LG 32LN578V No Backlight

[menlopete]

I got this TV free from someone who had this problem - namely sound is OK, but no picture.
With a torch I can see the LG logo on powerup and there is sound when I plug in a HDMI source, so it can't be too bad, I thought.
The guy who gave it to me had bought a replacement power board, but that behaves exactly the same way.
So I metered the connector from the power bd to the main bd.
Main voltages are OK. However there are three lines whose function I don't understand:
pwr on 3.5vdc 6.4vac (I measured with the multimeter set to DC and AC)
drv_on 3vdc 5.7vac
pdim 1.1vdc 2.1vac
I get no voltage out to the backlights.
What next? Should I get voltage at the backlight connector even if disconnected? (I don't)
Should I start looking for a new main board?

[dick_barton]

Re: LG 32LN578V No Backlight

Check the backlight voltages again and see if there is a voltage that rises and drop towards zero just as fast. The min/max function is extremely useful if you have it on your multimeter. The voltages will be DC not AC at the backlight connection.


DRV-On enables the backlight IC so it is looking like either faulty leds or IC801.
Since the power supply has already been replaced then I would be looking at a failed led(s).

[menlopete]

Re: LG 32LN578V No Backlight

Thanks for your reply.
I don't have that function on my meter.
It would appear that something happens when I power on - I get a couple of flashes of a reading on the meter, but nothing stable enough to read,

[neilc6]

Re: LG 32LN578V No Backlight

No backlights or a flash of the backlights is usually a symptom of bad backlights. A 32" is usually not too difficult to take apart with a low risk of cracking the panel.

https://techhometravel.com/lg-ln-ser...ed-backlights/

[menlopete]

Re: LG 32LN578V No Backlight

I see no flashing at all on screen. It would be nice to know for sure before ripping the screen apart - is it possible to connect an equivalent load to check that the circuit powers up?

[Diah]

Re: LG 32LN578V No Backlight

take the advice neilc6 gave to you.... after you take the screen apart you still can switch the set on and remain on with out LCD... you will see the BL how they flash for very short ms of time... remember all sheets you take out to access the BL should remain at same order when you put them back.

[petehall347]

Re: LG 32LN578V No Backlight

i recommend getting a back-light tester especially if you are going to be repairing more .
example i just opened a nice looking pc monitor that isnt working . tested the individual led strips with panel still in one piece . 2 strips shorted . needless to say its not worth spending any more time on .
2 strips test ok so will save panel in case i need the strips later .

[neilc6]

Re: LG 32LN578V No Backlight

If there is an open circuit in the string, the LEDs won't flash. That size of TV, the LEDS are likely wired in just one string.

[budm]

Re: LG 32LN578V No Backlight

So if you leave Black probe on the chassis, Red probe on the LED + pin, meter is set to 200VDC scale, then turn on the TV you do not see any Voltage at all? It should have at least the DCV feeding the LED driver boost converter.
Also take the reading with and without LED connector in place.

[menlopete]

Re: LG 32LN578V No Backlight

So if you leave Black probe on the chassis, Red probe on the LED + pin, meter is set to 200VDC scale

Ah now that is very different - I was measuring between the LED + and - previously and getting nothing. Measured from chassis to LED + gives 125V regardless of LEDs being connected or not.

[neilc6]

Re: LG 32LN578V No Backlight

For future troubleshooting, the main board sends two logic signals: PS-ON and BL-ON (sometimes labelled DRV-ON on LGs) to the power supply. If the BL-ON signal is sent and you have no backlight, the main board is likely good.

I have encountered some 4K LG models that turned on with no backlight and no DRV-ON signal. I usually search ebay for repair services for the main board to confirm if that model has a common main board failure (typically BGA issues).

[dick_barton]

Re: LG 32LN578V No Backlight

Ah now that is very different - I was measuring between the LED + and - previously and getting nothing. Measured from chassis to LED + gives 125V regardless of LEDs being connected or not.

The schematic shows the led voltage as ROW @ 80.4V, 400mA and POLA @ 70.3V, 400mA.
At 125V it looks as though you either have a faulty led(s) or a bad connection between the led strips within the panel.

[neilc6]

Re: LG 32LN578V No Backlight

Without a LED tester and suspected open LED, you will need to scratch to expose the copper on one side of each LED and put your mulimeter in diode test mode to check every LED.

[petehall347]

Re: LG 32LN578V No Backlight

Without a LED tester and suspected open LED, you will need to scratch to expose the copper on one side of each LED and put your mulimeter in diode test mode to check every LED.

i used to do that before i got the tester . the tester has actually fixed 2 tv.s somehow with no further work than testing and still working months later .

[menlopete]

Re: LG 32LN578V No Backlight

OK I worked up the courage to disassemble the screen, having never done it before.
And I discovered it's actually not too daunting a procedure.
So I have one bad LED strip, which I have now ordered.
Thanks for all your help guys - Peter

[neilc6]

Re: LG 32LN578V No Backlight

A 32" panel is relatively stiff and small so low odds of damage though you need to be careful with the flex connections. Wouldn't recommend 55" or over without some practice with broken screen TVs and suction cups.

[dick_barton]

Re: LG 32LN578V No Backlight

Do not make the fatal mistake of losing the order or direction the various panel sheets came off. You need to be put them back in the exact order they came off. Do not reverse any of the sheets.