UN55KU6270FXZA Backlight Flashing Briefly on Power On

[howardc64]

Probably LED string issue but wanted to check any more easy testing before ordering the LED strips.

Inspection/Probing

• No shorts around caps on main
• No bulging caps on PSU, no evidence of any damage

Disconnected PWM_BLU between main and PSU

• Backlight powers on when PSU gets power. Doesn't wait for main board power on.
• Remote turn on TV, display is perfect. White screen test shows no dark spots
• Remote turn off TV, backlight stays on until PSU power pulled
• Both LED strings reads 124+V. 14x 3v LED / strip x 3 strips for each string
  
• PWM_BLU line on PSU reads 3.1v

LED Tester

• LED strings disconnected from PSU. Tester only drive up to 109v. One string ramps up to 109 a little slower when powered off for awhile.

Anything else to check before ordering LED strips?

All pics shown are with PWM_BLU disconnected

[howardc64]

Re: UN55KU6270FXZA Backlight Flashing Briefly on Power On

PWM_BLU reading on main/tcon side with line disconnected to PSU (only line disconnected on the 12pin connector between main/tcon and PSU)

• ~0.3v and doesn't change as I change backlight menu between 0-20

Perhaps this suggest main/tcon board. Probed around and found the PWM_BLU connector solder joint just flaked off. Touched up the solder joint and no change. Perhaps a short has weakened the solder joint.

Reconnected PWM_BLU between main/PSU. Power on the TV and PWM_BLU is shorted the ground. So pretty definitive issue is main board. TV owner also mentioned game machine on HDMI sometime causes TV to reset. Perhaps first step is a toaster oven reflow before ordering a salvage replacement

[howardc64]

Re: UN55KU6270FXZA Backlight Flashing Briefly on Power On

Found a deterministic failure sequence. Its related to main board's video signal processing to adjust backlights via PWM_BLU.

As I understand it, Main and PSU has 2 backlight DIM signals

• ANA_DIM - This is direct backlight control. When changing backlight setting between 0-20 on the TV menu, backlight string voltage will change. From ~121 to ~128 per string on this TV with 42 3v LEDs per string.
  
• PWM_BLU - This seems to be main board's adjustment of backlight based on picture image brightness and maybe also ambient light.

PWM_BLU is the source of the problem on this TV. Seems quite frequent based on the comment list in this youtube how to video

https://www.youtube.com/watch?v=9R9e...onsElectronics

TV goes into a reset boot loop whenever its video input is dark. For example, set to HDMI without an input plugged in. And probably also when ambient light sensor detects total darkness (like facing the TV screen downwards on a bench while diagnosing it)

Common Problem Reproduction

Set inpuit source to an input without any imagery such as HDMI without a device. TV will boot and with black imagery. PWM_BLU signal become low causing backlight to flicker and eventually short and cause a reboot.

Solution

Above video suggest just to disconnect PWM_BLU and it certainly works. This messes up the proper boot signal sequence as PWM_BLU is now disconnected. But the TV boots fine (will see brief random lines, might be seeing tcon booting?) and during power off, backlight stays on for a little while longer (15-20s?) before shutting off (some comments in above video say backlight doesn't shutoff on their TV requiring pulling the power cord). Didn't check if everything is completely shut off but after everything cooled, thermal cam shows no activity on the main board and only heat source in PSU is the main transformer (but probably not putting out a lot of heat, maybe just standby voltage?)

Another solution is to make sure image source has a valid bright input when powering on the TV. In case TV was last left in an HDMI device, plug in something like a streaming stick already powered up to its main menu before firing up the TV will avoid the boot loop.

Perhaps replacing the main board (or find whatever is failing on the board for this brightness processing) is the proper solution. For this particular TV, salvage main (BN94-10810A) Version BJ02 is pretty rare and expensive. Unsure if other boards of the same design will work. Panel number on this TV seems to be unique to this model # (CY-GK055HGEV6H)

Probably will just add an inline switch to PWM_BLU so can have a reliable recovery.

Additional Notes

Did oven bake the main (400F 8min, used no residue liquid flux under the main BGA). Seems like it change nothing.

Got into service menu and set PWM _INI_2D, PWM_MOVIE_INI, PWM_Mid, PWD_Min all to 100. Seems to have no effect. Here is the directions on that

https://youtu.be/ov9niiau5x4?t=138

Unexplained

Strange enough, streaming a black screen stream from youtube doesn't cause the TV to reset. Perhaps the failure circuit is more sensitive during early boot process? Did see some backlight flickers in black transitions in a Dolby 4K youtube demo video at some point in the debugging session but seems to have gone away (perhaps PWM_* settings in service menu helped?)

Picture Mode in Dynamic seems to be worse than Standard in darker scenes (like picture menu with black source imagery in background)

[howardc64]

Re: UN55KU6270FXZA Backlight Flashing Briefly on Power On

UPDATE

This problem was ultimately diagnosed as bad backlight. Once backlight was fixed, problem went away.

[KYBOSH]

Re: UN55KU6270FXZA Backlight Flashing Briefly on Power On

UPDATE

This problem was ultimately diagnosed as bad backlight. Once backlight was fixed, problem went away.

I noticed you had a backlight tester at your disposal (pictured on the bed).
Was the problematic LED strip the one that ramped up to max voltage slower than the other?

[howardc64]

Re: UN55KU6270FXZA Backlight Flashing Briefly on Power On

I noticed you had a backlight tester at your disposal (pictured on the bed).
Was the problematic LED strip the one that ramped up to max voltage slower than the other?

My notes said 2 dead LED post LED bar extraction testing with LED tester. Unfortunately forgot to correlate if on the string that ramped up slower.

Kind of a PITA problem to diagnose as backlight doesn't clearly show error while installed which is why so many people use the PWM_BLU disconnect trick. But this trick probably also runs the backlight brighter.