Friends of mine have a Samsung 55" 4K TV, model UN55HU6840 (about 2014).
They say they can hear audio, but the screen is black.
Your approach is ok, but without a service manual and a
schematic, repairing a PCB might be problematic. Think
about isolating the problem to a particular PCB and
order a used replacement from the cannibals on eBay.
Friends of mine have a Samsung 55" 4K TV, model UN55HU6840 (about 2014). They say they can hear audio, but the screen is black. I'd like to take a shot at fixing it for them since they don't really have the money to buy
a new one. The encouraging thing is that until recently this has been an intermittent problem that's just gradually gotten worse, and now is permanent. But I'm encouraged to think it's not burned out LEDs, else it would never have been intermittent.
I just told you everything I know about repairing TVs, but am generally experienced in electronics as a hobbyist. Based on some Youtube videos
it seems I should take off the back cover and check all the connections. Then I would go to the power board and check the voltages on the lines
going to the backlight LEDs, and if low, start checking diodes and electrolytics, and of course look for bad solder joints.
Does this sound like a reasonable approach? Is there a typical cause of
this symptom?
I've had no luck finding a schematic. Does anyone know where I would
find that?
Well, any guidance would be appreciated.
This happened before, about three years ago, and they spent
$250 at a local repair shop, which replaced one of the
boards. But they don't know which board, and don't have the
paperwork that might tell. But they say the symptoms are
the same.
UN55HU6840
On Wed, 20 Jul 2022 21:45:35 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:
UN55HU6840
You might try a manual 'reset to factory', or a USB
re-flash. These procedures don't need an active screen
or remote.
I've yet to run across an audio-ok/blackscreen situation,
so can't advise.
You can take off the back and hunt for obvious problems
like bulging caps or iffy connectors.
To troubleshoot, you'll really need to get a service manual.--
This should tell you how to get the back off, without
bending something. It will also offer test points for
measurement.
RL
The HU series *does* have intermittent issues with LEDs,
either from individual LEDs or the interconnects on the
strips. Sometimes you can change just the bad LEDs or
hardwire the interconnects if there is a voltage drop
across them, but the best option is to order the
complete set of strips from someone like ShopJimmy.
Friends of mine have a Samsung 55" 4K TV, model UN55HU6840 (about 2014).
They say they can hear audio, but the screen is black. I'd like to take a shot at fixing it for them since they don't really have the money to buy
a new one. The encouraging thing is that until recently this has been an intermittent problem that's just gradually gotten worse, and now is permanent. But I'm encouraged to think it's not burned out LEDs, else it would never have been intermittent.
I just told you everything I know about repairing TVs, but am generally experienced in electronics as a hobbyist. Based on some Youtube videos
it seems I should take off the back cover and check all the connections.
Then I would go to the power board and check the voltages on the lines
going to the backlight LEDs, and if low, start checking diodes and electrolytics, and of course look for bad solder joints.
Does this sound like a reasonable approach? Is there a typical cause of
this symptom?
I've had no luck finding a schematic. Does anyone know where I would
find that?
Well, any guidance would be appreciated.
ohg...@gmail.com says...
The HU series *does* have intermittent issues with LEDs,Having no experience with these TVs, I don't know how the
either from individual LEDs or the interconnects on the
strips. Sometimes you can change just the bad LEDs or
hardwire the interconnects if there is a voltage drop
across them, but the best option is to order the
complete set of strips from someone like ShopJimmy.
LEDs are connected to power lines, or how the lines are
interconnected. Because of the level of disassembly needed
to deal with LEDs, I'm hoping that it's a power board issue.
I looked around at ShopJimmy, and they are out of stock of
most things. I don't know what might be available on Ebay.
ohg...@gmail.com says...
The HU series *does* have intermittent issues with LEDs,Having no experience with these TVs, I don't know how the
either from individual LEDs or the interconnects on the
strips. Sometimes you can change just the bad LEDs or
hardwire the interconnects if there is a voltage drop
across them, but the best option is to order the
complete set of strips from someone like ShopJimmy.
LEDs are connected to power lines, or how the lines are
interconnected. Because of the level of disassembly needed
to deal with LEDs, I'm hoping that it's a power board issue.
I looked around at ShopJimmy, and they are out of stock of
most things. I don't know what might be available on Ebay.
On Thu, 21 Jul 2022 15:36:36 -0400, legg <legg@nospam.magma.ca> wrote:
On Wed, 20 Jul 2022 21:45:35 -0700, Jeff Liebermann <jeffl@cruzio.com> >>wrote:
UN55HU6840
You might try a manual 'reset to factory', or a USB
re-flash. These procedures don't need an active screen
or remote.
I looked at the user manual and searched the internet for such a
procedure. There are two resets. One for resetting the smart hub and >another for resetting everything except the network settings. Both
are access from the setting menu which requires seeing something on
the screen. There are probably instruction for a factory reset
without using the remote or screen, but I couldn't find them.
I've yet to run across an audio-ok/blackscreen situation,
so can't advise.
Running across or otherwise trampling the TV is not advisable.
You can take off the back and hunt for obvious problems
like bulging caps or iffy connectors.
Not all electrolytic capacitor problems are easily recognizable as
bulging or leakage. I suggest an ESR tester to be certain.
To troubleshoot, you'll really need to get a service manual.
This should tell you how to get the back off, without
bending something. It will also offer test points for
measurement.
RL
On Thu, 21 Jul 2022 15:36:36 -0400, legg <legg@nospam.magma.ca> wrote:
On Wed, 20 Jul 2022 21:45:35 -0700, Jeff Liebermann <jeffl@cruzio.com> >>wrote:
UN55HU6840
You might try a manual 'reset to factory', or a USB
re-flash. These procedures don't need an active screen
or remote.
I looked at the user manual and searched the internet for such a
procedure. There are two resets. One for resetting the smart hub and >another for resetting everything except the network settings. Both
are access from the setting menu which requires seeing something on
the screen. There are probably instruction for a factory reset
without using the remote or screen, but I couldn't find them.
I'm just not clear how the LEDs are driven. Are they like the old
Christmas tree lights - all in series, so if one burns out they all go
dark? Or are they in parallel, or what?
Still no success finding a schematic or service manual.
An easy test on this model is to unplug the harness
between the main board and the power supply and plug in
the AC. With the main disconnected, the on/off line
pull-up resistor will put the power supply into
free-run, including the LED drive. If the back light
comes on, the main *or* the power supply board is bad.
If the LEDs come on, post that and I'll walk you through
isolating the main or the power supply.
If the LEDs don't come on with the main disconnected and
the AC applied which is what I'm confident you'll find,
measure between BD9101 (either side) and J858 on the top
side of the board near the LED harness. Should be more
than 150V-300V (depending on whether this uses 3V or 6V
LEDs) there between those jumpers. If it's there or
higher, the problem is inside the display - either a bad
LED or open interconnect between the LED strips.
No voltage or low voltage between those points? Bad
power supply.
ohg...@gmail.com says...
An easy test on this model is to unplug the harness
between the main board and the power supply and plug in
the AC. With the main disconnected, the on/off line
pull-up resistor will put the power supply into
free-run, including the LED drive. If the back light
comes on, the main *or* the power supply board is bad.
If the LEDs come on, post that and I'll walk you through
isolating the main or the power supply.
Ok, so the main board controls the brightness of the LEDs
through some kind of PWM signal to the power board. If so,
then it seems to me if the LEDs fire up when the main board
is disconnected, then the main board is the problem -
something is grounding that signal line. It seems the power
board has proved it's ok.
But it does occur to me that if nothing else on the main
board is bad, then you could replace the PWM signal with
something coming out of an Arduino. :-)
If the LEDs don't come on with the main disconnected and
the AC applied which is what I'm confident you'll find,
measure between BD9101 (either side) and J858 on the top
side of the board near the LED harness. Should be more
than 150V-300V (depending on whether this uses 3V or 6V
LEDs) there between those jumpers. If it's there or
higher, the problem is inside the display - either a bad
LED or open interconnect between the LED strips.
No voltage or low voltage between those points? Bad
power supply.
Yes, that makes sense. I don't have a lab power supply, but
if I could borrow one, it seems another test would be to
disconnect the LED strips and reconnect them to a power
supply. You could ramp up the voltage to see if the LEDs
light up at some point. If they do, then they aren't the
problem. If they don't, something is open.
In *this* model, they're all in series and it's a one
channel LED system - one LED or if one interconnect
opens, no light.
On Wednesday, July 20, 2022 at 11:01:15 PM UTC-5, Peabody wrote:
I've had no luck finding a schematic. Does anyone know where I would
find that?
Try here with this link:
https://www.samsung.com/ca/support/model/UN55HU6840FXZC/
Click on the Manuals and downloads link on the menu selection you can choose from.
ohg...@gmail.com says...
In *this* model, they're all in series and it's a oneOn the ShopJimmy page for this model, it shows the LED
channel LED system - one LED or if one interconnect
opens, no light.
package as seven strips of eight LEDs, plus seven strips of
five LEDs. That's 91 LEDs. If they're all in series, then
3V LEDs would need 273V. So perhaps the strips are wired in
some parallel configuration?
I'm starting on the diagnostics this morning with a neighbor
who I discovered used to work for TI back in the day.
One of the ShopJimmy videos say flatly that if you pass the
flashlight test, with audio, then a dark screen is either
the power board or the LEDs. I guess that's not absolutely
always true, but seems logical to me. I'm hoping for the
power supply to be the problem. I'm not sure I would be
willing to tackle the LEDs. I've watched those videos, and
it's a major undertaking, with lots of opportunities to break
things.
Well, I had a major setback trying to diagnose the repair. Last night I
set it on the dining room table, face down, and this morning I did the flashlight and audio test again before even taking the back cover off,
just to confirm the video and audio were still working. But after about 30 minutes, the TV started working again. All the backlights came on, and the picture is perfect. This is the behavior the owner reported - it might
get backlight at any time, or it might not.
I measured 267V between BD9101 and J858, so that's the benchmark in case it dies again.
So I don't know how you diagnose anything when it's working properly.
Well, I had a major setback trying to diagnose the repair. Last night I
set it on the dining room table, face down, and this morning I did the flashlight and audio test again before even taking the back cover off,
just to confirm the video and audio were still working. But after about 30 minutes, the TV started working again. All the backlights came on, and the picture is perfect. This is the behavior the owner reported - it might
get backlight at any time, or it might not.
I measured 267V between BD9101 and J858, so that's the benchmark in case it dies again.
So I don't know how you diagnose anything when it's working properly. We tried tapping on every part and connector on the power board, but not even
a flicker in the lights.
Both sides of the power board look perfect - not a hint of a bad solder, no domed capacitors, no brown marks. The connector to the LEDs (CLN802) has 7 black wires and one blue wire, and a legend that doesn't seem to match the connector.
In any case, I will let it cool down, and see how it powers up. I guess I could remove and test all the electrolytics, but there are a lot of them. Same for the diodes. But it seems it could be anything. Major bummer that it's working. :-)
So I don't know how you diagnose anything when it's working properly. We >tried tapping on every part and connector on the power board, but not even
a flicker in the lights.
I measured 267V between BD9101 and J858, so that's the
benchmark in case it dies again.
On Sat, 23 Jul 2022 11:33:53 -0500, Peabody
<waybackNO584SPAM44@yahoo.com> wrote:
So I don't know how you diagnose anything when it's working properly. We >>tried tapping on every part and connector on the power board, but not even >>a flicker in the lights.
Intermittents are a PITA. I'll spare you my guess of the day. The
way I used to troubleshoot intermittents is heating with a hot air gun
and cooling with freeze spray in a can. Whatever connection is
intermittent, is going to move when it gets hot or cold, either making
the connections, or as in this case, breaking the connection. Don't
use to much cooling or heating. Dropping the temperature below the
dew point will condense water all over the board. Heating the PCB too
much will melt plastic parts. If you do condense water on the PCB,
remove power from the TV, apply a little heat, and wait until the
water evaporates before trying again.
You missed a golden opportunity when you discovered that:
I measured 267V between BD9101 and J858, so that's the
benchmark in case it dies again.
The LED's are connected in series. That means you should see a
voltage equal to 267V divided by the number of strips, across each
strip. ><https://www.shopjimmy.com/samsung-bn96-34251a-bn96-34252a-led-backlight-strips-14/#mz-expanded-view-1444464375404>
Assuming they're all in series, that would be:
267 / 14 = 19V
Walk your DVM across each strip and see which one has the full 267V
across it. That's the defective (open) LED strip.
The LED's are connected in series. That means you
should see a voltage equal to 267V divided by the number
of strips, across each strip. <https://www.shopjimmy.com /samsung-bn96-34251a-bn96-34252a-led-backlight-strips- 14/#mz-expanded-view-1444464375404> Assuming they're all
in series, that would be: 267 / 14 = 19V Walk your DVM
across each strip and see which one has the full 267V
across it. That's the defective (open) LED strip.
The LED's are connected in series. That means you
should see a voltage equal to 267V divided by the number
of strips, across each strip.
Assuming they're all in series, that would be:
267 / 14 = 19V
Walk your DVM across each strip and see which one has
the full 267V across it. That's the defective (open)
LED strip.
Friends of mine have a Samsung 55" 4K TV, model UN55HU6840 (about 2014).____________________
They say they can hear audio, but the screen is black. I'd like to take a shot at fixing it for them since they don't really have the money to buy
a new one. The encouraging thing is that until recently this has been an intermittent problem that's just gradually gotten worse, and now is permanent. But I'm encouraged to think it's not burned out LEDs, else it would never have been intermittent.
I just told you everything I know about repairing TVs, but am generally experienced in electronics as a hobbyist. Based on some Youtube videos
it seems I should take off the back cover and check all the connections.
Then I would go to the power board and check the voltages on the lines
going to the backlight LEDs, and if low, start checking diodes and electrolytics, and of course look for bad solder joints.
Does this sound like a reasonable approach? Is there a typical cause of
this symptom?
I've had no luck finding a schematic. Does anyone know where I would
find that?
Well, any guidance would be appreciated.
I did try disconnecting the main board, and the LEDs did
indeed light up fully, with about 281V across the LEDs. But
unfortunately, it's still working,
Ok I think I've figured it out. One strip of LEDs across
the panel consists of one 8-LED section plus one 5-LED
section. So 7 strips times 13 LEDs is 91 LEDs. The
voltages show there are one three-strip (39-LED) and two
two-strip (26-LED) lines that are individually wired back to
the power board, but are simply jumpered together there.
This confirms what you guys said - in effect all the LEDs
are in series, so any problem with one LED, or one
connection, and they all go out.
I think this is the most relevant video on LED replacements.
It's not exactly the same model, but very similar:
https://www.youtube.com/watch?v=oI52B4ba2Dg
It doesn't look too bad, except I don't have the suction
cups. But as of this morning, ShopJimmy is out of stock on
these, with no indication of a restock date. I can get them
on Amazon for $100, or I can get knockoffs on Ebay for as
little as $40. I don't know if the Ebay Chinese ones are
any good. And I don't know if the Amazons are different
from the Chinese ones on Ebay.
https://www.ebay.com/itm/125141436851
I did try disconnecting the main board, and the LEDs did
indeed light up fully, with about 281V across the LEDs. But
unfortunately, it's still working, so there's still no way
to tell where the problem is. It could be the main board
(unlikely, but still possible), the power board, or the
LEDs. And I just can't narrow it down unless it goes into
failure mode again.
So I'm going to continue testing it for a couple days, but
if it doesn't fail again, I'll give it back to the owner.
After all, with all the messing around I did, I may have
fixed whatever was wrong, and it may go another five years.
I really appreciate everyone's help with this. I just wish
we had reached more definitive result.
ohg...@gmail.com says...
There is another possibility however. Most Samsung LEDsIf I connect my computer to the TV via HDMI, and display a
are built with a zener inside each die. Ordinarily, the
zener does nothing but if the LED fails open, the entire
voltage of the array will appear across the zener
causing it to conduct and (hopefully) short hard
effectively jumping out the bad LED. If it does, the
problem will fix itself until another wayward LED crops
up or if an interconnect gets resistive.
competely white (all FFs) BMP image full screen, would I be
able to detect an LED that's not lighting up?
ohg...@gmail.com says...
There is another possibility however. Most Samsung LEDsIf I connect my computer to the TV via HDMI, and display a
are built with a zener inside each die. Ordinarily, the
zener does nothing but if the LED fails open, the entire
voltage of the array will appear across the zener
causing it to conduct and (hopefully) short hard
effectively jumping out the bad LED. If it does, the
problem will fix itself until another wayward LED crops
up or if an interconnect gets resistive.
competely white (all FFs) BMP image full screen, would I be
able to detect an LED that's not lighting up? Would there
be a dark area on the screen, or is the failure of one LED
not enough to cause a detectable darker area on the screen?
When I try this, I don't see anything unusual. The display
is a little darker in the corners, but I think that's normal.
On Monday, July 25, 2022 at 1:41:31 PM UTC-4, Peabody wrote:white display. There will be *some* unevenness if an LED goes out, but it's generally not dramatic, but your all white bit map pattern might show it. Try different brightness settings when you're looking at it. I've also seen some older Samsungs with
ohg...@gmail.com says...
There is another possibility however. Most Samsung LEDsIf I connect my computer to the TV via HDMI, and display a
are built with a zener inside each die. Ordinarily, the
zener does nothing but if the LED fails open, the entire
voltage of the array will appear across the zener
causing it to conduct and (hopefully) short hard
effectively jumping out the bad LED. If it does, the
problem will fix itself until another wayward LED crops
up or if an interconnect gets resistive.
competely white (all FFs) BMP image full screen, would I be
able to detect an LED that's not lighting up?
Probably, but Samsung does a good job with their lens design on their back lit models. Think about it - there is a fair amount of dead space between LEDs as they're laid out and generally you don't see the individual LED illumination even on a plain
There is another possibility however. Most Samsung LEDs
are built with a zener inside each die. Ordinarily, the
zener does nothing but if the LED fails open, the entire
voltage of the array will appear across the zener
causing it to conduct and (hopefully) short hard
effectively jumping out the bad LED. If it does, the
problem will fix itself until another wayward LED crops
up or if an interconnect gets resistive.
One of my employees brought in a Samsung 55" UN55JU7100 that has a
shorted primary side of the switching supply. Hard short at the output
of the first bridge rectifier. Noticed a few small value caps that are slightly bulging - is there anything else to check/replace on this
series other than the shorted MOSFets and suspect caps?
I'm going to suggest he add a small fan to the backside of the TV to get longer life...
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