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Old 04-02-2020, 09:40 PM   #41
budm
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Default Re: Samsung UN50F6300AF No Backlights

"This tester reads 255 when plugged in. So the LED tester when connected to the appropriate pins at the power supply could read 255, 0 or somewhere in between. Is a reading somewhere in between the forward voltage drop? Then what does 255 and 0 mean? I can only see light through tiny holes and will have no idea what is going on in there so the reading on the tester must be the key.

You do not connect the LED TESTER to the power supply pins, you remove the LED connector from the power supply board then test each one LED string, you have four LED string, 1 ~ 4.
The LED tester will force the current through the LED string then the LED tester Volt meter will show the Vf of the LED string.
The LED tester only puts out about 30mA so the Vf of the each LED will be only about 2.7 ~ 2.8V.
You do not use the Voltmeter of the LED tester to check the output of the LED power supply! The LED tester has 30mA constant power supply so do not hook it up to another power output.
Bottom line: use the LED tester to test each LED string individually first, yo have four strings to test.
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Basic LCD TV and Monitor troubleshooting guides.
http://www.badcaps.net/forum/showthr...956#post305956

Voltage Regulator (LDO) testing:
http://www.badcaps.net/forum/showthr...999#post300999

Inverter testing using old CFL:
http://www.badcaps.net/forum/showthr...er+testing+cfl

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http://www.badcaps.net/forum/showthread.php?t=24809

Last edited by budm; 04-02-2020 at 09:50 PM..
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Old 04-02-2020, 09:49 PM   #42
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Default Re: Samsung UN50F6300AF No Backlights

Series circuit:
"Before that I would like to know what the voltages at CNL802 should be approximately on a working system. R_J says "Channel1 voltage is 215vdc and channel2 voltage is 274vdc". That would be the voltage at the anodes of the first strip in each string. I am guessing the reading at the cathode of the first string would be the voltage drop across the first strip which would be applied to the anode of the next strip and so on throughout the string? So, if you know the number of LEDs per string, couldn't you calculate the approximate voltage at the cathode of the string? R_J says "Channel2 has 80 leds, Channel1 has 64 leds. On the other hand, if you know the voltage at the anode of the string and the voltage at the cathode couldn't you calculate the approximate number of LEDs in the string? Or am I making the wrong assumptions. Please, this is important to me"

You will not see any Voltage drops of the first LED string which is connected in series to the second LED string if the second LED string has open circuit.
https://www.swtc.edu/Ag_Power/electr...s_circuits.htm

Last edited by budm; 04-02-2020 at 09:52 PM..
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Old 04-03-2020, 01:17 AM   #43
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Default Re: Samsung UN50F6300AF No Backlights

Thanks budm and neilc6 for help on the LED tester among other things. I will try to test the strings as you instructed and get back with findings.

budm - I am curious how you can use that LED tester as a voltmeter, not that I would want to. I thought it was just a low current power supply.
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Old 04-03-2020, 10:02 AM   #44
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Default Re: Samsung UN50F6300AF No Backlights

Your Fluke meter should not have a problem measuring the correct voltage, some people have cheaper meters (lg/goldstar as an example) that can have an issue where there are high frequencey pulses on the dc line.
As for the channel voltages: they are marked on the power supply board itself.
Since you have an led tester, which pins did you connect it across to do the checks?
If I had to guess, you should be able to check the strips connected to pins:
3~4
5~6
9~10
11~12
I should add that even IF the leds are all ok, it seems that there is a current monitoring line (pin15 & pin16) that is comes from the panel. If there was a break, poor connection on this line it could also shut down the led's.

Last edited by R_J; 04-03-2020 at 10:23 AM..
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Old 04-03-2020, 04:55 PM   #45
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Default Re: Samsung UN50F6300AF No Backlights

Quote:
Originally Posted by MEB View Post
Thanks budm and neilc6 for help on the LED tester among other things. I will try to test the strings as you instructed and get back with findings.

budm - I am curious how you can use that LED tester as a voltmeter, not that I would want to. I thought it was just a low current power supply.
You don't.
My post #41:
You do not use the Voltmeter of the LED tester to check the output of the LED power supply!
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Old 04-09-2020, 08:53 AM   #46
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Default Re: Samsung UN50F6300AF No Backlights

Ok, I used the LED tester to check the 4 strings. 3 of the 4 showed open and the other showed a voltage drop from 255 to 248. I guess it is time to open it up and test the LEDs and try to correlate the readings from the meter to the condition of the strings. Thanks to everyone. You have been most helpful. I will be back with the results.
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Old 04-09-2020, 12:52 PM   #47
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Default Re: Samsung UN50F6300AF No Backlights

Take your time to disassemble the panel. This is a 50" chances of breaking the LCD is moderately high here. Larger screens are more of a PITA.

Here's my suggestion:
0. Take pictures along the way. Document what screws go to which locations.
1. With the screen facing DOWN - remove all of the boards (T-CON, Main, SMPS).
2. Disconnect the ribbon cables to the skinny PCBs that were connected to the T-CON.
3. Remove any wires taped to the back of the TV, remove the TV mounts and the power button board. You want the TV as flat as possible.
4. Turn the panel over. Lay it flat. Remove the front bezel.
5. CAREFULLY release the long/skinny PCBs from the chassis so they are hanging free.
6a. If you HAVE suction cups, CAREFULLY place them on the screen, lock and move the screen out of the way.
6b. If you DON'T have suction cups, CAREFULLY lift one corner of the screen and slowly insert a foam board under the LCD panel. This is what i usually end up doing with larger screens. You'll likely need two foam boards - one for each side. Be careful about the flex cables on the bottom edge of the screen. Once you have the LCD shimmed up with the foam boards, slide it off the chassis.

I highly recommend wearing a pair of gloves to keep finger prints off of the diffuser when you continue the disassembly. Make a small mark on the chassis and the diffuser to indicate the orientation of them. Remove all of the diffuser pieces together. Do not make a bend or kink in them. You'll end up seeing this crud on the screen.
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Last edited by ngth82; 04-09-2020 at 12:53 PM..
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Old 04-09-2020, 03:36 PM   #48
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Default Re: Samsung UN50F6300AF No Backlights

Thanks for the advice. I will let you know how it goes
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Old 04-13-2020, 06:27 AM   #49
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Default Re: Samsung UN50F6300AF No Backlights

Finally got in there and it was like everyone said. It kind of looked pretty much like neilc6ís photos. Many shorted and open leds. Will order new strips today from SJ. While waiting I want to trace out those strips back to the pins as it is not readily visible to me about which strings are in series which each other. Their are 9 rows of 16 leds. There were 2 rows which were working and all the others were totally out or a few lit here and there. Will be back in a few days. Thanks.
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Old 04-13-2020, 06:39 AM   #50
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Default Re: Samsung UN50F6300AF No Backlights

When repaired ensure that the backlight level is reduced as much as possible in order to reduce the strain on the leds. This will reduce the problem of them burning out and should increase their lifespan.
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Old 04-14-2020, 03:05 AM   #51
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Default Re: Samsung UN50F6300AF No Backlights

Thanks, I will turn them down before I give it back. Strips are on order from SJ.
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Old 04-14-2020, 08:33 AM   #52
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Default Re: Samsung UN50F6300AF No Backlights

On F model years, you can apply picture settings to all inputs. I would turn off eco sensor, set picture mode to standard, backlight level to 12, and turn off all motion smoothing/lighting settings that cause the soap opera effect.
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Old 04-26-2020, 07:02 PM   #53
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Default Re: Samsung UN50F6300AF No Backlights

Sorry for the long delay, Work has been crazy. I put the new LED strips in and all is good, I turned the back lights down and my friend picked it up. I thank everyone and I learned some good stuff.

But I still have one question.

3~4 String 1 = 32 LEDs
5~6 String 2 = 32 LEDs
9~10 String 3 = 32 LEDs
11~12 String 4 = 48 LEDs

There are 2 Channels (Power supplys) so like bud said, 2 strings are connected in series. I can use the LED tester at the connector and test each string. But if I connect to the anode of string 1 and the cathode of string 2, I expected both strings 1 and 2 to light.but I get nothing. Are those 2 strings connected via the PS when the LED connector is attached? I thought about jumpering the pins, but thought I'd better wait. Anyway, thanks so much to all. This was a really good learning experience.
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Old 04-26-2020, 10:13 PM   #54
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Default Re: Samsung UN50F6300AF No Backlights

Glad to hear that you got it working.

As i recall, the four strings are connected as two channels on the power supply board.

The SMPS silk screen shows the output of CH1 as 219V @ 245mA, and CH2 as 274V @ 245mA.


How are you conneting the annoted of string 1 to the cathode of string 2? By a wire? Or by plugging it in to the respective connector?

The LED strip 1, and LED strip 2 are connected on the power supply board once you have it hooked up with the wiring harness. This makes output channel 1

Same with strip 3 and strip 4. This makes output channel 2
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Old 04-27-2020, 07:25 AM   #55
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Default Re: Samsung UN50F6300AF No Backlights

Quote:
Originally Posted by MEB View Post
Sorry for the long delay, Work has been crazy. I put the new LED strips in and all is good, I turned the back lights down and my friend picked it up. I thank everyone and I learned some good stuff.

But I still have one question.

3~4 String 1 = 32 LEDs
5~6 String 2 = 32 LEDs
9~10 String 3 = 32 LEDs
11~12 String 4 = 48 LEDs

There are 2 Channels (Power supplys) so like bud said, 2 strings are connected in series. I can use the LED tester at the connector and test each string. But if I connect to the anode of string 1 and the cathode of string 2, I expected both strings 1 and 2 to light.but I get nothing. Are those 2 strings connected via the PS when the LED connector is attached? I thought about jumpering the pins, but thought I'd better wait. Anyway, thanks so much to all. This was a really good learning experience.
Yes, exactly. If you look at the underside of the PS where the led driver connector is you will see those pins connected. Removing the harness from that connector means they are no longer connected in series.
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