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Old 02-03-2015, 08:17 PM   #1
lowercasek
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Question LG LCD 42LD750 Backlight flashes 4 times, then tv off.

i have a faulty lg tv. When i turn it on, the LG logo comes up fine, about 2 seconds in, the screen goes black, then two seconds later it comes back on. it does this about 3-4 times. Each time the backlight comes back on, the tv screen is perfect in brightness. then finally it turns off and the standby light flashes red constantly. when the backlight turns off, the TV is still booting up like normal, and by the 4th time it actually shows the input no problem. but then the tv turns off entirly and flashes the standby light red.

I have tried desoldering the transformers on the inverter/power board. There are two for this model, and when i ohms test the windings they are both very simliar. The primary on both is exactly 1.6ohms. and the secondaries vary between 36ohms - 37.2ohms.

i have read through http://www.badcaps.net/forum/showthr...lashes+3+times but nothing there seems to of helped.

Any ideas or suggestions would be much appreciated. Cheers!
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Old 02-16-2015, 05:57 AM   #2
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Hey there,
I got exactly the same issue with same tv here. I expact ic501(str w6053n) and ic301 (bd... cant read it. Its burned Can u help me?) to be bad. Ic501 is supporting voltage for the coils and is in communication with error and standby circuit.

For other lg models there are repair kits.

Did u see any strange behavior of led standby light? For this tv here its always red and doesnt turn to blue anymore. Tv starts as yours... thats not right

Let me know if this helps u in any way..

Good luck
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File Type: jpg 2015-02-16 12.52.29.jpg (922.6 KB, 83 views)

Last edited by stormanimal83; 02-16-2015 at 06:00 AM..
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Old 03-06-2017, 04:37 AM   #3
Adriatic
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Hello.
First thank you for accepting me on this forum.

If someone can help me. I have the exact same tv. The previous owner (thru repair shop) did try to repair the PSU but failed. I got this tv for 40$ and now I'm trying to repair it myself. I'm a hobby / learning type.
However to the point. On the picture attached there is something missing (thought by the color it is a thermistor but not sure). Because of lacking schematics of this PSU board I'm almost blind and I can only guess.
If somebody can help me and check the type of missing element. I would be very

Have a nice day to all of you.
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Old 03-06-2017, 04:53 AM   #4
dick_barton
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

What is the board marking alongside it. It looks like a capacitor.
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Old 03-06-2017, 05:17 AM   #5
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

It is marked as VA100 (EL42 and EL20 on the empty holes/pins)
Here is a picure of board where this element is missing.
Thank you
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Old 03-06-2017, 05:18 AM   #6
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Btw it is betwen N and L... by the memory .. not sure

Last edited by Adriatic; 03-06-2017 at 05:48 AM..
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Old 03-06-2017, 05:27 AM   #7
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

VA100 appears to be a ceramic transient suppressor fitted across the mains. However that shouldn't stop the set from working but does need to be replaced.


https://www.engineersgarage.com/cont...pike-protector

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Old 03-06-2017, 06:28 AM   #8
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

dick_barton
Thank you, hope you are right. Btw that on the picture is green I have spare blue ones 10D471K , hope it will do the job.

Last edited by Adriatic; 03-06-2017 at 06:29 AM..
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Old 03-11-2017, 06:20 AM   #9
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Now this is spooky ?
I recovered PSU board from the dead but only to the point when it starts three times and then stops.
But here is the catch. If I stick probes of DMM to the input of ccfl lamps and set it to AC than TV set starts normally , no shutdown no error etc. It is not pressure or contact related btw and that is what is bothering me. Immediately when I pull out probes from DMM (from DMM not from TV set !) the TV shuts off and show error. Reading are avg at 340V, at start there is spike up to 440V, one thing I noticed is when tv was already running 5 mins or so there was a spike over 1040V ?? DMM complained about it and played a music for me and TV shuted down.
I cannot even tell what am I measuring and why this is happening .
However from the ohm readings, the inverters are fine (confirmed by my 6 year daughter ) but there is one little transformer which is suspect to me but cannot find datasheet for him.
The markings on suspected transformer is LIP-100.SC091111C (third photo)
It shows different ohm reading from second one (all messed up) but it is also in reverse from second one so hmm.

Long story short: Why is DMM causing TV to start normally ???
Can anybody help me. Ohh and sorry for long post.
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Last edited by Adriatic; 03-11-2017 at 06:28 AM..
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Old 03-11-2017, 07:02 AM   #10
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

You possibly have faulty capacitors. There are two blue ceramic caps alongside the pink connector to the backlights. Check them out.

You shouldn't use your meter to check the High Voltage output which can be as high as 2000V at startup and then run between 1000V - 1200V when the tubes are lit unless you wish to damage your meter

Last edited by dick_barton; 03-11-2017 at 07:04 AM..
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Old 03-11-2017, 08:06 AM   #11
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Will check thank you.
One is SL6KV 10J .. I thinkn 10pF
Second SL6KV 27J .. I think 27pF
Correct me if i'm wrong, <<<<<<<<<<<<<<<<btw having trouble measuring them because resolution of my DMM is 10pF
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Old 03-11-2017, 08:22 AM   #12
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Those values appear correct.
Does the backlight come on without them in place?
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Old 03-11-2017, 08:26 AM   #13
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Don't know, pulled out all 4 of them for both inverters.
Will try to test them at friend's house with his DMM with extra digit resolution. Fingers crossed.
Can I try to start the board without them ?
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Old 03-11-2017, 08:56 AM   #14
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

You can give it a try but don't leave it on longer than needed for a quick check.

Last edited by dick_barton; 03-11-2017 at 08:58 AM..
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Old 03-11-2017, 08:59 AM   #15
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Just checked capacitors. All in ranges. So it is not that.
Hmm.
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Old 03-11-2017, 09:11 AM   #16
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

OK so placing your probes into the High Voltage socket at the back of the panel makes the set light up. If it's a connection problem then possibly by you inserting the meter probes you are helping to make a good connection.
Put your capacitors back in circuit and insert the meter probes as you have earlier but remove the leads from your meter. What happens?
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Old 03-11-2017, 09:27 AM   #17
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

@dick_barton
Sorry but I already wrote about this and it is not the case. Without DMM on, TV wont start even if there are probes in TV. Tried several times, because of that I wrote that is some kind spooky. If the probes are in and the DMM on, the TV starts and keep running until I unplug probes from DMM ( from DMM not TV !) the same happens if I turn off DMM.
So my conclusion is that DMM is some kind influencing on TV behavior. How ?
Don't know.
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Old 03-11-2017, 09:31 AM   #18
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

BTW
If I check resistance (TV off ) the same way I get 0.3 Ohms... not sure that is ok ?
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Old 03-11-2017, 09:42 AM   #19
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Run a soldering iron over all the pins on the inverter transformers to make certain that all the solder joints are good. I have had a similar problem in the past where the solder joints looked good but tubes would not light but after soldering them my backlights worked.
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Old 03-11-2017, 09:44 AM   #20
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Already done that no luck
Actually done that not only on inverters but also elsewhere

Last edited by Adriatic; 03-11-2017 at 09:45 AM..
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