LG 32LN578V - Picture; No Backlight

[vinceroger69]

Re: LG 32LN578V - Picture; No Backlight

Hi all
Sorry to butt in on this one
But does anyone have a service manual for this set ?
I have just had one of these in with the same issue no backlights.
I pulled p801 and applied 60v using a variable p/s going from 0v to 60v very slowly directly to backlights still no backlights.
I then disassembled the screen to gain access to the led backlights there are three strips of 7 backlights I then applied 20v to each strip using variable p/s and found one led on two of the strips had blown I replaced them then applied 60v slowly to all three strips again and all worked fine. I then reassembled the screen and powered up the TV with all connected but still no backlights! I then pulled p801 and applied 60v to backlights while TV was powered up I have a perfect picture and sound.
So I could now do with a service manual covering the p/s or does anyone know what causes this issue on this model. ?
Much appreciate any help.

This link seems too have the same psu and a schematic
https://schematicscom.blogspot.co.uk...er-supply.html

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

Thank you vinceroger69

[dick_barton]

Re: LG 32LN578V - Picture; No Backlight

What is the IC part number as written on it.
What voltages do you have on the led connector pins when you power up. With meter on the first pin , power up and see if it goes to a peak before dropping then repeat for the other pins.

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

The IC MAP3202
Voltage on pins of p801 taken in 200v range
pin 1=led - 2.8v in st/by no change when powered on
pin 2= 00.2v in st/by no change when powered on
pin 3= 00.2v in st/by no change when powered on
pin 4=led + 58.1v in st/by decreases to 53.2v slowly when powered on

[dick_barton]

Re: LG 32LN578V - Picture; No Backlight

It looks as though the led booster is not kicking in since the voltage should be around 70 - 80V. It will say on the power supply board.

Can you give the voltages on the MAP3202 pins please.

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

All voltages taken in 20v range
pin 2 vcc power supply input = 0.00v in st/by 12.25v when powered on
pin 8 ref reference voltage pin = 0.00v in st/by 0.02 when powered on
pin 11 pwmi power gate driver input = 0.00v goes to 3.05v then stables at 1,32v
What other pins need checking ?

[dick_barton]

Re: LG 32LN578V - Picture; No Backlight

VCC supply (Pin 2) is within the specified range of the IC to run.
The reference voltage (Pin 8) should be 5V and is internally generated by the IC.
The enable voltage (Pin 1) should be between 2.0 & 5.5V for the IC to operate.

I would check Pin 1 to see if the IC is switched on.

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

pin 2 looks ok
pin 8 should be 5v
pin 11 looks ok

[dick_barton]

Re: LG 32LN578V - Picture; No Backlight

Is Pin 8 at 5V which it should be. What voltage is on Pin 1, not Pin 11

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

pin 1 =0.00v in st/by then goes to 2.98v when powered up .

[dick_barton]

Re: LG 32LN578V - Picture; No Backlight

So VCC (Pin 2) goes to 12.5V and Enable (Pin 1) goes to 2.98V when powered up and stay at those voltages.

The REF (Pin 8) which should be at 5V is at 0.02V when powered up and stays at that voltage while on.

If the above is correct can you measure the resistance (set off) between pin 8 and pin 4

Edit
Is there a capacitor connected from the REF pin to ground?

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

pin 8 is 0.00v in st/by then it only shows 0.02v when powered on if this should be 5v and it is generated internally does this mean the ic is faulty ?

[dick_barton]

Re: LG 32LN578V - Picture; No Backlight

Quite possibly or there is a fault around that circuit. See edited post #31

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

I am getting 0.06 in 200 range between pin 8 and pin 4

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

C811 is shown on circuit diagram but no values and it is a smc

[dick_barton]

Re: LG 32LN578V - Picture; No Backlight

I am getting 0.06 in 200 range between pin 8 and pin 4

Sorry Ron but do you mean 0.06 ohms ?

If it's 0.06 ohms then you have a short circuit somewhere possibly the smd capacitor.

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

Re measured this morning after tv off over night in the 200 ohms range and I am getting 02.5 ohms .

[ronhayes]

Re: LG 32LN578V - Picture; No Backlight

This capacitor.

[vinceroger69]

Re: LG 32LN578V - Picture; No Backlight

That capacitor is certainly a odd colour compared too the other ones maybe it has been getting hot etc
in this thread there is a service manual and this set also uses the same ic as budm was helping with a issue with the leds on this set which looks like dry joints on the led driver ic? but hopefully it will tell you the capacitor value you need
https://www.badcaps.net/forum/showth...ge=2&styleid=8

[dick_barton]

Re: LG 32LN578V - Picture; No Backlight

You need to lift off the capacitor and check if it still measures low resistance. Capacitors are normally over-range when measuring resistance.
If the resistance is very high then measure the resistance across the two pads the capacitor was soldered. If that is still very low resistance then I suspect the IC at fault.