lg rt26lz50

[GodG]

hi all,
i saw the post: "Lg 23lx1rv" from PlainBill that talked about an 1,8 volts noisy power source .

I have an LG RT-26LZ50 lcd that when power up (after some time off) shows vertical lines only on half the screen. After some minutes all screen goes black.

i have checked the several power sources and all of them seem stable, either way some caps of 47 were replaced by 100uf . Today i will confirm with an osciloscope if there is noise on the lines.

LCD uses "gm5221". pulling and moving the cable that connects the t-con board to the main board, sometimes, solves the problem and shows the image normaly, but only for a sec or two.
it seems that something desoldered itself. maybe a combination of not well solderes with bad caps.
could someone please, advise me on what to do next?
thanks in advance

[selldoor]

Re: lg rt26lz50

Remove clean and reseat the lvds cable checking for any shorts , bent or broken pins.
Welcome to the forum-
We can usually help a lot more if you please post a good clear picture of the whole chassis, and then pictures of each board, front and back and close up of connectors, (max resolution 2000x2000 and 2MB) using the manage attachments button, which is found by clicking "go advanced" under quick reply.

Please do not post inline and offsite as they slow down the loading of pages.

Examples of what is needed
https://www.badcaps.net/forum/attach...1&d=1290283049

https://www.badcaps.net/forum/attach...7&d=1280167246

[GodG]

Re: lg rt26lz50

hi selldoor
thanks.
i have already checked the cable and eventually ended up by bying a new one, but that did not solve the problem.
i was at work at the time of the first post, thats why i did not attached pictures.

[GodG]

Re: lg rt26lz50

could someone help me please?

[tom66]

Re: lg rt26lz50

Check the 1.8V regulators (in fact all) for out of tolerance readings - anything too low could be an oscillating regulator, you'd need a scope to be sure though.

[selldoor]

Re: lg rt26lz50

Ok not sure why you would replace 47uf caps with 100uf
one step up to 68 might be acceptable but double??

Pictures are not really sharp enough for troubleshooting.
I am a bit concerned by the browning all over the Main/AV board, particularly in the area on the back of the gmchip, has someone tried to reflow it? or do you know what has happened.
Have you changed out the brown SamYoung cap near the big 5221 chip- Sam young are poor.

Just above the 5221 chip in your pic there are IC 905 &906.
Can you get the part numbers off them - are they voltage regulators?
On the Tcon there is another U423 I think it says?

[GodG]

Re: lg rt26lz50

the reason from 47 to 100 was that i only had 100 (at that time, does not seem much) and i read on another posts similar caps change.

the tv has been repaired onde, but as far as i can see it was on 1.8/3,3v regulator. the browning seem residues from previous soldering i think.
that was mus have beem from the previous reparing. i could change it again.
later this night i will check the ic 905 and 906 part number.
i will check the tcon board also for the U423 (or similar).

thanks

[GodG]

Re: lg rt26lz50

HI,
sorry fot the absence but i have been on vacations.
i was able to find the problem but i dont know how to solve it.

it appears the problem occurs on the output LC network of the MP1583DN (5v).
I have a perfect square coming out the DC-Dc converter. But after the coil the 5v rail gains 400mv of oscilating noise.

i have replaced the 2- 1000u 16v caps but no news.
my next step would be to disconnect the 3,3v converter to see what happen.

could some one please help me.

thanks.

[tom66]

Re: lg rt26lz50

Replace the input 470u/35V cap too. Did you use low esr caps?

[GodG]

Re: lg rt26lz50

i have compared the ic1300 and 1301 outputs before the the coil and they appear the same. do you still advise me to replace this cap also?
the cap i used is a branded [M] (blue ones) probably that dont help much. I have aluminium versions but they are smd.

[tom66]

Re: lg rt26lz50

Possibly Panasonic caps. How did you measure ripple - with a scope?

[GodG]

Re: lg rt26lz50

yes, with a scope. i could show u a pic but i dont have the usb driver for the scope here.

[tom66]

Re: lg rt26lz50

I would give replacing them a go, if not, then it could be the power supply noise getting in.

[GodG]

Re: lg rt26lz50

finally i had time to replace the the 450uf on the input. but the problem persists. do you have any other alternative?
im wondering if the problem is not from the 5 to 3,3v dc-dc converters.

thanks for your time.