Hi
I have a QN65QN90A which was purchased around launch I guess making it about 1.5-2 years old... had been working fine until a few days ago, simply went black and would no longer turn on (at all). No standby light and no typical PSU relay click etc. Absolutely dead.
Pulled it apart without doing much research first, suspecting the power supply as it had a faint buzz (I guess due to the forthcoming found short), investigated PSU for some time and concluded there was no fault that I could spot with the board removed.
Came to learn after a bit more research that it would have been smarter to do the following first:
A) Remove the mainboard power cable - turns out this enabled the PSU to turn on and backlight to light.
B) Remove TCON LVDS ribbon and/or TCON ribbons to panel - no apparent effect.
So then went on to pull out the mainboard, began checking various rails and found that 3.3v rail was shorted - ie VCC/GND on eeprom 25Q40CLSIP were dead short. Eeprom IC just happened to be one of the first places I discovered the short, so unsoldered VCC leg on eeprom and determined short was still there.
Next I used a LM338T adjustable reg set to approx 3v and a 1-2 amp supply to try to determine where the short was by feeding 3.3v rail external power, this is after having exhausted checking all the obvious components and starting to clutch at straws...
Found an MLCC to be getting hot. Removed the MLCC - confirmed to be a dead short. After removing the MLCC, short was cleared... 1.25kohm or there abouts from memory across 3.3v rail.
No idea what the value of the shorted MLCC was and the other MLCC that was in parallel with it, I am unable to measure the good one as don't have a meter capable of such. I pulled out the second MLCC that was in parallel with it (they're probably 10uf-ish I'm guessing) and replaced them both with 47uf M2102 6.3v X5R off a random RPI Pico board I had lying around. Caps are probably a bit too high in value but didn't *seem* like it would hurt if its right on the 3.3v output from the buck converter.
Put board back in the TV and it now starts (auto starts actually, no initial standby mode) and I see the initial NEO QLED splash screen with the backlight lit for about half a second, then PSU goes click and 1-2 seconds, it attempts to boot again.
I have a replacement TCON/Mainboard and PSU coming from US (I'm in New Zealand, but it's a imported US TV) but it will be some days away still. Also starting to become dubious it will solve the problem having done even more research and seeing how often the panel can be responsible for issues like this.
Beginning to think the failed MLCC is merely a result of whatever caused the actual fault, perhaps panel? - altho unplugging the LVDS between the TCON and the panel does not change the reboot behavior, I did not try unplugging the panel ribbons from the TCON.
Does anyone have any thoughts on what should be investigated next? Eeprom?
Does anyone know at what point that "NEO QLED" text on the screen is generated? TCON? or by the mainboard?
Also can't quite explain the yellow staining under the mainboard, nothing (not even the shorted MLCC) looks burnt or damaged, I do smoke, but I find it peculiar that it only is around that board. Was also present when it went black and turned off, no smell at all when it failed.
See attachments...
I have a QN65QN90A which was purchased around launch I guess making it about 1.5-2 years old... had been working fine until a few days ago, simply went black and would no longer turn on (at all). No standby light and no typical PSU relay click etc. Absolutely dead.
Pulled it apart without doing much research first, suspecting the power supply as it had a faint buzz (I guess due to the forthcoming found short), investigated PSU for some time and concluded there was no fault that I could spot with the board removed.
Came to learn after a bit more research that it would have been smarter to do the following first:
A) Remove the mainboard power cable - turns out this enabled the PSU to turn on and backlight to light.
B) Remove TCON LVDS ribbon and/or TCON ribbons to panel - no apparent effect.
So then went on to pull out the mainboard, began checking various rails and found that 3.3v rail was shorted - ie VCC/GND on eeprom 25Q40CLSIP were dead short. Eeprom IC just happened to be one of the first places I discovered the short, so unsoldered VCC leg on eeprom and determined short was still there.
Next I used a LM338T adjustable reg set to approx 3v and a 1-2 amp supply to try to determine where the short was by feeding 3.3v rail external power, this is after having exhausted checking all the obvious components and starting to clutch at straws...
Found an MLCC to be getting hot. Removed the MLCC - confirmed to be a dead short. After removing the MLCC, short was cleared... 1.25kohm or there abouts from memory across 3.3v rail.
No idea what the value of the shorted MLCC was and the other MLCC that was in parallel with it, I am unable to measure the good one as don't have a meter capable of such. I pulled out the second MLCC that was in parallel with it (they're probably 10uf-ish I'm guessing) and replaced them both with 47uf M2102 6.3v X5R off a random RPI Pico board I had lying around. Caps are probably a bit too high in value but didn't *seem* like it would hurt if its right on the 3.3v output from the buck converter.
Put board back in the TV and it now starts (auto starts actually, no initial standby mode) and I see the initial NEO QLED splash screen with the backlight lit for about half a second, then PSU goes click and 1-2 seconds, it attempts to boot again.
I have a replacement TCON/Mainboard and PSU coming from US (I'm in New Zealand, but it's a imported US TV) but it will be some days away still. Also starting to become dubious it will solve the problem having done even more research and seeing how often the panel can be responsible for issues like this.
Beginning to think the failed MLCC is merely a result of whatever caused the actual fault, perhaps panel? - altho unplugging the LVDS between the TCON and the panel does not change the reboot behavior, I did not try unplugging the panel ribbons from the TCON.
Does anyone have any thoughts on what should be investigated next? Eeprom?
Does anyone know at what point that "NEO QLED" text on the screen is generated? TCON? or by the mainboard?
Also can't quite explain the yellow staining under the mainboard, nothing (not even the shorted MLCC) looks burnt or damaged, I do smoke, but I find it peculiar that it only is around that board. Was also present when it went black and turned off, no smell at all when it failed.
See attachments...
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