LG 43UK6500AUA no LCD image, backlight and sound okay

[howardc64]

Similar to another LG UK series here

https://www.badcaps.net/forum/showthread.php?t=93943

Have backlight and sound. no LCD image.

• main board has integrated T-Con
• main board T-Con has LCD DCDC/PMIC RT6950GQW QFN68 package. Richtek chip. Can't find datasheet. Chip generates VADD (10.44V), VGH (~28V), VGL (~-14.5V)
• RT6950 pin 18-19 (clockwise) appeared to be bridged. Unlikely from my probing, looks like ball squeezage so it probably got hot
• Cleaned up bridging. Pin 17 is VGL (-14V) Pin 20 is VGH (28V). Pin 18 (I think 19 too) jumps between VGL and VGH voltage endlessly. Probably should be either VGH or VGL so likely chip failed.
• Disconnect right (looking at TV from front) panel driver board flex will get black image on left 1/2 and white image on right 1/2. VADD/VGH voltages looks good
• Connect right panel driver board flex (and the left) VGH and VADD all gets pull down to 1-1.5V.
• right and left panel driver board not symmetrical. Right board seem to be master (or have more signals) For example, 3.3V only supplied on right board and not left from main/tcon board (checked the left board's 3.3 signal trace. left board 3.3V dead ends after entering main+tcon from flex.
• Tested all caps on panel edge board. No shorts (all > Ks OHM on power rail side). Injected 1.5V 1A into AVDD and VGH rail, no current consumption, no FLIR hotspots.
• Voltage inject between different main rectangular ground pads on the edge boards only shows tiny current consumption. Shows full 1A consumption of +- injected on the same pad of course (as a reference to confirm properly injecting voltage) These ground pads are probably connected by panel matrix? So providing more confidence no panel matrix / COF tab shorts?
• All 6 drive chip on flex appears to not have burn marks (can only easily see the side towards the panel edge driver board. Other 1/2 hidden behind bezel which I didn't remove)

Looks like bad T-Con section and perhaps RT6950. My question is

• Anyone has datasheet for RT6950?
• Reasonable to conclude RT6950 pin 18 alternating between VGH and VGL is incorrect?
• What are the chances my panel + edge driver board failed? Seems like no short offers some confidence its good? But right edge board pulling down VGH and VADD is concerning. Of course if RT6950 is bad, then could just be that. I guess cost a main+tcon board purchase to find out if panel is good. haha.

[b0ba]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Similar to another LG UK series here
Reasonable to conclude RT6950 pin 18 alternating between VGH and VGL is incorrect?

There should be another pin alternatig VGL<->VGH. Those are VGH_ODD and VGH_EVEN alternatig at about half a second period.

[howardc64]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

There should be another pin alternatig VGL<->VGH. Those are VGH_ODD and VGH_EVEN alternatig at about half a second period.

Yes, put in another main+tcon board and same result so something is pulling down VGH and AVDD on the right side of the panel. Have to diagnose deeper.

Also removed (hotair) and reflowed the RT6950 on original main+tcon, seems to have killed it (presumably with too much heat). Chip gets 13V input but no more AVDD, VGH, VGL, VGH_ODD, VGH_EVEN.

[b0ba]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

There are five 0 ohm resistors on both sides of the panel dubbed CL-1 through CL-5. Try removing them all one side first (I shift them with two soldering irons) on the left-hand side first (observe results) then do the other side. Both ribbons t-con->panel should be connected. There will be no image if one of the t-con<->panel ribbons are disconnected.

[howardc64]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

There are five 0 ohm resistors on both sides of the panel dubbed CL-1 through CL-5. Try removing them all one side first (I shift them with two soldering irons) on the left-hand side first (observe results) then do the other side. Both ribbons t-con->panel should be connected. There will be no image if one of the t-con<->panel ribbons are disconnected.

Didn't do this yet but got the panel up by masking out pin 1 of t-con cable to the right panel driver board (looking from front of TV)

• Masking pin1 will get black lines
• Masking pin1-4 will get rid of black lines
• Have a 2+ second interval subtle brightness change. Don't know yet if panel or backlight.
• pin 1 & 3 reads ~2.8v when panel is on. pin 2-4 fluke ohmmeter voltage display shows OL but flickers slightly. Probably changing too fast.
• pin 1 is continuous to probe point A

Without pin1-4 masking, power disconnected, both t-con cable attached

• all resistors around point A reads ~100ohms in circuit
• all resistors reads ~3.3Mohm to ground point B. But sometimes seems to be 1.7Mohms. What causes 1.7Mohms. Left panel edge board (likely good, doesn't pull down VADD, VGH etc) always reads 3.3Mohms.

Resistors aren't labeled CL-1 through CL-5 but I presume these resistors are near the problematic areas. Will remove them on right edge board and see any results.

[howardc64]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

UPDATE from last post

Been able to determine the 2s+ brightness flicker correlates to when black lines aren't present. So

• No masking pin1 get no image
• Masking pin1 get black lines and no flicker
• Masking pin1+2 get no image
• Masking pin1-4 get rid of black lines and get flicker
• Tried various combo of masking pin1 + pin3-4. Can't get no black line and no flicker.
• Masking pin1 + 5/6/7/8/9 all get lines (5/6 seems least objectionable) and no flicker

[howardc64]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Think I found the failure point and perhaps its the reason why so many of these have panel failures.

Right side panel last COF looks like may have some damage compared to the other 5 COFs on the panel. Here is a comparison.



There are also scrape marks on the flex (remaining 5 doesn't have any). These models have a metal external bezel with sharp edges. If this was assembled first before the metal protective shield, its likely the source to scrape the flex. Naturally the most difficult step to assemble this bezel would be the corners. Probably much safer to assemble upper corners last to avoid scraping the flex cables. After scraping, the additional metal protective shield could also be touching this flex (which could be compromised with scrap marks) and eventually causing COF damage. Maybe a good preventive maintenance is to add electrical tape covering the metal flex shield? Of course no owner will ever do this...



Several posts outline masking pin1-6 is a fix. Not sure if they noticed the slight brightness flickering.

https://www.badcaps.net/forum/showpo...12&postcount=2
https://www.badcaps.net/forum/showpo...0&postcount=17

Will consider desoldering the bank of resistors one by one and see if can recover better image on the panel.

[howardc64]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Got full picture (no black lines) and no flickering. Solution is as follows. All on the right panel board (looking at front of TV)

• removed 1 resistor (same as masking off pin1 on T-Con connector). Get black lines. But this removes the short that takes out the picture.
• checked the remaining 2 rows x ~6-7 resistors and all beep briefly in diode mode against a couple of points (didn’t on the other panel driver board). These resistors corresponds to many pins on the T-con connector. Decided to just mask off additional pin 2-24 and got full pic with no flicker.
• Decided to remove all resistors all the way to VGL1 line = least amount masking required. Works perfect and TV has slightly less buzzing than masking off pin 1-24.
• 1 resistor measured 100ohm out of circuit, probably true for all.

Unnecessarily killed the original main board’s t-con section by hot air removal of the DCDC/PMIC (Didn’t know PMIC produced an alternating VGH VGL signal and thought the chip was bad. Not so well skilled with hot air yet, definitely can be a dangerous tool haha) Will order one from AliExpress for $5 shipped and see if can rescue the board.

[nomoresonys]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Glad you sorted it, looks like a very nice picture on that one.

[howardc64]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Glad you sorted it, looks like a very nice picture on that one.

Indeed, these LG IPS panels are the best I've seen across the lower price points. 43inch's tighter dot pitch makes it look even better. I'll use this one in the den to ensure fix longevity until fixing something else better haha.

Recently fixed Samsung's QLED was a better picture but these Samsung snap on rear cover all pop+crackle after removal+reassemble... Is there some trick? Even taped all the edges and clip points without fixing the noise. Really unpleasant.

[b0ba]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Good job. Cheers.

[nomoresonys]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

So it only did it after reassembly? there's something here about QN95B but might be different: https://eu.community.samsung.com/t5/...5399588/page/2

[howardc64]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

So it only did it after reassembly? there's something here about QN95B but might be different: https://eu.community.samsung.com/t5/...5399588/page/2

Its like this one

https://www.youtube.com/watch?v=NtQs...l=Paul%27sTVCo

Got TV used so don't know if it had random pop/cllicks before disassembly. But seems to have a fair bit of complaint about it from consumers online. Basic construction isn't that rigid.

Anyway, I'll see if can resolve it next time I get one of these haha.

[howardc64]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Good job. Cheers.

Thanks, your input was crucial to help me understand those resistors were really just fuses? and most were connected directly to t-con flex connector. I guess my FLIR would have caught the COF if it wasn't hiding under the bezel haha.

[nomoresonys]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Ah ok, the screwless backs, hmm, I haven't heard of those backs being noisy but knowing samsung they prolly did something wrong.

[b0ba]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

[QUOTE=howardc64;1221091] just fuses? /QUOTE]
No idea tbh. Guess LG put them for "debugging" purpose. It takes cutting on other manufacturers panels.

[nomoresonys]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Have seen the cutting done on youtube but knowing what and where to cut looked confusing without a step by step guide or is there a simple method of what to cut, I wonder.

[howardc64]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Have seen the cutting done on youtube but knowing what and where to cut looked confusing without a step by step guide or is there a simple method of what to cut, I wonder.

Here is a guess on the process...

As noted, removing resistors on this panel is same as cutting the lines (but of course easier to restore any cut lines). So how to choose which lines to cut?

Basically, I guess these lines at the very edge of the panel on both edge driver board are the row gate control signals (VGH, VGL, VCOM) These signals must travel up both side of the panel and apply row control from both sides (I guess for less voltage drop across the row?) Here is a good diagram (high/low voltages are VGH/VGL)

http://lh6.ggpht.com/--pf66yN_UQE/U9...5B5%25255D.gif

Both left and right edge board's row control signal board patterns are mirror images (see pics in post #1). Therefore, I decided to just remove the resistors (ie cut the line) on the failed side that didn't match the good side in diode test. The first pic in post #8 shows 2 probe points that beeped briefly with the 2 rows of resistors (no beep on the good side of the panel) so I guess these were causes by the failed COF and "cut" these lines.

Since removing these resistors / cutting these lines were identical to masking T-CON ribbon connection (confirm by checking which line/resistor = which pin on T-Con connector) It provided more confidence that cutting these lines are effectively same as masking T-Con signals. But cutting the lines are more logical since they reflect how the COF failed whole masking a continuous group of T-Con signals might remove more signal than necessary.

Anyhow, I'm not professional and spend 2 days learning (didn't even know what VGH/VGL was in advance haha) and doing tests. Not sure the time spent is profitable for a pro. But I suppose experience would have enabled much faster progress in the following sequence

- disconnect one side t-con at a time. Check VADD/VGH/VGL/VCOM
- This TV showed right side panel was puling own all these signals
- Didn't find any obvious shorts on the edge board
- Inspect the COFs would have reveal 1 suspect COF
- Diode test compare lines around the suspect COF to its mirror on the other edge board would have revealed the lines with differences. Some lines labeled with VGH, VGL.
- Then it would offer a clue on which lines to cut to get rid of diode test differences.

Just a thought on the process after going through it once. Perhaps the experience pros can chimed in on the procedure.

[nomoresonys]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

Thanks Howard, great explanation.

[b0ba]

Re: LG 43UK6500AUA no LCD image, backlight and sound okay

quick guide