QN75QN85BAFXZA no backlight

[howardc64]

2+ year old TV with many (720? 65" version has 720) mini LED dimming zones. TV boots, has sound, panel picture can be seen with flashlight. No backlight. No TV model sticker on rear panel (Maybe Samsung service removed it, read below). Found the model # through the PSU and zoned LED driver board numbers.

PSU has a click near LED output circuit at 1 second interval, sounded like the 4x barrier rectifier area. Protective pastic liner under PSU shows heat signature on the Schottky Barrier Rectifier in this circuit. Found a shorted diode (2 diode per rectifier, failed rectifier labled #2 on the pics) on 1 of 4 rectifiers. Device datasheet here ( link )

Normally I would replace this and test. However, given this TV's history, like to double check a few things. Here is what I know of the history

• Owner said Samsung service came 3 times to attempt repair. Eventually refunded the customer.
• During a repair attempt, backlight worked but only for 1 hour
• TV rear panel was removed previously

This is all the info I have. Presumably Samsung may have replaced the PSU to make it work (Just a guess) but then PSU failed 1 hours later (also a guess) and the replacment PSU remained in the TV (also a guess) until I picked it up.

zoned LED driver baord appears to not have been removed previously (judging by the sticky tape to secure all the connectors/flex, but can not be certain) Checked this board ( incase the short on PSU caused failures on the LED driver board) for shorts and found shorted caps on all 10 mini LED flex cable connectors (red arrow in picture). Both side of all 10 caps measures near 0 ohms to ground. Board is removed from TV for testing.

Greatly appreciated if anyone has this zoned LED driver board and can check these 10 caps for continuity to ground on both sides. Or any advice. I'm thinking to repair the PSU by replacing the failed PSU barrier rectifier and replace the LED driver board. Don't know any way to check mini LED array for faiure in case that is causing the PSU to fail somehow (through LED driver board feedback circuit to the PSU requesting higher voltage and/or current?)

[lotas]

If these capacitors are on the same bus, then one of the capacitors is faulty (short circuit), then all of them will be short circuited, you can carefully examine each of them under a magnifying glass, sometimes you can see that it has a microcrack,if you know what voltage this bus is, you can apply voltage from a laboratory power supply to this bus and, by increasing the current, see which one starts to heat up, that one will be the culprit.

[lotas]

There are no shorted capacitors in this area, rather this power supply should be formed from these converters, check the resistance on the inductors relative to ground.

[howardc64]

If these capacitors are on the same bus, then one of the capacitors is faulty (short circuit), then all of them will be short circuited, you can carefully examine each of them under a magnifying glass, sometimes you can see that it has a microcrack,if you know what voltage this bus is, you can apply voltage from a laboratory power supply to this bus and, by increasing the current, see which one starts to heat up, that one will be the culprit.

I should have mentioned I checked all 10 caps under 10X loupe and found no fractures or obvious discolorization. One end of the cap goes to large board traces (I'm assuming ground plane) and flex to mini LEDs. The other side hard to trace (goes into board internal layers) But I'd imagine they are all on the same power rail. Unfortunately recently sold my bench PSU so can't do simple voltage injection test currently even though I own a FLIR camera. I guess time to invest in another benchtop PSU

There are no shorted capacitors in this area, rather this power supply should be formed from these converters, check the resistance on the inductors relative to ground.

Yes no shorted caps here. Here are inductor resistance to ground measurements.

[lotas]

And if you don't have a milliohmmeter, sometimes it helps to measure the resistance on each capacitor and where there is the least resistance, that is what needs to be removed, I would also like to know where this voltage comes from through the connector from the power supply or is it formed on the board itself.

[Davi.p]

Hi, the first things to check , for me, are the electrolytic caps on the power board (psb), especially those at 160v (blue) and secondarily the "cigarette" ones on the primary (450v).. both capacity and esr.. one can think those caps made an high ripple that caused main power rail for the leds to break the power diode 2 and i think unfortunately also some leds, plus the small ceramic caps.. get a decent multimeter, check each of 10 smd caps in ohms, stay still until the measure stabilizes, take note of the decimal part, compare the decimal parts of the caps and remove the one with lower value.. PS: for a repair man, the bench psu is the most important thing after the multimeter and the PC.. Thermal camera is for the "glamour guys"...

PS: on the 160v caps, suggested an overvoltage suppressor like a varistor, if it was put by samsung, the fault was probably very easy to restore..

PPS: on the led board, bottom right, there is a big diode by Little Fuse (LF printed on it), check it, if zero ohms remove it and check again it and the caps..