LG 55LD520 wont turn on

[burner6999]

My LG 55LD520 recently stopped turning on. The red light at the bottom still comes on and when I push the power button it still turns blue but there is nothing on the screen. I took it apart and there doesn't seem to be anything burnt or any bulging caps. I started checking voltages and i was getting only 1 wire going to the main board with 3.5v so I unplugged the backlight inverter boards and I started getting all 12v and 3.5, but still there is no 24v at any of the pins going to the main board or to the back light inverter boards. what is my next step? I have attached pic. If you need any more let me know. Please help and thanks in advance.

[selldoor]

Re: LG 55LD520 wont turn on

Please can you repost your pictures using manage attachments jpgs less then 3mb should work, then ask a mod to remove the offline ones.
Maybe help to dust the board down and retake the pictures in better light without flash so we can see the components better.

[burner6999]

Re: LG 55LD520 wont turn on

Here are the pictures. I can not take pics without flash my camera doesn't work well without it. but here are some better pics.

[selldoor]

Re: LG 55LD520 wont turn on

Has someone been resoldering the caps on the inverter board?

Did you read this thread?
https://www.badcaps.net/forum/showth...t=33900&page=1

[burner6999]

Re: LG 55LD520 wont turn on

No they are original.

[selldoor]

Re: LG 55LD520 wont turn on

OK as voltages drop off when inverter is connected it may be a shorted mosfet.
This is mentioned in the other thread and there is a link to testing.
I suspect there is no 24v because it will be stopped from starting by a protect circuit.

[burner6999]

Re: LG 55LD520 wont turn on

There is no 24 even when the inverter boards are unplugged.

[selldoor]

Re: LG 55LD520 wont turn on

There is no 24 even when the inverter boards are unplugged.

Yes but unplugging the boards may be interpreted as a fault so protection might kick in shutting off the 24v

Need to check the mosfets for a short.

[Ltank]

Re: LG 55LD520 wont turn on

You have a shorted mosfet on either the master or slave board. Bad design there are two types mosfets tied to switch neg/pos for the ac to transformers. Half the mosfets on each board switch positive and the other half switch the negative. If the timing gets screwed up both sets will turn on at the same time and burns up. Mine actually had cought fire. Look up the part numbers on the two types, you will see that they are opposite and the drains of all are tied together. I did not keep my notes, I remember it when I see that model tv

[burner6999]

Re: LG 55LD520 wont turn on

I dont believe that is the case cause I was reading another post about the same tv where the guy was having problems with inverter boards and he said he had 24v when they were unplugged. Also it is wierd because when I first probe the 24v points it jumps up to 19v and then 0

[budm]

Re: LG 55LD520 wont turn on

The 24VDC should be present (with or without the load) when TV is on and the PS_ON is present. What DC voltage do you get on the filter cap C613? it should be around 360~400VDC measure at the two legs of the cap. You can use two 12V car lamps in series as the load for the 24V.

[burner6999]

Re: LG 55LD520 wont turn on

I measured the voltage at those at all 3 caps and it was 393vdc. I have a question. If it was a mosfet on the slave or master board wouldn't unplugging them one at a time change something?

[budm]

Re: LG 55LD520 wont turn on

You should have the 24VDC without having the inverter boards connected, so you need to fix that 24VDC SMPS section first.

[burner6999]

Re: LG 55LD520 wont turn on

That is what I am trying get figured out. Any other ideas about what to check? On the bottom side of the board the traces that runto the 24v go to the long bank of caps by the heatsink.

[selldoor]

Re: LG 55LD520 wont turn on

Yes - according to Ltank - post13 in the linked thread I gave earlier
https://www.badcaps.net/forum/showth...t=33900&page=1

[budm]

Re: LG 55LD520 wont turn on

Yes, in that link he does have 24V when inverter boards are not connected, but with the inverter boards connected the 24V went into shutdown due to over current protection kicked in. So that is why you need to get the 24V back first without the inverter boards connected. Then check the inverter boards for shorts.
Post 1:
" so I unplugged the backlight inverter boards and I started getting all 12v and 3.5, but still there is no 24v at any of the pins going to the main board or to the back light inverter boards."

[burner6999]

Re: LG 55LD520 wont turn on

Well when I do that it changes nothing. Just master plugged in or just slave it doesn't matter. That is why I am thinking it has to do with the 24v loss on the power supply. Hey bud any other ideas. I am a noob to this so any help from here is appreciated.

[selldoor]

Re: LG 55LD520 wont turn on

Ok - but when you get the 24v back you can try again to narrow down which mosfet might be shorted.
How about posting a picture of the bottom of the power supply. Is there any voltage on any of the caps in the row.

[burner6999]

Re: LG 55LD520 wont turn on

I can get the pic posted and check the caps. I will post back when I find out.

[burner6999]

Re: LG 55LD520 wont turn on

Ok after a few minutes of checking I have narrowed it down to master 2. With it completely out the tv tries to turn on. The back light flickers. The weird thing is that it has to be taken completely out off the chassis. Even when it is just unplugged it still kills the 24v. How do I test the mosfets?