LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried.

[triplefour]

So I was trying to repair this tv. Board independent tests prove Psu is good. Backlights are good. But mainboard does not turn on or respond in any way to button or remote. I dig around and find some parts I've stashed from other LG tvs and find the box for 55UH6090 which says all parts were good. Taken from bad panel tv. It looks different and takes a different tcon but the connections to the Psu and to the tcon are conpatible. I figure what the hey just try it. So I do. At first I don't connect the tcon. It powers up fine, backlights and everything. Turn it off, plug in the tcon and turn it back on and I am greeted by one of the components on the mainboard going poof with flames and smoke. It is marked Q2504 and is a 3 pin device right near the LVDS connector. Checking for voltage I can see I have 13v going to the component but nothing coming out so no voltage to the tcon so no picture. TV runs fine otherwise. So my question is this: was it a bad idea to swap mainboards even tho connectors looked same and it's from a very similar tv? Has this happened to any of you? Was it just going to fry anyway for other reasons? The parts were sitting for a long time but other than that I can't see what went wrong! Of course now that the component is fried I can't read the markings on it so I will need to track down a picture of the board or a schematic to get the replacement part....but what are the chances it will just fry again?? All components around it seem to have survived. Nothing reads short except for the fried component. Your wise input would be much appreciated!

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

Here's some pictures. There's 3 more but I have to resize them

[neilc6]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

You connected the main board from a 4K TV to a non 4K one. U in the model number is an indication of 4K (UHD). Sometimes ok to try swapping mains from same/similar model but different screen sizes and the connections and board match.

[R_J]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

The transistor Q2504 supplies 12v to the t-con and likely the panel as well, which is likely different between these models.
The transistors, collector would normally connect to the t-con fuse via the lvds cable
It looks like the supply for the t-con is on different pins on the t-con boards.

I suspect your main board tried supplying 12v to ground (or some other component on the t-con or the panel which may now be damaged) via the transistor

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

Dang...so lesson learned. Just because it looks like it matches doesn't mean it won't fry something. I've done a lot of risky swaps but this is the first time I've seen something fry as a result. I would like to see if replacing the burned component would fix the problem but I guess I'll need to wait till I have the appropriate panel to test with. It doesn't look like anything on the tcon got damaged and I'm hoping the panel is fine. I guess I should have focused on fixing the original main board...which just sits there not responding to button press or remote...

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

Here's those other pics. This is the original tcon. I guess I assumed the lvds pinout would be same if similar model same brand and connectors match...feel I've done this before in other tvs with no harm. Maybe doesn't work but no harm...come to think of it I had already tried a different psu and main board combo on this panel. One from a 50" lg that also had a different tcon but same connections. I had no visible problem other than the TV just displayed some vertical lines. Though who knows maybe doing that damaged the panel already.

[Diah]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

the blown IC are stepdown . just you have the MB now toasted... T-con maybe toasted too as the VCC pin are at different align .. panel will not effected.

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

I accept that i may have fried that mb too bad to repair but i still would like to replace that component just to see. I have a spare tcon of the same kind if it took that out as well. For now I'm gonna focus on getting the original main board to boot up. It just sits there not responding to button or remote but the light is on. Lights are on but no one's home! I see 3.5v on pin 3 of the button board connector and it goes down to zero when I push the button so that part of the circuit is working. I see 3.1v on the PSON pin which I think needs 5v to tell the psu to turn on all the way? Not sure about that because all 13v voltages are present. Backlight on pin sits low, so mb is not turning it on...I guess I'll try to trace where both of thise pins lead to.
This is all a learning experience. I'm working with scavenged tvs and parts so no real damage to anything but my ego...and that poor mainboard!

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

You connected the main board from a 4K TV to a non 4K one. U in the model number is an indication of 4K (UHD). Sometimes ok to try swapping mains from same/similar model but different screen sizes and the connections and board match.

Thank you for clarifying this. I will watch out for the U from now on!

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

The transistor Q2504 supplies 12v to the t-con and likely the panel as well, which is likely different between these models.
The transistors, collector would normally connect to the t-con fuse via the lvds cable
It looks like the supply for the t-con is on different pins on the t-con boards.

I suspect your main board tried supplying 12v to ground (or some other component on the t-con or the panel which may now be damaged) via the transistor

Yes upon closer inspection I now know that the mainboard sends 12v via the leftmost 8 pins but on the tcon for this panel it only receives 12v on the leftmost 4 pins so the 12v went to 4 more pins than it should have!

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

the blown IC are stepdown . just you have the MB now toasted... T-con maybe toasted too as the VCC pin are at different align .. panel will not effected.

I hope you're right about the panel but honestly there's a good chance the panel was bad already. When I hooked up parts from yet another TV, nothing fried and I was able to see some lines on the panel but no picture. Could be tcon is incompatible with that main board too but just not catastrophically so. I need to check pinouts before trying this stuff from now on!
Oh I checked on the MB that did work and showed me some lines on the display. It only sends 12v on 4 pins so that's probably why it didn't fry anything. I guess I should see on the tcon where that extra 4 pins of 12v could have gotten to

The TV had been sitting out in the elements for some time. The psu was broken, had quite a few rusty components, and the main board won't boot. And probably the panel is bad too. Good TV to learn on!

[R_J]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

I checked a different schematic and found the panel_power transistor is a SSM3J332R,LF mosfet, I suspect this would work for Q2504

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

I checked a different schematic and found the panel_power transistor is a SSM3J332R,LF mosfet, I suspect this would work for Q2504

Thanks I'll try order some
Where should I look for schematics ?

[R_J]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

I doubt you will find the exact schematic, the one I used is for a 49LH5700-UD (LA66L chassis). The panel power circuit is not that critical and similar between models

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

Would be nice if I could find a schematic for the mainboard..

[williej]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

Here is the manual.

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

Here is the manual.

Wow thank you! It has the schematics! I have a chance to fix this board now

[Diah]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

so SSM3J332R panel VCC of sure because of false pin align went short.

BTW: yours Photo is ths belong to original MB.. if yes it work and booting 100%. if you feed it with HDMI signal it will of sure hear sound. if no video then T-CON. .. or panel to start one side ribbon plug out.

[triplefour]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

so SSM3J332R panel VCC of sure because of false pin align went short.

BTW: yours Photo is ths belong to original MB.. if yes it work and booting 100%. if you feed it with HDMI signal it will of sure hear sound. if no video then T-CON. .. or panel to start one side ribbon plug out.

Thank you. The orginal mainboard does not boot. Does not respond to button or remote. Backlight doesn't come on. Red standby light is on and light from optical audio out is on. I haven't had a chance yet to study the schematic but I will soon

[Diah]

Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried

Thank you. The orginal mainboard does not boot. Does not respond to button or remote. Backlight doesn't come on. Red standby light is on and light from optical audio out is on. I haven't had a chance yet to study the schematic but I will soon

i am confused... the photo i mentioned before of mainboards you uploaded .. belong for what then ???
we can see on it the optical port lit on mean this mainboards booted complete ... !!!