LG 60PA5500 - strange piture faliure after board swap

[STEAMER629]

Hy!

I have a problem with the mentiond tv.

The story:

The tv had a blown y-sus when i brought it, i replaced all the blown mosfet's/IGBT's. I turned it on, had a picture but the white test pattern had a very bad mal-discharge, red, green and blue was fine. So i brought a complete board kit from a display damaged tv. But sadly the y-sus came as defective (blown). So i just swaped the z-sus to rule out that, same problem. I replaced the 2 buffer board, and i had no picture (no, the buffer ic's not shorted). Then after 2-3 restart it started doing like that in the video. I put it back all the original boards (2 y buffers and the z-sus), and doing the same. Maybe the original Y-sus mal-discharge problem escalated, and a drive is dead? Should i try to reapir the second blown y-sus? All the voltages are good as before this failure, and no bad solder joints. Ive never seen this faliure, repaired a lot of the 60" LG's, but this is new. I hope somebody also faced this issue, and maybe helping me out

Video:

https://drive.google.com/file/d/1L2oYVYpBWHidb9uiNWkp_KdXtmxJOPc3/view?usp=sharing

Maybe the video is still not porcessed by google, so sorry for the quality.

Thank you!
Istvan Vaczik

[Biruslapio]

Re: LG 60PA5500 - strange piture faliure after board swap

Take a look at the VSC and -VY voltage while it runs these tests, if they float around instead of a fixed DC value that's bad.

All of the IGBTs gate drivers and resistors must be checked, they fail often when IGBTs short, it's a P105.

https://www.badcaps.net/forum/showthread.php?t=94627

A 2R2 resistor went open on my Y-SUS when the top buffer failed.

[STEAMER629]

Re: LG 60PA5500 - strange piture faliure after board swap

HY! Thank you! I checked everything, It was the same resitor like yours. R513 mine went to 800 ohm from 2.2. So the probem on the video now fixed.

Soooo back to the original problem...

All test patterns are ok, no mal discharge or any problem. But on the full white i have mal discharge. I noticed on the full wihte patter the VSC and VY are dropping 0.8-1V.

Update, new problem!!!: after few minutes the tv turned itself off. Now when i turn it on its immitietly turn back off. No short on the two buffer, and no short on the y-sus...

[Biruslapio]

Re: LG 60PA5500 - strange piture faliure after board swap

That's not good, the 2R2 resistor fails when there is an abnormal current surge (buffers shorting for example), it's linked to the closest P105 optocoupler that drives IGBTs.

Y-buffers: Are you testing them correctly? Negative probe on each of the display output pins and positive probe on the floating ground, can be the negative pad of the SMD aluminium capacitors, resistance or diode mode, in 60 MEGAOHMs scale my meter shows 7.2M to 7.6M between the chips that are good, when they short it's usually 20 OHMs or below.

Disconnect the main board from the power supply to rule it out.

Problems with full white signal may be lack of VS, before that, are all the main voltages, VS, VA, VSC, -VY and ZBIAS matching the panel?

[STEAMER629]

Re: LG 60PA5500 - strange piture faliure after board swap

All voltages are maching to the panel. I checked mutiple times. And yes, i measured the buffers like you said (i had a shorted buffer before on a 60pk950). So, no clue. I have no idea why it has mal-discharge on white. Maybe the replaced IGBTs/MOSFETs are fake?

[Biruslapio]

Re: LG 60PA5500 - strange piture faliure after board swap

Now it's working again?

On the white wash test, does it get better when VS is increased?

I recommend enabling energy saving to minimum or medium, it will last very little with bright images on maximum brightness possible.

[STEAMER629]

Re: LG 60PA5500 - strange piture faliure after board swap

Yeah, i did absolutly nothing, and its working again. Maybe some protection triggered last time. I didnt increaced the VS or other voltages because they are all good from the frist day i started playing with a repair. Only the VSC and VY droping around 0.6-0.9V on full white, rest is fine. At the moment i have only that problem with the white (i made a picture).