LG 42LB5610 BackLight problem.

[budm]

Re: LG 42LB5610 BackLight problem.

That sure sounds like bad LEDs.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

It does but...

Today i entered the menu and reduced backlight to zero. The voltage to the leds is now 55.5 V but still didnt help. I get the same problem and at the same time frame (a minute or two).

What confuses me is that some times when the leds go dark, i turn the tv off and right back on and they will come on again for a minute or two while other times when the leds go dark, i turn the tv off and back on and the leds will stay off.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

I will dissasemble the panel to reach the LED's remove each strip and with a constant voltage supply i will compare each strips current consumption. Hopefully that will give me a picture of the led's condition

[dick_barton]

Re: LG 42LB5610 BackLight problem.

What confuses me is that some times when the leds go dark, i turn the tv off and right back on and they will come on again for a minute or two while other times when the leds go dark, i turn the tv off and back on and the leds will stay off.

It could well be that when current is flowing something is heating up, a led or perhaps a joint and possibly going open circuit. Switching off cools it down and then it starts all over again.

[Andrew F. Ali]

Re: LG 42LB5610 BackLight problem.

Looks light bad LEDs to me too. LG is noted for over-driving their LEDs and so a lot of LG TVs' main problem is bad LEDs.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

As soon as i make some time to take the panel apart and test the leds, ill write back. Hopefully i will have some solid evidence about who is gulty.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

Good news brothers ! i discovered 3 shorted leds.

Thank you all once more for your valuable insight !

[Trevtron]

Re: LG 42LB5610 BackLight problem.

Sorry to hijack your thread @HellasTechn1 but I have the same set here with a slightly different problem.
It came to me with no backlight at all, turns out every LED was short-circuit. All 32 of them! Assuming one or two had gone thus forcing more current through the others causing catastrophic failure (xmas tree light scenario) I replaced all 32 LED's with 6V ones.
But I still have no backlight. I've checked the strips with a cheap LED tester and they all work OK (although my tester toils to get up to the 200 or so volts to run them all the strips together in series).
The +ve terminal of the LED output from the PSU is sitting at 101.6V, clearly not enough to fire up the LED's. Tracing this back to the rectifier D201 still shows just 102.4V, the difference being lost in D801. (These voltages are much the same even with the LED's disconnected).
The other outputs to the main board all seem OK.
Any clues as to why only this rail is low?
PSU board is a EAX65423701 (2.0)

[Andrew F. Ali]

Re: LG 42LB5610 BackLight problem.

Perhaps posting pictures of the PSU and especially close up focused shots of the LED Driver section Top and Bottom of the board will help us.

[budm]

Re: LG 42LB5610 BackLight problem.

Sorry to hijack your thread @HellasTechn1 but I have the same set here with a slightly different problem.
It came to me with no backlight at all, turns out every LED was short-circuit. All 32 of them! Assuming one or two had gone thus forcing more current through the others causing catastrophic failure (xmas tree light scenario) I replaced all 32 LED's with 6V ones.
But I still have no backlight. I've checked the strips with a cheap LED tester and they all work OK (although my tester toils to get up to the 200 or so volts to run them all the strips together in series).
The +ve terminal of the LED output from the PSU is sitting at 101.6V, clearly not enough to fire up the LED's. Tracing this back to the rectifier D201 still shows just 102.4V, the difference being lost in D801. (These voltages are much the same even with the LED's disconnected).
The other outputs to the main board all seem OK.
Any clues as to why only this rail is low?
PSU board is a EAX65423701 (2.0)

We need pictures of your board.
The 101.6V is the feeding Voltage to the Voltage boost converter that boosts the Voltage up to higher Voltage fro driving the LED string.
You need to leave the Black probe on the chassis, Red probe on the LED+ then turn on the TV while keeping an eye on the meter to see how high the Voltage jumps up to before settling back down to 101V.

[Trevtron]

Re: LG 42LB5610 BackLight problem.

Thanks for the quick replies, and apologies for not posting pic's.

I checked the +ve terminal with respect to chassis and even with a reasonably decent meter (Avo with bar-graph below the digital read-out) I could not see any peaking of the voltage, it just rises to the 101V fairly quickly and sits there. It drops to around 74V in standby. But this is where it gets confusing. The voltage on the -ve pin stays low at about 1.6V. BUT if I measure the voltage between the two terminals the meter shows 0V. Which would certainly explain why the LED's don't light up, but I can't get my head around why the voltage shows 0.00V when there appears to be a difference of over 100V when measured separately. (I'll upload a video if you don't believe me!!)
Anyway here are the pic's.

[budm]

Re: LG 42LB5610 BackLight problem.

OK, it has just one LED string, so remove the LED connector and then retest the Voltage between GND and the LED + again on the LED connector of the board when you turn on the TV, I expect to see the Voltage jumps up close to 200VDC or more so make sure to set the meter to be able to see that high of the Voltage.
If the meter can be trusted and you do not see the Voltage jumps up to that very high level then it means the Voltage boost converter is not working or the LED driver is not getting the BL-ON signal to turn on the circuit.

[Trevtron]

Re: LG 42LB5610 BackLight problem.

OK, it has just one LED string, so remove the LED connector and then retest the Voltage between GND and the LED + again on the LED connector of the board when you turn on the TV, I expect to see the Voltage jumps up close to 200VDC or more so make sure to set the meter to be able to see that high of the Voltage.
If the meter can be trusted and you do not see the Voltage jumps up to that very high level then it means the Voltage boost converter is not working or the LED driver is not getting the BL-ON signal to turn on the circuit.

Thank you for the clear explanation. Happily my meter has a manual range selector (otherwise it can get a bit confusing) and on the 2kV range it definitely did not 'peak', even with the string disconnected. I will check the BL-ON signal from the main board later when I get a minute and see if I can follow its path.

[Trevtron]

Re: LG 42LB5610 BackLight problem.

OK, I checked the voltages on the big connector, labelled as below.

There are two 'variables' (apart from power-on):

Those are P-DIM (Standby : 0V, On : 2.8V, reducing to 1.1V after a few seconds)

And DRV-ON (Standby : 0V, On : 0.11V).

All the other voltages are pretty much as labelled.

So I cut the DRV-ON wire and checked the return voltage from the main board and it is the same. Then I spied an old button cell lying on the bench & thought... why not? So I used it to provide 1.4V to the DRV-ON pin on the PSU and bingo!- the LED's came on. A quick check of the outputs revealed 215V on the +ve terminal and 45 on the -ve (wrt chassis).

So safe to say the DRV-ON signal is missing from the main board. Why would this be?
And is there any reason why the DRV-On can't be derived from another source (such as PWR-ON) that goes down when the set in in standby?

[budm]

Re: LG 42LB5610 BackLight problem.

OK, I checked the voltages on the big connector, labelled as below.

There are two 'variables' (apart from power-on):

Those are P-DIM (Standby : 0V, On : 2.8V, reducing to 1.1V after a few seconds)

And DRV-ON (Standby : 0V, On : 0.11V).

All the other voltages are pretty much as labelled.

So I cut the DRV-ON wire and checked the return voltage from the main board and it is the same. Then I spied an old button cell lying on the bench & thought... why not? So I used it to provide 1.4V to the DRV-ON pin on the PSU and bingo!- the LED's came on. A quick check of the outputs revealed 215V on the +ve terminal and 45 on the -ve (wrt chassis).

So safe to say the DRV-ON signal is missing from the main board. Why would this be?
And is there any reason why the DRV-On can't be derived from another source (such as PWR-ON) that goes down when the set in in standby?

DRV-ON (BL-ON) is from the main board and it looks like it is being kept low, it should be 2 ~ 3 VDC range, <1V = OFF, so more troubleshooting needs on the main board.
The BL-ON is turned on after the power supply is turned on so it will not cause the current surge by having the backlights on and the power supply to come on at the same time, the power supply is turned on first so it will have time to stabilize first and then the backlights are turned on next.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

I replaced all 32 LED's with 6V ones.
But I still have no backlight.

No, no, the correct type for this TV set is 3Volt 1Watt LED's.

Use this and it maυ very well work !

Though having all leds shorted is very weird. are you sure you have checked them correctly ?

[Trevtron]

Re: LG 42LB5610 BackLight problem.

No, no, the correct type for this TV set is 3Volt 1Watt LED's.

Use this and it maυ very well work !

Though having all leds shorted is very weird. are you sure you have checked them correctly ?

I pondered the 3V/6V question, and figured the LED output rating of the power supply being ca 200V divided by 32 LED's was a lot closer to 6V/LED than 3.

And yes, I also thought it strange that every one was shorted but I checked and double-checked. The resistance at the plug was less than 3Ω, all strips in series. I even checked the individual strips just in case there was a hidden short somewhere.

DRV-ON (BL-ON) is from the main board and it looks like it is being kept low, it should be 2 ~ 3 VDC range, <1V = OFF, so more troubleshooting needs on the main board.
The BL-ON is turned on after the power supply is turned on so it will not cause the current surge by having the backlights on and the power supply to come on at the same time, the power supply is turned on first so it will have time to stabilize first and then the backlights are turned on next.

I have the set running to soak-test the new LED's (thank you @freakaftr8 for that idea!) so I will take a look at the main board tomorrow. If all else fails I could put a 100μF across the BL-ON pin to ground & power it via a 10K resistor to delay the backlight. (I'm an analogue man, can you tell?)

[budm]

Re: LG 42LB5610 BackLight problem.

You can use level shifter on the switched 12V T-CON to turn turn on your backlights circuit.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

I pondered the 3V/6V question, and figured the LED output rating of the power supply being ca 200V divided by 32 LED's was a lot closer to 6V/LED than 3.

How many stripes does your tv cosist of ? if i remember correctly the one i fixed had 5 stripes of 8 leds each. But i am not 100% sure.
Still my tv would run the leds on 200Volts with brightness set to100 and about 55Volts with Brightness set to zero.

It would not hurt to check with the manufactuter or service manual about the leds rating.

I just find it very weird my tv to work with 3V leds and yours to work with 6v leds. Unless if i have mistakenly replaced the shorted leds with 3Volt leds and now the tv would just work because the psu can barely balance the current flowing in them. could that be the case ?

And one last question... assuming we have no idea if a tv uses 3V or 6V leds how can we test that ? can a 6V led run on 3Volts ? because if the led is 6V and i supply it with 6V to test it, i will burn it for sure...

[budm]

Re: LG 42LB5610 BackLight problem.

His LED for his panel are rated at 6V.
As you can see, the filter cap C224 (68UF 160VDC) for the power supply that feeds the Voltage booster so the 101V reading at the LED connector due to lack of BL-ON is correct, and when you force on the BL-ON and you get over 215V (the filter cap for the boost converter out put is 33uF 350V) on LED+ and about 45V on the LED- (this Voltage will go down as you turn up the backlights level, this is the Voltage drops on the PWM controlled MOSFET and the current sensing resistor on the return line (LED-) of the LED string to maintain the constant current through the LED string.

http://assets.shopjimmy.com/media/ca...071901-top.jpg
The printed rated Voltage on LED connector of the board is 208V 235mA that is why the cap is rated at 350VDC.