careful when handling as the break very easy. If you want along tem fix, change all the strips even those that appear to be ok. They will fail in the short term and you will have to rip her open again.

+1

[SdlewisSr]

Re: LG 49LB5550-UY with backlights that flicker on and off

I would have expected the voltage at the led connector to be steady at some value when the led's are disconnected. I see there are 4 electrolytic capacitors around the connector. Have you checked them for value and ESR readings.

ESR values (out of circuit)
C801 2.1ohm
C805 1.9ohm
C224 1.8ohm
c219 1.6ohm

[SdlewisSr]

Re: LG 49LB5550-UY with backlights that flicker on and off

https://www.badcaps.net/forum/attach...4&d=1530592637
If you look at the Voltage chart as printed on the board, you will see the TWO output Voltages for the two LED strings which are not the same, one LED string has more LED than the other string. The loaded Voltage for the smaller string is 113V 205mA, the larger LED string is 170V 205mA.
How steady are the Dimming control Voltages?
The two 47uF 160vdc caps (C223?, 224) are for the filter caps power supply that feeds the two Boost converter circuits, so you need to check the Voltage between the two legs of those two caps to see if they are steady or not.

-------------
Dim 1: 2.739v to 2.78v
Dim 2: 2.4v to 2.78v

--------------

Voltages across capacitors LED's not connected
______High_____Low
C801__178.24v__163.52v
C805__147.37v__136.68v
C224__93.47v___90.15v
C219__93.42v___89.86v

Voltages across capacitors LED's connected
______High_____Low
C801__130.61v__121.55v
C805__145.98v__128.41v
C224__92.76v___90.3v
C219__92.38v___89.7v

[dick_barton]

Re: LG 49LB5550-UY with backlights that flicker on and off

ESR values (out of circuit)
C801 2.1ohm
C805 1.9ohm
C224 1.8ohm
c219 1.6ohm

You don's give the value of the capacitors and I can't make out from the photo which one is which. Find attached a chart which is a guide for expected ESR

[SdlewisSr]

Re: LG 49LB5550-UY with backlights that flicker on and off

You don's give the value of the capacitors and I can't make out from the photo which one is which. Find attached a chart which is a guide for expected ESR

ESR values
C801 2.1ohm (Top Left) 33uf 350v
C805 1.9ohm (Top Right) 33uf 200v
C224 1.8ohm (Bottom Left) 47uf 160v
C219 1.6ohm (Bottom Right) 47uf 160v

[dick_barton]

Re: LG 49LB5550-UY with backlights that flicker on and off

The 47uF, 160V should have a maximum ESR of around 0.46 ohms. Your readings are considerably higher.

The 33uF, 200V approximately 1 ohm max

Not certain of what the 33uF 350V would be but I suspect around 2 ohms.

Have you also checked their capacitance and do they match what is on them.

[Andrew F. Ali]

Re: LG 49LB5550-UY with backlights that flicker on and off

Those caps should be replaced. Is the BL-ON signal stedy when the Backlights are flashing? I still suspect bad LED backlight strips.