QN75QN85BAFXZA no backlight

[howardc64]

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 !

Yes, as noted in post #1, this TV had prior repair attempt by Samsung. What they did is unclear but I guess they likely changed the PSU and failed again after working 1 hour. So have suspicion downstream hardware (LED driver board, mini LED array) has fault that is causing PSU to fail quickly.

Anyway, so far LED driver board have no evidence of clear fault so I will probably just repair PSU shorted diode soon to start testing.

[howardc64]

UPDATE : Got the backlight to turn on...

Studied the circuit of the shorted barrier diode rectifier array. There are 2 arrays (2x 150V 20A diode in each array) with total of 4x 150V 20A diode working in parallel. Since only 1 diode failed, I guessed 3x parallel is reasonable design margin to test the TV. Cut the shorted diode leg and TV backlight turns on.



Now I notice likely the original problem. LED driver voltage is 69V while the board has printed ~64V with up arrow. I also notice its 69V constant regardless of content brightness. Ran black video and dimming zone test on youtube and dark area backlight does not dim/turn off. This maybe the original problem that caused the PSU to fail.

It appears zoned dimming is not working and mini LED backlight maybe on full brightness. Changing brightness doesn't change any brightness. Tried local dimming with standard and high (settings are low, standard, high) doesn't have any local dimming with above youtube tests.

Found firmware disabling local dimming is a massive problem on this TV ( link link ) If without dimming is full brightness, then LED driver would feedback to PSU to provide max current and voltage. Updated to the latest firmware (1661.6) no difference on backlight dimming. Probably need to find a way to downgrade firmware if thats possible.

[nomoresonys]

Wonder if they just didn't use components stout enough to handle the loads, those samsung bean counters like to save pennies at the consumers expense.

[Diah]

you need now to replace the diode complete not just cut..

[howardc64]

you need now to replace the diode complete not just cut..

yes of course. Just did this to test until replacement part arrive.

I don’t know this diode would cause 69v instead of ~64v spec printed on the PSU. Don’t fully understand the circuit. The 4 diode’s anode are on the PSU <-> driver board ground plane. Also I’ve seen wrong voltages printed on PSUs before although this board prints 1/10 volt resolution.

Will go through service menu in detail. Other Samsung’s have settings for lower backlight level.

[howardc64]

Looks like many Samsung mini LED backlight TV owners are struggling with local dimming not working. Some owners (gamers?) dislike local dimming so I wonder if Samsung may have turn it off as default in a way that no one can easily turn back on in recent firmware updates.

Tried lowering PWM_INI_2D and PWM_MOVIE_INI (both 100 default with latest firmware) to 20, no difference with youtube dimming/black screen test videos. Did find someone posting their SubSetting menu (in service menu) and a couple of differences from mine

• Mine has no PWM_MIN
• My 2D_DimPosi1 is 2x bigger. Didn't dare to change this not knowing if this is related to different number of mini LEDs of different size TVs.

Here is my Service Subsetting menu



Here is another persons ( link )

Anyway, nothing is causing LEDs to dim and PSU LED voltage rail is always 69v

[lotas]

LEDs do not work from voltage, but from current strength. Less current is supplied to the LED - it glows weaker, more current is given - the LED starts to glow brighter, heats up more, thus the crystal degrades faster...
That's why, when they change a new LED backlight, they reduce the current in the backlight driver (increasing the resistor resistance) so that the LEDs last longer, and all TV companies set it from the factory to the maximum permissible current (so that the picture is as bright as possible) and thus, after two or three years, the backlight becomes unusable, just when the warranty ends.

[howardc64]

LEDs do not work from voltage, but from current strength. Less current is supplied to the LED - it glows weaker, more current is given - the LED starts to glow brighter, heats up more, thus the crystal degrades faster...
That's why, when they change a new LED backlight, they reduce the current in the backlight driver (increasing the resistor resistance) so that the LEDs last longer, and all TV companies set it from the factory to the maximum permissible current (so that the picture is as bright as possible) and thus, after two or three years, the backlight becomes unusable, just when the warranty ends.

Yes, I always avoid max brightness on LED TVs. 6V LEDs (typical TCLs) seems more easy to fail.

On this TV, I wonder why have 69v instead of ~64v printed on the PSU. Maybe printed # is wrong. Maybe LED driver baord is requesting higher current which should show higher voltage if resistance path doesn't change?

But PSU LED + - isn't final individual mini LED current on this zoned mini LED TV design. So I wonder why driver board needs to request more current from the PSU? Anyway, still trying to understand the design.

LED driver board does have a feedback signal to the PSU. Will monitor this soon to try and understand the design. PSU LED + - isn't final individual mini LED current on this zoned mini LED TV design. So I wonder why driver board needs to request more current from the PSU?

===

BTW, eliminating the shorted diode solution (just temporary until replacement part arrive) actualy reduced the design capacity by 50%. LED + and - rail have 4x diode on each rail. The 4x diode has 2x with current flowing in one direction and 2x flowing in the opposite direction. So eliminating 1 diode reduces 1 direction from 2x 150V 20A diode to just 1. But I don't think this eliminated diode can change the voltage from 64v to 69v (as long its under diode spec) These diodes are just noise barriers between the both side of the diode.

[howardc64]

Probed all pins between PSU and LED driver board

pin 4 = Vdrv On = 3.067v
pin 7,8,15,16,20,21,22 = GND = -0.011v
pin 9,10,11,12,13,14 = Vdrv = 69v
pin 17 = B13V = 12.83v
pin 19 = FB = 2.139v

FB signal looks like goes through side B of the LV358G op amp (IC 280), then to the main board connector (CN 250) and the Samsung chip nearest that connector (IC 2100)

[lotas]

As the LED degrades, voltage increases, which in turn leads to increased current and heat generation. This is due to the deterioration of the semiconductor material, which means that higher voltage is required to maintain the same brightness. Increased current and temperature accelerate further degradation of the LED, creating a "snowball" effect.
Degradation and Voltage Rise:
As an LED ages, its internal resistance increases. This means that more voltage is required to pass the same current as it did at the beginning.
This is what is happening with you, what is written on the power supply, the output voltage, has now increased and over time it will gradually grow.

[howardc64]

As the LED degrades, voltage increases, which in turn leads to increased current and heat generation. This is due to the deterioration of the semiconductor material, which means that higher voltage is required to maintain the same brightness. Increased current and temperature accelerate further degradation of the LED, creating a "snowball" effect.
Degradation and Voltage Rise:
As an LED ages, its internal resistance increases. This means that more voltage is required to pass the same current as it did at the beginning.
This is what is happening with you, what is written on the power supply, the output voltage, has now increased and over time it will gradually grow.

very good summary. Another variable is any high heat solder connections thermal cycle weakening over time. I fixed about 20 apple 27 thunderbolt displays by reflowing ithe edge lit LED bar connector. Original lead free solder weakened and resistance increase push PSU beyond its limits. On this TV I haven’t disassembled to know what the led system looks like.

I guess Samsung firmware updates (default auto update) made it much worse by disabling dimming or even any ability to adjust backlight to lower brightness.