LG 32LH20R No backlights

[Biruslapio]

The CCFLs are not lighting up at any moment, when the fault occoured the lamps shut down and only sound still played, there's image on the screen, with a flashlight I tried turning energy saving off, no change, PWM_DIM signal is 0.2V, I isolated the pin and soldered a 3.3V regulator at it but it still does not work.
All the main voltages 24V/5V/12V are ok, the mainboard is not sending enough PWM_DIM signal voltage.

1~5 - 24V - 25.01V
6~10 - GND
11 - BRI - 1.54V
12 - INV_ON/OFF - 5V
13 - PWM_DIM - 3.3V (with an AZ1084 3.3V regulator)
14 - ERROR - 0.04V

The only signal left to manipulate would be BRI, but I do not know what voltage it's supposed to be.
Disconnecting the ERROR pin does nothing.
That floating IC next to the inverter cable is the 3.3V reg, currently disabled(no Vin).

[Robinsosa]

Re: LG 32LH20R No backlights

I had a similar problem. After confirming the CCFLs are good and inverter board is working. I changed the T-con board and TV displayed a picture.
Check the training manual.

[Biruslapio]

Re: LG 32LH20R No backlights

Nothing suspicious with the t-con, no shorts, fuse is fine, since there is image and sound, I've managed to turn off energy saving and put backlights at 100, this way the PWM_DIM went to 2.6V. still no backlights.

Disconnecting the t-con or just the panel changes nothing either, the "BR1" signal is ok according to this training manual for a similar model.

Powering the PSU via resistor jumper only sends 24V to the inverter, 0V on pins 11~14, no backlights.

[Robinsosa]

Re: LG 32LH20R No backlights

From your readings pin 12 INV on/off(P203) is 0v, hence the back light is OFF.
it should be HIGH to switch the inverter circuit. The signal to turn inverter ON comes from the main board. When you were performing the test, the main board is not active since no power is being supplied.

Try to do this:

Disconnect power cable from power supply board to the main board since we want to check power supply and inverter circuit only.

On connector P201 (on power supply board), Connect jumper 10k from pin 11 to pin 19 (PWR) AND pin 20 (INV ON/OFF). This will turn the power supply ON and also the inverter circuit ON.
See attached picture.

[Biruslapio]

Re: LG 32LH20R No backlights

Yes the PSU does not produce any signals to the inverter, all the traces are straight from the main board to inverter and the PSU just acts as a bridge.

With an 1k resistor INV ON/OFF gets 5V, still nothing happens.

I have already inspected the board and tested the fuse, the four switching mosfets and transformer windings, they are fine.

There's a 56 OHM resistance between the lamps output and ground, is that normal? Never messed with an inverter board before.

[Robinsosa]

Re: LG 32LH20R No backlights

Check voltages on P203 on the inverter board per manual.
Pin 1-4 = 24v
Pin 12 inv on/off= High
pin 15 error= 0v

If there are correct then check connections from inverter board to CCFL and for dry joints. If you have CCFL tester you can check the lamps. If the lamps are faulty or the electrodes have blackened, they won't turn on and inverter will shut down.

Does the ballast start Det LED turns on?
If not, then inverter board is probably faulty.

Check the continuity of the transformers, normally they go open on the legs of the transformer or develop dry joints. Also check the connectors for the CCFL they tend to burn or develop dry joints

[Biruslapio]

Re: LG 32LH20R No backlights

The pinout in that training manual is different from my TV, but the error pin is at 0V during PSU+Inverter only test, with the mainboard it is at 0.03V, basically zero.

I did the dreaded thing and opened up the TV to check on the lamps, they are fine and no sign of bad soldering, it looks perfect, unfortunately the only other 32" LCD TV I have uses "U" shaped lamps, so no way I can test them.

The inverter transformer does not output anything, it has one pulse at the startup and nothing else, like the controller detects an short or something, there's that low resistance to ground on the lamps output pins, is it part of the design of the board?

[Biruslapio]

Re: LG 32LH20R No backlights

Anyone has any idea? I'm stumped, the most out of order thing would be the PSU 24V rail measuring 25V, but that seems insignificant.

[R_J]

Re: LG 32LH20R No backlights

Does the led on the inverter board light when the tv is first turned on?

[Biruslapio]

Re: LG 32LH20R No backlights

Wow thanks, I can't believe I forgot about the LED, I've seen it and kept ignoring for some reason, it does not light up at all, there's 1.65V forward voltage on it.
Does the inverter means that no LED = Bad? Since it has all the necessary voltages to operate.

[Biruslapio]

Re: LG 32LH20R No backlights

Reading that training manual with more attention this time, the board in it is very similar, just different layout and main IC, it points that the led should start as the Inverter (or Ballast) is on the boot up process, and when the backlights were supposed to work, it would turn off, the one on this TV never emits light, so I will order a replacement board.

[Robinsosa]

Re: LG 32LH20R No backlights

If you check post #6 I asked you “Does the ballast start Det LED turns on?
If not, then inverter board is probably faulty” The training manual shows clearly where the LED is situated.
Anyway good luck.

[Biruslapio]

Re: LG 32LH20R No backlights

Fixed, just replaced the inverter board, it was very cheap and effective.

When the TV powers on the LED lights up bright blue, unmistakable if you know it should be lit during startup.

[Robinsosa]

Re: LG 32LH20R No backlights

Nice of you to give us feedback of your success

[TVAddict]

Hi everyone, apologies for reviving this old thread but i have the same exact issue with the same model LG tv, i would appreciate any guidance.
Basically i've tried the procedure outlined in post #4 but i can't seem to grasp the results which shows 0 volts on pwm-dim , 0 volts on error and about 1.5 volts on inv on/off the other voltages are present (24v, 12v) the 5v which is where i soldered one end of the 10k resistor to drops off to 1.5 - note p201 pin configuration on my board seems slightly different to the connector shown in post#4

As was the case with the member Biruslapio i too have not seen that led on the ballast board light up at all.

Do you guys think i've done the procedure correctly??

[nomoresonys]

Looks like same issue, Vis-à-vis acording to advice and post #11 quote from manual, it probably needs a replacement inverter. Led of inverter never lights up. I think another clue is the inverter on/off voltage seems too low.

[TVAddict]

Inverter on/off stays at 4.9v with everything hooked up, just no backlights or the ballast led. should i take a risk and replace the ballast board, if i can find one??

[lotas]

There are also PWR_DIM and BRI (brightness) commands.

[nomoresonys]

Inverter on/off stays at 4.9v with everything hooked up, just no backlights or the ballast led. should i take a risk and replace the ballast board, if i can find one??

I think I would, there should be a fuse on the inverter that you can test for continuity. Had a bad one a few times.

[nomoresonys]

If you do you'll want to be careful, good policy is never work on inverter with tv plugged in. these ccfl backlit tv inverters can put out over 1000 volts and can kill instantly. Leave it unplugged a few days before replacing.