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..

[Diah]

i didn't read any about Driver_ON signal on PSU pin 4 if you monitored it or if it exist or simulated manually to drive PSU start BL circuit ?

[howardc64]

i didn't read any about Driver_ON signal on PSU pin 4 if you monitored it or if it exist or simulated manually to drive PSU start BL circuit ?

Thanks. Since PSU has shorted diode on backlight power output, have to fix that first before next level testing. Will have to report back after receiving parts.

[howardc64]

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.

Searched for other chips that produces voltage. Found IC300 VPM2SM ( 56pin QFN : Samsung parts say this is DC/DC Converter link ) pin 39 (small yellow arrow in pic) shorted to ground. Pin 39 trace stops just before the ground plane so its probably not suppose to be ground. Unfortunately no datasheet found.

[lotas]

Here is someone who created the pinout of this chip, it looks like the output goes to this resistor.

[howardc64]

Here is someone who created the pinout of this chip, it looks like the output goes to this resistor.

Thank you. That R100 (0.1 ohm) resistor explains why I saw a short. Just very low resistance to ground. I removed it and continuity to ground from pin 39 was gone. So false alarm.

I also remove it due to faint image of a trace on the bottom of R100 (2nd picture, enhanced to show this faint image). This faint trace image cleaned off with Q-Tip so I thought maybe R100 resistor had failed to cause this false image. But R100 appears to test good (0.6 ohms on my multimeter, probably just additional probe resistance)

LED driver picture retaken in outdoor light to reduce shadows.

LED driver board is ~$20 on US ebay. I'll probably just buy one to eliminate any faulty variables on this board. Pair replacement LED driver board with repaired PSU and test.

[lotas]

Got it, the fuse is alive?

[howardc64]

Got it, the fuse is alive?

Yes fuse Y has continuity on the 13V rail.

Also checked the power rails into the LED driver board from PSU. 64V LED power rails goes directly (0.5 ohms which is just probe resistance) to all the mini LED connector (pin 4-6, 46-48 on 51 pin connector) after 8R2 2213 inductor. I guess the 5x Samsung chips probably modulate LED current (and therefore brightness) via PWM ground or variable resistance.

Ground pins (7,8,15,16,20,21,22) from PSU are the only pins with continuity to board ground.

Board seems good other than the shorted cap (remained shorted when I took off R100) on each mini LED connector mystery haha. Anyway, Will probably have parts in a week and hopefully mini LEDs don't have faults that damage LED driver and/or PSU.

[lotas]

Ok, let's wait for the new driver...

[howardc64]

Someone send me following info why a cap might be connected to ground on both ends. Largely to reduce high frequency noise on one ground plane from contaminating another.

So if the LEDs brightness are adjusted by PWM the ground, this would have some frequency component. Maybe conecting a cap to a very low frequency ground is helpful? My background is not an EE or even board design engineer ( software and digital computer architecture )

So maybe those mini LED connector ground cap is properly designed...

=====

⚡ Reasons to Connect a Capacitor Across Two Ground Planes
1. High-Frequency Isolation or Filtering

• Even if two ground planes are nominally “ground,” they can have different high-frequency characteristics.
• A capacitor can provide a low-impedance path for high-frequency noise between them, helping suppress differential-mode noise or EMI.
• Often used between analog and digital ground planes to prevent digital switching noise from contaminating sensitive analog circuitry.

2. Controlled Return Path

• In multi-layer PCBs, different ground planes might serve distinct purposes (e.g., signal ground vs. chassis ground).
• A capacitor helps steer high-frequency currents safely across those boundaries without introducing ground loops.
• Particularly useful in mixed-signal designs or RF applications.

3. Mitigating Transient Voltage Differences

• If the planes are connected through high inductance paths (like narrow traces or vias), they can momentarily diverge in potential during transient conditions.
• A capacitor provides a fast-reacting shunt path to equalize those differences.

4. EMC Compliance Strategies

• Capacitors can help meet radiated and conducted emissions requirements by reducing the voltage differential between grounds at high frequencies.
• This is sometimes seen in audio equipment, RF designs, or devices undergoing CE/FCC testing.

⚙️ Considerations

Capacitance Value	Must be chosen based on target frequency range (noise band, etc.)
ESR & ESL	Low ESR and ESL preferred for high-frequency decoupling
Placement	Should be close to noisy sources or signal return paths
Isolation Requirements	Ensure it doesn’t violate isolation boundaries in sensitive designs

[lotas]

Yes, there is such a thing, as described, two grounds, one can be digital, the other analog (the layout of these grounds on the boards), but ultimately they are connected to each other, the board can be three-four layers.

[Diah]

Thanks. Since PSU has shorted diode on backlight power output, have to fix that first before next level testing. Will have to report back after receiving parts.

i don't know which short diode you are talking on, and what you wrote mini LED no way for them in set.. being used ribbon cable mean LED designed for several zone , but they remain same normal LED with limited live hours.

[howardc64]

Yes, there is such a thing, as described, two grounds, one can be digital, the other analog (the layout of these grounds on the boards), but ultimately they are connected to each other, the board can be three-four layers.

I'm not expert but looks like 4 layer board? Tracing 1 ground path on the mini LED connector cap disppears on both side of the board so surely > 2 layers.

[howardc64]

i don't know which short diode you are talking on

The shorted diode is labled #2 in post #1 underside PSU picture (#4 pic) Its a 2x barrier rectifier diode array rated at 150V 40A. I've extracted the component and confirmed 1 shorted diode. Will order replacement soon.

, and what you wrote mini LED no way for them in set.. being used ribbon cable mean LED designed for several zone , but they remain same normal LED with limited live hours.

The mini LED array wiring circuit is mysterious to me. I have seen older larger LED zoned backlight flex on SONY 65" XBR TV, it seems to use matrixed addressing method (line to send row and column data) to reduce wire/trace count. So multiple LED brightness controls are multiplexed over a single wire to reduce wire numbers. On this TV, this is surely similar when I look at the 10x mini LED connectors. Not enough signal pins to dedicate 1 pin / mini LED (I don't know the exact mini LED count but seems to be 500-600+)

But yes, I was thinking if mini LEDs fail... Its almost unrepairable. I've seen LEDs fail open (most of the time), shorted and just passthrough, and worst is melted connecting + to underside of the LED stripe connecting to TV chassis ground. I wonder how the LED driver + PSU will behave on this TV with LED opens and shorts.

[Diah]

if you found shorted Diode #2, then why you searched for driver boards and lotas went with to look forward new driver boards.. ? if you removed the shorted diode r´the circuit will work in kess one zone !

mini words not exist , if they use ribbon cable or other kind,,, LED+ would be shared with many zone.. but the LED- will play roll to adjust the zone light on beside the bright of them.