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Old 03-11-2017, 09:49 AM   #21
Adriatic
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

If I understand correctly those two wires should both be L and on the other side the tv where are only one wires per ccfl is ground ? So I only measured difference voltage between those two hot wires and not between ground and live (hot) ones ? Am I correct ?

Last edited by Adriatic; 03-11-2017 at 09:50 AM..
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Old 03-11-2017, 09:49 AM   #22
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Sorry but I'm out of ideas.
Last thought.
Have you measured the resistance of the windings of the inverter transformers, both primary and secondary to see if they match each other.

You have 4 connectors from the two inverter transformers. If you have your meter on another connector does the set still come on ?

Other than that can anyone else think why loading the inverter output with a DVM would bring on the CCFL backlights?
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Old 03-11-2017, 10:09 AM   #23
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

If you remove the mainboard and carry out the checks as given in the post below do your backlights come on and stay on. Here you jumper PSon to 5V standby using a 1K resistor and also BLon to 5Vstby using another 1K resistor. That should power up the whole PSU and the backlights.

http://www.badcaps.net/forum/showthread.php?t=57487
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Old 03-11-2017, 10:12 AM   #24
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

If you think this small ones (picture) than yes I did. They are not the same. First one is ok I think all 4 showing 0.62 Mohms. But here is strange thing .. one time I get same readings on first one and not second one(some are's 88 kohms) and vice versa, strange can not get same readings twice ? Not sure what to think about it. They are however reversed on the board. Why I don't know.
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Last edited by Adriatic; 03-11-2017 at 10:14 AM..
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Old 03-11-2017, 10:15 AM   #25
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Quote:
Originally Posted by dick_barton View Post
If you remove the mainboard and carry out the checks as given in the post below do your backlights come on and stay on. Here you jumper PSon to 5V standby using a 1K resistor and also BLon to 5Vstby using another 1K resistor. That should power up the whole PSU and the backlights.

http://www.badcaps.net/forum/showthread.php?t=57487
Will try that ...thank you very much for your time BTW
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Old 03-11-2017, 11:30 AM   #26
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Quote:
Originally Posted by dick_barton View Post
If you remove the mainboard and carry out the checks as given in the post below do your backlights come on and stay on. Here you jumper PSon to 5V standby using a 1K resistor and also BLon to 5Vstby using another 1K resistor. That should power up the whole PSU and the backlights.

http://www.badcaps.net/forum/showthread.php?t=57487
Tried and yes the backlight stays on. You are pointing on ... ?
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Old 03-11-2017, 04:45 PM   #27
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

So they do come on and without having to have the meter connected to the connector as shown in you photo.
If that is true can you check what is happening to the PSon & BLon voltages.

I'm suspecting that an Error signal (pin 24) is being sent from the mainboard turning off the backlights.
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File Type: jpg Error Pin.jpg (601.7 KB, 15 views)

Last edited by dick_barton; 03-11-2017 at 05:19 PM..
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Old 03-14-2017, 12:39 PM   #28
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Quote:
Originally Posted by dick_barton View Post
.....

I'm suspecting that an Error signal (pin 24) is being sent from the mainboard turning off the backlights.
Thanks but this tv is a mystery for me. If I put phase detector (picture 1) or DMM on inverter output wires than the TV works (picture 2). It works for hours flawlessly ! It doesn't matter where I put it, on the PSU board or CCFL input jack doesn't matter ...it works every time for hours.
WHY ? WHY ? WHY ? Anybody smart enough ? Oh and yes there is no error signal from the main board in this case ! Strange !
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File Type: jpg 20170314_190148.jpg (987.9 KB, 22 views)

Last edited by Adriatic; 03-14-2017 at 12:40 PM..
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Old 03-14-2017, 01:18 PM   #29
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Can you get image that is all whites to see if you will see light gray band on the screen?
I have a feeling that one of the lamp is not fully lit so when you add that small load to it, it stays on without tripping the shutdown.
What is the LCD panel model number?

I use CFL as the probe for high Voltage probe, for locating dead output of inverter transformer/circuits or dead lamp.
http://www.badcaps.net/forum/showthr...x+dx-40l261a12
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Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

Tear down pictures : Hit the ">" Show Albums and stories" on the left side
http://s807.photobucket.com/user/budm/library/

TV Factory reset codes listing:
http://www.badcaps.net/forum/showthread.php?t=24809

Last edited by budm; 03-14-2017 at 01:27 PM..
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Old 03-14-2017, 01:32 PM   #30
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

@budm
Tnx. Will try that tomorow. By the way this small load mystery fix only works for down CCFL if I put this "fix" on upper CCFL there is no fun, tv wont work. So you could be right.
It is LG 42LD750 as mentioned in title.

Last edited by Adriatic; 03-14-2017 at 01:34 PM..
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Old 03-14-2017, 01:45 PM   #31
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Quote:
Originally Posted by Adriatic View Post
@budm
Tnx. Will try that tomorow. By the way this small load mystery fix only works for down CCFL if I put this "fix" on upper CCFL there is no fun, tv wont work. So you could be right.
It is LG 42LD750 as mentioned in title.
That is the TV model number, I need the LCD panel model number.
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Old 03-14-2017, 01:49 PM   #32
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Will check tomorrow. Guess it writes on the panel itself.
Thank you budm.
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Old 03-14-2017, 01:56 PM   #33
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

http://www.badcaps.net/forum/attachm...3&d=1489234821
BTW, do not try to take high frequency AC at the output of the Inverter, the start up Voltage can be as high as 2000V! which will ruin the meter.
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Old 03-14-2017, 02:00 PM   #34
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Thx ... I know that, but to late already tried with cheap Vici vc99 (from EEVBlog only morons buy this meter ) but hay it survived yupiii
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Old 03-15-2017, 11:44 AM   #35
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

Quote:
Originally Posted by budm View Post
Can you get image that is all whites to see if you will see light gray band on the screen?
I have a feeling that one of the lamp is not fully lit so when you add that small load to it, it stays on without tripping the shutdown.
What is the LCD panel model number?

I use CFL as the probe for high Voltage probe, for locating dead output of inverter transformer/circuits or dead lamp.
http://www.badcaps.net/forum/showthr...x+dx-40l261a12
I did try with a bright background as you suggested and the result is very very good, the bottom half of the screen is perfect (picture 1) ...Seriously I did with white bckg (picture 2) and it looks to me no problem, evenly lit (backlight on max and min).

Did also try with cfl lamp, don't know if I did it correctly IMHO but it seems ok, btw cfl was 18W. At static image cfl shines evenly and at movie it also shines evenly with a small hiccups at the end of the bulb, guess this is ok. Both upper and down are the same. (picture 3 and 4)

Oh and yes panel is LC420WUH (SC)(M1) LG Display (picture 5)

Any solution to suggest ? Do Not wanna put phase detector inside tv because this would be an addon ha ha. And btw thank you for help.
Attached Images
File Type: jpg 1.jpg (806.4 KB, 17 views)
File Type: jpg 2.jpg (412.3 KB, 17 views)
File Type: jpg 3.jpg (460.8 KB, 25 views)
File Type: jpg 4.jpg (454.1 KB, 23 views)
File Type: jpg 5.jpg (427.9 KB, 16 views)

Last edited by Adriatic; 03-15-2017 at 11:47 AM..
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Old 03-15-2017, 01:37 PM   #36
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

OK, your LCD panel uses EEFL so the lamps can be connected in parallel without using the Ballast cap per lamp like CCFL.
So at this point I have no idea why the protection kicks in, but if it has added small load then it stays ON. More further troubleshooting is required.
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File Type: pdf LC420WUH.pdf (1.49 MB, 18 views)
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Old 03-15-2017, 02:08 PM   #37
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

I wonder if the two blue ceramic capacitors are the problem and that adding items to the output of the transformer is giving just sufficient capacitance to bring up the supply!!!
I suppose you could swap them over with the other pair of capacitors and see if there is any difference Clutching at straws here.

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Old 03-16-2017, 03:07 AM   #38
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

[QUOTE=dick_barton;733475]I wonder if the two blue ceramic capacitors are the problem... QUOTE]

Blue ceramic capacitors were already desoldered previously and are well in their specs...
Banging my head with this
Btw as last resort.. What would be replacement for phase detector ..something I would solder on ? Hmm
Not a expert in this , sorry
Don't wanna put my dmm inside ha ha nor the phase detector but I guess there would be some equivalent stuff maybe..just saying as last resort..

Last edited by Adriatic; 03-16-2017 at 03:12 AM..
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Old 06-20-2019, 01:06 PM   #39
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

[QUOTE=Adriatic;733588]
Quote:
Originally Posted by dick_barton View Post
I wonder if the two blue ceramic capacitors are the problem... QUOTE]

Blue ceramic capacitors were already desoldered previously and are well in their specs...
Banging my head with this
Btw as last resort.. What would be replacement for phase detector ..something I would solder on ? Hmm
Not a expert in this , sorry
Don't wanna put my dmm inside ha ha nor the phase detector but I guess there would be some equivalent stuff maybe..just saying as last resort..
My same Tv does exactly the same. I increased value of R403 and 404, 20ohm, 1W by few more ohms. Those are shunts for ccfl current, it worked for me. he set is on now almost 2 years without issues.
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Old 11-15-2019, 02:06 AM   #40
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Default Re: LG LCD 42LD750 Backlight flashes 4 times, then tv off.

[QUOTE=torentino;902154]
Quote:
Originally Posted by Adriatic View Post

My same Tv does exactly the same. I increased value of R403 and 404, 20ohm, 1W by few more ohms. Those are shunts for ccfl current, it worked for me. he set is on now almost 2 years without issues.
Hi sir, how much increase? 30 Ohms or more?
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