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!
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LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried.
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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.
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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 transistorLast edited by R_J; 04-02-2022, 12:26 PM.
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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...Don't fear the repair...
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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.Last edited by triplefour; 04-02-2022, 03:42 PM.Don't fear the repair...
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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!Last edited by triplefour; 04-02-2022, 07:04 PM.Don't fear the repair...
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Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried
Originally posted by neilc6 View PostYou 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.Don't fear the repair...
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Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried
Originally posted by R_J View PostThe 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 transistorDon't fear the repair...
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Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried
Originally posted by Diah View Postthe 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.
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!Last edited by triplefour; 04-02-2022, 07:25 PM.Don't fear the repair...
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Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried
Originally posted by R_J View PostI checked a different schematic and found the panel_power transistor is a SSM3J332R,LF mosfet, I suspect this would work for Q2504
Where should I look for schematics ?Don't fear the repair...
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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 modelsLast edited by R_J; 04-02-2022, 07:39 PM.
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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.Last edited by Diah; 04-03-2022, 03:31 AM.
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Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried
Originally posted by Diah View Postso 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.Don't fear the repair...
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Re: LG 55LF6000-UB main board swap from 55UH6090 powered up but component on MB fried
Originally posted by triplefour View PostThank 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
we can see on it the optical port lit on mean this mainboards booted complete ... !!!
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