Riddled TV Repair Forum

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

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.

Google

JTS1957 · Joined: 21 Jan 2009 Posts: 2380 Location: Far, Far Away

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

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: 2380 Location: Far, Far Away

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

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.

Google

JTS1957 · Joined: 21 Jan 2009 Posts: 2380 Location: Far, Far Away

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

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

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: 2380 Location: Far, Far Away

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

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: 2380 Location: Far, Far Away

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.