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Second opinion] I have an LG LB7100 That has a backlighting…

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Second opinion] Hi,I have an...
Second opinion] Hi,I have an LG LB7100 That has a backlighting issue.Backlights appear to be ok when powering up as I get the LG logo and it will show input for a second before backlights go out.Opened it up and see the power board has a green led on it that flashes 4 times stops and then repeats the 4 flashes and stops on and on
Submitted: 1 month ago.Category: TV
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Customer reply replied 1 month ago
Video is ok--can see when flashlight used. Sound is still working.
Answered in 24 minutes by:
3/17/2018
TV Technician: Louie, Technician replied 1 month ago
Louie
Louie, Technician
Category: TV
Satisfied Customers: 13,217
Experience: 25+ yrs. experience in the field and 5+ yrs. online, down to component level.
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Hi and welcome to JustAnswer; an independent site with members providing product information, repair tips and technical assistance as a form of paid Q & A service.
My name is ***** ***** I'll be assisting you with your requested second opinion question.

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TV Technician: Louie, Technician replied 1 month ago

Initially, the LB7100 is a series. Which particular model please?

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Customer reply replied 1 month ago
60"
TV Technician: Louie, Technician replied 1 month ago

Are you then considering a more involved DIY (do-it-yourself) repair approach? If yes, do you have access to and familiarity with a multi-meter (and even perhaps a soldering iron)?

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Customer reply replied 1 month ago
I'd be willing and able depending on how involved and I do have a multi-meter
TV Technician: Louie, Technician replied 1 month ago

Great on the DIY!
When able, please post back with very clear pictures of the "...circuit boards..." Click HERE.
http://ww2.justanswer.com/help/how-do-i-send-photo-or-file-expert-hl
These pictures would then serve as common references allowing me to see what you are seeing.

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Customer reply replied 1 month ago
Ok--but I'm going to need to do that tomorrow. Had the TV on the floor and now it's back on the stand to keep it away from kids. I'll need to wait until tomorrow morning to get it down again. Touch base with you tomorrow, Thx.
TV Technician: Louie, Technician replied 1 month ago

I understand and yes please, when conveniently able.

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TV Technician: Louie, Technician replied 1 month ago

Apologies for delays; mobile/fieldwork with sluggish Internet if at all.

Try reading the voltages off of the pins of the connectors twice. Compare between the 2 above conditions; i.e. when backlights are on and when off. Note down readings on a table.

The idea is to find which pin has a significant change in value between the 2 above conditions. This pin then would be responsible for the "...backlights go out..."

Potential suspects are:

  • • faulty main board/missing LED trigger (least likely);
  • • low or missing LED driver voltage (likely);
  • • over current/auto-protect due to faulty LED strip/s (most likely).

A friendly reminder: voltage measurements by its very nature requires that the unit be plugged in and turned on. It is therefore LIVE! Please exercise due care and caution.

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Customer reply replied 1 month ago
Thx for getting back to me.2 questions concerning testing:[1] Referencing the pictures I sent, the red/black ribbon cable is for the LEDs and the printing on the power board would mean the red wires are the positives, the black the negative, is this correct? [I ask since I've run into mislabeled things before :( ][2] Where should I connect the ground to, the metal of the TV chassis, or directly to each black wire? If the chassis is there any preferred location?Thx for the help!
TV Technician: Louie, Technician replied 1 month ago

Appreciate the understanding. Actually, the fieldwork is with my class/students.

Good questions; for standard voltage check, use the common ground (almost always); i.e. the same ground used by the metal frame/chassis.

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Customer reply replied 1 month ago
These are the readings I got for each red wire [LED+] on the LED [red/black] ribbon:178v when TV is turned on and Backlights come on
125v when TV Backlights go off [TV still turned on]I did it both with no signal going to TV and a signal going to HDMI 1--same resultsAll voltages climbed steadily on voltmeter and dropped steadily
TV Technician: Louie, Technician replied 1 month ago

How long (seconds/minutes) between 178V and 125V?

Please repeat above test with the ribbon wire connector removed. This would be the no-load condition.

If dive from 178 to 125 persist, then it would most likely be the power supply/LED driver board.

On the other hand if voltage remains high, then it would be the LED strip.

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Customer reply replied 1 month ago
It was a couple of seconds--no minutesI'll check the no-load condition later tonight and let you know.Thx again!
TV Technician: Louie, Technician replied 1 month ago

Copy that and would check back when able.

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Customer reply replied 1 month ago
No-load I got:On start I got 233v then 154v for all. [Time difference between different voltages *seemed* the same as the time the LED Backlights would stay on and then go out]Thx!
TV Technician: Louie, Technician replied 1 month ago

Offhand, it would appear that the power supply / LED driver is exhibiting the voltage drop even without a load hence would be suspect.

Replacing it would then be both an attempt of a fix and as a test.

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