LG 42la620v Backlight Issue

[thejellyfox2]

I have a LG 42la620v tv with a back light issue. The set powers up as normal displaying the splash screen with the back light working properly but the back light cuts out after a second and the power led flashes three times. The logic board is functioning a picture can be seen and functionality confirmed by holding a flashlight up to the screen.

I have measured voltages from the PSU and the 3.5 12 and 24v rails all measure correctly and have little to no ripple or transients as measured with a scope. The model of the PSU is EAX64905401(1.7) I can post pictures if needed.

Additionally the PWR_ON and DRV_ON pins both read 3.5v when the set is initialing and running. The PDIM1 and PDIM2 pins both read ~1.2V They are something to do with brightness control. Im not sure what the appropriate reading is for these pins I don't think that an analogue voltage reference would be used for back light control.

The led driver outputs ~35v for a split second during startup then immediately drops off to nothing this behavior is repeated regardless of whether or not the back light is connected.

I an unable to isolate the cause of the fault either the back light or back light driver is defective. Im unsure as to how to test the back light I tried hooking it up across my bench psu but coud not reach the necessary voltage. It would be helpful if someone with some experience with these sets could shed some light on my situation.

Thankyou for your time.

[Andrew F. Ali]

Re: LG 42la620v Backlight Issue

From what I read, this appears that the backlights are faulty.

[nomoresonys]

Re: LG 42la620v Backlight Issue

Try unplugging mainboard from powersupply board, see if backlights come on and stay on.

[sordids999]

Re: LG 42la620v Backlight Issue

from what i read, this appears that the backlights are faulty.

+1

[thejellyfox2]

Re: LG 42la620v Backlight Issue

If the back lights are faulty why do they function for a split second on startup is there any general resistance that I should see across the led arrays.

Also will the back light driver shutdown if it detects no load?

I cant really disconnect the psu from the logic board as its relying on PWR_ON and DRV_ON pins staying high. I can try manually jumpering these pins too be brought high all the time and see what that does.

[nomoresonys]

Re: LG 42la620v Backlight Issue

If the back lights are faulty why do they function for a split second on startup is there any general resistance that I should see across the led arrays.

Also will the back light driver shutdown if it detects no load?

I cant really disconnect the psu from the logic board as its relying on PWR_ON and DRV_ON pins staying high. I can try manually jumpering these pins too be brought high all the time and see what that does.

Ok, I think that means no pullup resistors. sounds like it's worth a try, I'm not sure of the proper procedure.

[thejellyfox2]

Re: LG 42la620v Backlight Issue

I tried manually jumpering the PWR_ON and DRV_ON pins to be high. This resulted in the 12 and 24v rails of the psu becoming active but still no output from the back light driver regardless of any load attached.

[nomoresonys]

Re: LG 42la620v Backlight Issue

Guess it could be a protection circuit shutting it down maybe due to a backlight fault.

[nomoresonys]

Re: LG 42la620v Backlight Issue

Is this a late model tv, can't seem to find any parts for it.

[thejellyfox2]

Re: LG 42la620v Backlight Issue

How would I go about diagnosing a fault with the back light led array not the driver?

In my experience leds usually fail open circuit and there seems to be very even illumination across the panel when the back light is running.

This leads me to believe the driver is at fault and failing under load.

I have not disassembled the panel to that point yet but I assume that the diffuser may hide individual led failures.

Browsing parts available I cannot see any additional led driver board so I assume that the leds are being driven directly from the psu board.

[nomoresonys]

Re: LG 42la620v Backlight Issue

Might need a schematic to make sure that power supply is doing everything it should. I can't read them very well but the pros on here can.

[thejellyfox2]

Re: LG 42la620v Backlight Issue

I got a schematic from:

The driver uses the BD9483f IC if anyone knows about it.

All Caps tested good on the psu so ill go into more depth with it tomorrow with the schematic.

I can buy a good psu board on ebay I can also get a new driver ic if i can isolate it as the cause of the fault.

[nomoresonys]

Re: LG 42la620v Backlight Issue

Sounds like a solid plan.

[sordids999]

Re: LG 42la620v Backlight Issue

So what voltages do you have on this connector when tv is switched on etc

[budm]

Re: LG 42la620v Backlight Issue

Can I see good clear straight shot pictures of the whole top and bottom side of the board?
You TV has two LED strings, so we need to verify the Voltage of Anode and the Cathode of both LED strings.
1) TV OFF, set your meter to higher than 200VDC scale, then keep the black probe of the meter on the chassis.
2) Keep the red probe of the meter on pin 1 (LED 1) of connector P801.
3) Turn on the TV while keeping an eye on the meter to see how high the Voltage goes up to, record that number, then wait 10 seconds to see how low it goes down to and then turn the TV off.
4) Repeat stepS #2 and #3 on pin 3 (LED+), PIN 5 (LED -), and PIN 7 (LED +).

[thejellyfox2]

Re: LG 42la620v Backlight Issue

Im getting roughly 35 to 50 volts on both led strings the refresh rate of my meter isnt very fast so there is variation between readings. This happens regardless of whether I have the leds plugged into there connector.

I know that between 70 and 105 vdc @ 0.313A is nominal for this board.

[nomoresonys]

Re: LG 42la620v Backlight Issue

n/a

[nomoresonys]

Re: LG 42la620v Backlight Issue

Any pics? what are the numbers on your inverter board if you have one, can't seem to find much on this model, hardly see any parts or pictures.

[budm]

Re: LG 42la620v Backlight Issue

Im getting roughly 35 to 50 volts on both led strings the refresh rate of my meter isnt very fast so there is variation between readings. This happens regardless of whether I have the leds plugged into there connector.

I know that between 70 and 105 vdc @ 0.313A is nominal for this board.

So both boost converters are not working at all then.
I need good clear pictures of the whole top and bottom side of the board.
Did you look up the spec sheet of the LED driver IC yet to see if it has the VCC and the Enable signal to turn on the IC?
"The led driver outputs ~35v for a split second during startup then immediately drops off to nothing this behavior is repeated regardless of whether or not the back light is connected."
This does not make sense to me, it should show the input Voltage at the LED connector if the Boost converters are not working, I do not expect it to show 0V