Need some help troubleshooting circuit


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Need some help troubleshooting circuit

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chrisjmatheis
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Sep 5, 2015, 2:30 AM

Post #1 of 10 (3446 views)
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Need some help troubleshooting circuit	Can't Post

Hello everybody! I have recently tried my hand at fixing flat screen TV's and have been pretty successful or probably just lucky :-) 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.

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

LED Driver Output: 100V, 111V 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 3.5 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.

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.

jts1957
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Sep 5, 2015, 3:47 AM

Post #2 of 10 (3443 views)
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Re: [chrisjmatheis] Need some help troubleshooting circuit [In reply to]	Can't Post

Make sure each and every backlight LED is coming on. It only takes one bad to kill lighting. http://www.shopjimmy.com/...2-strips-1-board.htm

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Location: Far, Far Away

chrisjmatheis
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Sep 5, 2015, 4:02 AM

Post #3 of 10 (3436 views)
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Re: [jts1957] Need some help troubleshooting circuit [In reply to]	Can't Post

Those are the output voltages that are supposed to be there and the ones I was reading are without the backlight hooked up are to high, I jumped out the connector without the main or tcon boards hooked up, I tested this theory on another TV, which performs in the way it should. the backlights light up.

jts1957
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Sep 5, 2015, 11:30 AM

Post #4 of 10 (3424 views)
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Re: [chrisjmatheis] Need some help troubleshooting circuit [In reply to]	Can't Post

I'll repeat: It only takes ONE bad LED to kill lighting.

Just like old school Christmas tree lights.

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Location: Far, Far Away

chrisjmatheis
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Sep 5, 2015, 1:17 PM

Post #5 of 10 (3421 views)
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Re: [jts1957] Need some help troubleshooting circuit [In reply to]	Can't Post

I don't know if I am explaining this correctly, but the LED driver circuit turns on and outputs too much voltage with and without the LED panel attached to power supply board then shuts off to protect itself, when I turn on the LED driver circuit I should get a steady 100VDC and a 111VDC on the 4 pin connector. It is firing with more than 150VDC and shutting off and returning to 77VDC priming state.

jts1957
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Sep 5, 2015, 2:01 PM

Post #6 of 10 (3416 views)
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Re: [chrisjmatheis] Need some help troubleshooting circuit [In reply to]	Can't Post

Quote

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?'

Quote

I tested this theory on another TV, which performs in the way it should. the backlights light up.

I'll believe you IF ...
BOTH TVs are the Same Brand/Model ...
BOTH Use Same power/LED board number ...
BOTH boards behave the SAME in the Working set ... &
BOTH boards behave the SAME in the NON-Working set.

The one thing you have NOT said is that when the board from the Working set is installed in place of the one in the NON-working set, it produces a working set.

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Location: Far, Far Away

(This post was edited by jts1957 on Sep 5, 2015, 2:34 PM)

jts1957
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Sep 8, 2015, 2:00 AM

Post #7 of 10 (3378 views)
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Re: [chrisjmatheis] Need some help troubleshooting circuit [In reply to]	Can't Post

You keep on checking in? Looking for other opinions? I thought you were obtaining another PS/LED board? Get it, install it, and prove or disprove your or my theory.

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Location: Far, Far Away

chrisjmatheis
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Sep 8, 2015, 2:44 AM

Post #8 of 10 (3375 views)
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Re: [jts1957] Need some help troubleshooting circuit [In reply to]	Can't Post

Trust me, I check back everyday hoping someone with more expertise will respond to my quandary to no avail, I will be sure to post comments w/ pictures of my conquests, hold on, I'm checking my funds and ordering a power supply board while taking pics right now, see you soon.

chrisjmatheis
New User

Sep 17, 2015, 2:30 AM

Post #9 of 10 (3325 views)
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Re: [chrisjmatheis] Need some help troubleshooting circuit [In reply to]	Can't Post

Dear JTS1957:

So this is me apologizing for being so crass. I apologize. I received the new power supply in the mail, slapped that baby in the TV like it was on fire, plugged it in, pressed the power button on the TV, and the same thing happened, shouted some expletives (luckily it was just me at home), remembered your gentle instruction, disassembled TV where I had just the LED strips on the frame and power supply on the back, forced the power supply to output voltage to LED driver circuit, 3 rows of LEDS were not firing, removed each LED strip from TV of the rows that were not firing, wired combinations of 9V and 1.5V batteries in series to test LED strips and found the faulty strip.

So in short, I have ordered the replacement strip and awaiting delivery, I could've gone about like nothing happened, but I feel I owed you this at least after acting like a know-it-all.

Thank you

jts1957
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Sep 17, 2015, 12:50 PM

Post #10 of 10 (3302 views)
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Re: [chrisjmatheis] Need some help troubleshooting circuit [In reply to]	Can't Post

I accept your apology. You admitted to being a NOOB in your initial post.

IMHO, Manufacturers are building their sets using the least expensive materials, putting them together in the cheapest ways and pushing them out into the market as soon as possible so they can 'makith the money' as quickly as possible.

They adjust the sets to 'defaults' that produce the best looking picture to the vast number of people. In particular backlight intensity and brightness are usually set to 100%. (Imagine if they shipped with Volume default of 100%.)
With defaults at 100%, things may 'look' better, but things will run hotter, use more power and parts / sets longevity will be reduced.
You may wish to check this on all your TVs.

Be aware that the replacement bulbs/strips are 'used' ... which is actually good and bad, as they will match your originals 'used' condition, but is anybodies guess as to whether next failure will be in the original or replacement strips. (Some people replace BOTH headlights on their car when one becomes defective.)

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Location: Far, Far Away

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