LG 49lf510V no backlight

[sordids999]

Hello
Having problems with this tv no backlight at all

If I connect backlight tester to led legend and turn on tv I get a good backlight and image
But I measured the led legend and it's only measuring 49 volts even turning the tv off and on the voltage does not go up

Thanks

[budm]

Re: LG 49lf510V no backlight

What is the AC adapter OUTPUT DC rating, 19V, 24V?
It sounds like the booster circuit (L501/502, Q502) is working since you do have 49V (but we do not know what it should be at this point). The filter caps for the backlights ae re rated at 100VDC so 49V may or may not be right.
What Voltage do you have on each pin of the LED connector (all connectors are in place) with ref to GND?
When Voltage reading does you LED tester show when testing the LED string?

[sordids999]

Re: LG 49lf510V no backlight

19 volt ac output
Led legend pins 1-4
pin 1 00.0 pin 2 0.00 pin3 0.00 pin 4 49.1 volts
Led tester is showing 96.6 volts

Thanks

[neilc6]

Re: LG 49lf510V no backlight

Did you check if all your LEDs are lit with the panel removed? It may be hard to spot one or two shorted LEDs but it will cause the baclights to not stay on. Backlights usually flash on power up or plug in on LGs.

[budm]

Re: LG 49lf510V no backlight

19 volt ac output
Led legend pins 1-4
pin 1 00.0 pin 2 0.00 pin3 0.00 pin 4 49.1 volts
Led tester is showing 96.6 volts

Thanks

If the total Vf is shown as 96.6V @30mA of drive current, that means the total VF of the LED string will be over 100V at over 100mA of drive current but the caps are only rated for 100V, something is not quite right.
If you remove the LED connector and have the meter probes to GND and pin 4 of the LED connector and then turn on the TV, what do you get?
Also pin 1 the return line is not showing any thing.

[sordids999]

Re: LG 49lf510V no backlight

With led connector removed I'm still only getting 49 volts on pin 4 and pin 1 is 0.00

[budm]

Re: LG 49lf510V no backlight

I believe that these are your LED's strip, it has 36 LED's total so at 30mA the total Vf will be = 36 x 2.7 V = 97.20 V, your tester shows 96.60V so that is about right.
Looking at the picture, I am beginning to think that two caps (33uF 100V) are connected in series to handle over 100V for the LED Voltage.
You panel model is NC490.
The LED strips are:
https://www.shopjimmy.com/lg-nc490du...d-strips-8.htm

Now we need to find out why you are only getting 49V instead of over 100V.
Can you read the P/N of the LED driver IC 501?
You can also try heating those caps in the LED driver section to see if it has any affect on the LED output Voltage.

[sordids999]

Re: LG 49lf510V no backlight

I have uploaded a picture with details of the ic 501

[sordids999]

Re: LG 49lf510V no backlight

Also I have tried heating up the two caps by the led legend but no luck

[budm]

Re: LG 49lf510V no backlight

Also I have tried heating up the two caps by the led legend but no luck

Have to look up the spec sheet and application circuit for that IC then we can start checking Voltages.
Check the D502 in Ohm mode and Diode mode.
Also check the DCV between the two legs of the lytics cap (33uf 100 Vdc) to the left C537, that Voltage will be the feeding Voltage to the LED Boost converter circuit.

[sordids999]

Re: LG 49lf510V no backlight

D502 tested fine
C537 reading starts at 26 volts but slowly decreases
Also noticed Q502 N-Channel MOSFET SMN630LD has taken a lot of heat voltage reading of the gate is also 49.1 volts

[budm]

Re: LG 49lf510V no backlight

Please take the resistance reading between Cathode of D502 and GND.
C537 reading starts at 26 volts but slowly decreases according to the service manual, that Voltage should be 50V for the first stage Boost converter, the first stage takes 19VDC input and boosts it up to 50V then that 50V is fed to the second boost converter.
So you need to start checking boost converter to find out why it is only putting out 26V.


LED driver IC DT1641.
I found the service manual for set.

[cheeky2]

Re: LG 49lf510V no backlight

Have you checked R551 10ohm as it looks like it has heat damage to it?

[budm]

Re: LG 49lf510V no backlight

BTW, that over heated Q502 checked out OK?

[sordids999]

Re: LG 49lf510V no backlight

Q502 mosfet
If I connect positive lead to drain and negative to source no reading then if I quickly touch gate then back to drain I should have a reading but I'm getting nothing ?

[budm]

Re: LG 49lf510V no backlight

The second booster SEEMS to be working because it is getting 26V in and boosted it up to 49V (double the input), but the first booster is getting 19V but the output is only 26V instead of 50V per schematic, may be that is the section to check.

[sordids999]

Re: LG 49lf510V no backlight

So you think Q502 is good ?

[budm]

Re: LG 49lf510V no backlight

Well, it is not shorted out and looks to be switching because you are are getting the 49V. You need to find out why your are not getting the 50V output from first stage, look at the schematic and start checking the parts in that section.
BTW, the ac adapter output is 19VDC NOT AC, right? You indicate in post 3 as:19 volt ac output.

[sordids999]

Re: LG 49lf510V no backlight

Ok thanks

[budm]

Re: LG 49lf510V no backlight

I just look at the schematic again and so please clarify something for me, please measure the Voltage between the two legs of cap C513 (33uF 100V) to see what you get.