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LG 47LN5700-UH - LED backlight issues   Goto page 1, 2  Next  
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chrisjmatheis



Joined: 05 Sep 2015
Posts: 6
Location: Leesburg, FL

PostPosted: Sat Sep 05, 2015 8:43 am    Post subject: LG 47LN5700-UH - LED backlight issues Reply with quote

Hello everybody! I have recently tried my hand at fixing flat screen TV's and have been pretty successful or probably just lucky Smile But now I have one that is giving me a hard time but I have isolated it as far as I can and could use a point in the right direction. So I will give you a run down of what I did and where I am at.

Model LG 47LN5700-UH LED TV that was given to me
Take it home, plug it in, standby light comes on, press power button on TV, LG splash screen appears, goes black, shine flashlight at screen, press menu button on TV, menu appears, unplug, take off back, note: power supply and LED driver on same board, remove power supply from TV.

These are what the input and output voltages that are supposed to be coming out of power supply and LED driver

Input: 120VAC
Output: 3.5VDC, 12VDC, 24VDC

LED Driver Output: 100VDC, 111VDC Note: has a 4 pin connector for two separate outputs

P201:
GND GND
12V N.C. (not sure what this is)
12V 12V
GND GND
24V 24V
GND GND
3.5V PDMI2
3.5V PDMI1
PWR-ON DRV-ON

Check standby voltages of 3.5V OK, closed 3.5V to PWR-ON circuit, 12V and 24V rails OK, closed 3.5V to DRV-ON and got 76V at LED driver output, closed 12V to PDM1 and another 12V rail to PDMI2, LED output over 150V and then drains to 76V.

This is where I need help, I don't know where to go next, it seems as the circuit is overloading itself. I can solder and can use multimeter and have both, there are no bad caps, have checked some diodes on the board - in and out of circuit, checked all transformers for continuity on the primary and secondary sides, and definitely checked all fuses.

Any help would be appreciated and am comfortable with most component level repairs. I would really like to fix the board as I know that it is the problem but I have a line on another one and they are plentiful and I have plenty of TV's
so this is a challenge that I would like to conquer)

If there is any more information you need please feel free to contact me and I hope I was specific enough without sounding like too much of a NOOB.
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JTS1957



Joined: 21 Jan 2009
Posts: 2266
Location: Far, Far Away

PostPosted: Sat Sep 05, 2015 9:44 am    Post subject: Reply with quote

chrisjmatheis wrote:
Take it home, plug it in, standby light comes on, press power button on TV, LG splash screen appears, goes black, shine flashlight at screen, press menu button on TV, menu appears, unplug, take off back,

What happens to Power LED when set is turned On? Standby Red - Operating White?
Changing to a valid working input (ie. built in tuner with antenna, tuned to an active station / component / composite / HDMI) produces sound?


chrisjmatheis wrote:
Check standby voltages of 3.5 OK, closed 3.5V to PWR-ON circuit, 12V and 24V rails OK, closed 3.5V to DRV-ON and got 76V at LED driver output, closed 12V to PDM1 and another 12V rail to PDMI2, LED output over 150V and then drains to 76V.

What do YOU mean by 'closed?'

N.C. usually means Not Connected.

Between this and the other forum, I see we have some problem(s). (Any kin to bjlasota?)
chrisjmatheis



Joined: 05 Sep 2015
Posts: 6
Location: Leesburg, FL

PostPosted: Sat Sep 05, 2015 10:49 am    Post subject: Reply with quote

Tv operates normally including sound and changing inputs using a flashlight to see the screen except the back light comes on and goes off and by closed I mean connected and no relation bjlasota and your the same guy from the other forum SMH and LOL
JTS1957



Joined: 21 Jan 2009
Posts: 2266
Location: Far, Far Away

PostPosted: Sat Sep 05, 2015 11:08 am    Post subject: Reply with quote

Sort of sounded like you were taking the 3.5V standby voltage and hooking it to POW-ON & DRV-ON, and then for some unknown reason connecting the 12V switched source to the PWM dimming pins (whether you were 'closing' the 3.5V for this or not, you don't say). Wouldn't ALL that be the MAIN boards job?

http://imgur.com/EekJi8G
chrisjmatheis



Joined: 05 Sep 2015
Posts: 6
Location: Leesburg, FL

PostPosted: Sat Sep 05, 2015 3:05 pm    Post subject: Reply with quote

You are correct that the main board handles the switching, I am not hooking anything to the PWM dimming circuit for whatever unknown reason to you, I am manually switching circuits on in the power supply and LED driver to troubleshoot the board because the LED driver output voltage is too high and then shuts itself off thus causing the backlight issue.
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JTS1957



Joined: 21 Jan 2009
Posts: 2266
Location: Far, Far Away

PostPosted: Sat Sep 05, 2015 5:25 pm    Post subject: Reply with quote

Well, I'm gonna stick with the bad LED(s) theory then.
chrisjmatheis



Joined: 05 Sep 2015
Posts: 6
Location: Leesburg, FL

PostPosted: Sat Sep 05, 2015 5:38 pm    Post subject: Reply with quote

I know your the veteran and I respect that but I do not think you are listening, if the LED panel is not hooked up it still does the same thing, leading me to believe it is a problem with the LED driver circuit. I am going to get a new board and I will take pictures for posterities sake of old and new just to show the Vet that it is not a problem with the LED panel.
JTS1957



Joined: 21 Jan 2009
Posts: 2266
Location: Far, Far Away

PostPosted: Sat Sep 05, 2015 5:57 pm    Post subject: Reply with quote

Does LED voltage exceed the working voltage of the electrolytics filtering it?
Does PWR_ON & DRV_ON both come on high AND STAY high? (See chart)

http://imgur.com/nE8o2MD

Be sure to get pictures/video of the LED strips in their entirety as they attempt to light.

Be sure board provider has GOOD boards (and more than one of them) as they are likely to be 'new-to-you' (pulled from service, promised working), NOT 'NEW.'
chrisjmatheis



Joined: 05 Sep 2015
Posts: 6
Location: Leesburg, FL

PostPosted: Sat Sep 05, 2015 7:10 pm    Post subject: Reply with quote

To answer your question, NO, just the LED driver circuit goes high (over 150VDC and back to 76VDC) everything else is just fine.

But after revising my work I have a better description but tells the same thing:

Closed 3.5VDC to PWR-ON, 12VDC and 24VDC rails ok, read 76VDC at LED driver output (priming voltage) being a 4 pin connector with a +-+- because it takes about 200VDC to fire the LED's so I had 2 rails that had 76VDC, closed the other 3.5VDC to DRV-ON, voltage at LED driver output at over 150VDC on both rails and then goes down to 76VDC on each rail when the voltage should be 100VDC and 111VDC respective to their rails (don't know which one because it is malfunctioning) constant and steady
JTS1957



Joined: 21 Jan 2009
Posts: 2266
Location: Far, Far Away

PostPosted: Sat Sep 05, 2015 7:45 pm    Post subject: Reply with quote

You may want to review panel information. http://www.datasheetspdf.com/PDF/LC470DUE-SFR1/906956/1
Especially in regards of strips/strings/arrays. R1/L1 & R2/L2 go together as per: http://www.shopjimmy.com/lg-lc470due-sfu1-led-strips-12-strips-1-board.htm

The most likely reason (IMHO) your 'power on' and 'panel on' are present at first and then go away so you're left having to provide your own 'on' voltages, is that one (or more) LEDs in one (or more) of the series strings are failing as soon as the set tries to turn on. The b-plus voltage rises without a load.
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