LG 42LW4500 no picture no backlight but near all lines good

[disorder]

hello, i have the same issue as this thread :

https://www.badcaps.net/forum/showth...8668&styleid=8

it's a 42LW4500 LG tv, previously it had the classical L407 classical issue : desoldering due to bad resin that gave me no 24V. work great for one month

yesterday : sudenly, green light then screen shut down. white led ok

open it and test : first put a resistor between 3,5 and PWR-ON, another one between 3,5 and inv_on : tv start, backlight ok and stable.

but when the connector between motherboard and PSU is connected : i have 3,5V 12V 24V but NO INV_on, a_dim and p_dim.

if i test ic1002 at inv_ctl : got 3.5V, r421 is good and Q405 no short.

so i think my issue is due to P_dim (power dimming so)

all pins joints were resoldered

during all this tests, once the tv works for some seconds then shut down.

thank you

put here service manual .

[nomoresonys]

Re: LG 42LW4500 no picture no backlight but near all lines good

Makes me think the same thing as the thread you refer to...that the mainboard is not sending the inverter on signal to the powerboard, I would try a replacement mainboard.

[zen]

Re: LG 42LW4500 no picture no backlight but near all lines good

I do recall working on one of these sets some time ago .

The back light themselves can be very intermittent .

In fact , on the one i had , set would run for 2 - 3 day without problem , before back lights cut out .

[disorder]

Re: LG 42LW4500 no picture no backlight but near all lines good

hello , some news : problem seems to be solved

after analysed a-dim p-dim and inv_on, many filters missing compared to schematics.
so it was easier to find the culprit

removed Q405 by another NPN transistor and since the tv works normaly
(i took a npn with same values except cutoff at 0,1A )

hope this will help someone else later :-)

[zen]

Re: LG 42LW4500 no picture no backlight but near all lines good

Great result , & thanks for the information .

[disorder]

Re: LG 42LW4500 no picture no backlight but near all lines good

failure !!!

the tv worked many time in my garage, just plug in in the house. worked for 20 minutes and then shut off and does the same weird issue.

before shut off , screen goes like snow image as CRT screen

maybe cold joints ? will look,

[disorder]

Re: LG 42LW4500 no picture no backlight but near all lines good

finally got it.
i baked it in oven and now it works great , maybe for some days or years.
i have tried it for more than 15 hours wthout any problem.

some bad solder joint so.

Issue solved :-)) !