Samsung UN50F6300AF No Backlights

[CapLeaker]

Re: Samsung UN50F6300AF No Backlights

Ah! Well, I looked at a 612b board on SJ and it was the same than the board from op. So yeah, looking at a different board and schematic will do it all right. Lol

[R_J]

Re: Samsung UN50F6300AF No Backlights

You should only be getting +275 volts for the 50" model power supply board, not 440v.
It's going to be a bad string or bad connection on the interconnect board in the panel.

[budm]

Re: Samsung UN50F6300AF No Backlights

OK, I trace out the LED connector CNL802 of the BN44-00612B. I think I trace it out correctly this time.
CNL802 pin out:
LED string set #1 (consist of LED string # 1 and string # 3 connected in series).
Pin 12 is 1+ which is the Anode of the LED string #1 which is fed by the LED power supply #1.
Pin 11 is 1 - which is the Cathode of LED string # 1.
Pin 6 is 3 + which is the Anode of the LED string #3, it is connected the Pin 11 1 -.
Pin 5 is 3 - which is the Cathode of the LED string #3, it is connected to the return circuit of the LED driver #1.

LED string set #2 (consist of LED string # 2 and string # 4 connected in series)
Pin 10 is 2 + which is the Anode of the LED string #2 which is fed by the LED power supply #2.
Pin 9 is 2 - which is the Cathode of LED string # 2.
Pin 4 is 4 + which is the Anode of the LED string #4, it is connected the Pin 9 2 -.
Pin 3 is 4 - which is the Cathode of the LED string #4, it is connected to the return circuit of the LED driver #2.

So check the Voltage when TV is turned on each time at
Pin 12 (1+), Pin 5 (3 -) for LED string set #1.
Pin 10 (2+), Pin 3 (4 -) for LED string set #2.

440V reading indicate open circuit in the LED string.

[MEB]

Re: Samsung UN50F6300AF No Backlights

Thanks budm and everyone. Once again I am at work and will get back to you.

I do understand checking the strings with a tester from the connector on the board, and I will, but right now I am learning great things which I want to understand first. I am a little stubborn, but mean no dis-respect. Please bear with me.

[CapLeaker]

Re: Samsung UN50F6300AF No Backlights

You should only be getting +275 volts for the 50" model power supply board, not 440v.
It's going to be a bad string or bad connection on the interconnect board in the panel.

yep

[ngth82]

Re: Samsung UN50F6300AF No Backlights

Thanks budm and everyone. Once again I am at work and will get back to you.

I do understand checking the strings with a tester from the connector on the board, and I will, but right now I am learning great things which I want to understand first. I am a little stubborn, but mean no dis-respect. Please bear with me.

Sure, we'll be here. But honestly, the LED string checking will quickly and visually "tell" you which part is the issue - good first check when you don't have backlights. Let us know when you get a chance with that test. We're here to learn and share knowledge.

[neilc6]

Re: Samsung UN50F6300AF No Backlights

The OP wanted to be able to test the backlights before opening up the panel and has an LED tester. In this scenario, you guess the string pairs on the connector (by color code or location) and compare the voltages on each string and see if they light up.

Checking the voltages on power up at the connector is a technique if you DON'T have a LED tester but you need to know how the circuit is wired. In this case measuring the max voltage and not seeing it settle at a lower voltage indicates an open circuit in the string. I have a LED tester so don't bother doing this.

I typically see shorted LEDs >90% of the time vs open LEDs. Almost every Samsung TV with bad backlights I repaired has about 50 to 90% bad LEDs and needed full replacement of all strips. With the lack of undervoltage protection shutdown, people will keep using their TVs until the screen gets too dim, the power supply is damaged, or a LED goes open. In a few cases, a LED failing open early will protect the rest of the string. A small percentage cause power supply damage with shorted LEDs (more in some models like UN58H5202 where the FET is damaged).

[MEB]

Re: Samsung UN50F6300AF No Backlights

OK, still at it. Still confused. Please see my little drawing. That is how I think this is how it works, in my mind. if that is wrong let me know.

I took the measurements from the back of the board using the pin locations shown by budm. i don't understand this, but this is what i got. All measurements were taken with black on chassis and red on pins tuning power on each measurement.

Pin 12 - jumps up to about 350 V then drops to nothing.
Pin 10 - same as 12
Pin 5 - jumps to 400 V then climbs to 438V and remains steady.
Pin 3 - same as 5.

The backlights did fire up once for a few seconds when I was probing the CNL802 connector, but only once. Thanks again.

[R_J]

Re: Samsung UN50F6300AF No Backlights

Why don't you go to shopjimmy, search your model number and look at the led's that are available for this model, then MAYBE it might make a bit more sense.
The chances of the power supply being at fault 5% bad led(s) 95%
The supply voltage for all the leds comes from CM882, the return lines FROM the leds connect to Q9151C and Q9251C, there are also 2 current monitoring lines IF1 and IF2 which connect to the strips inside the panel. The two seperate strings have a different amount of leds, Channel1 voltage is 215vdc and channel2 voltage is 274vdc

Channel2 has 80 leds, Channel1 has 64 leds

[neilc6]

Re: Samsung UN50F6300AF No Backlights

LEDs are driven by current sources and not voltage sources. Your LED tester outputs a constant current and displays back the voltage across the load. The diagram of the DMM tied to the output directly is incorrect since it should be drawn as a current source connected to a load and the DMM can measure the voltage across the load.

A shorted LED will cause an undervoltage situation and an open LED will cause an overvoltage situation. The voltage drop should be constant going through a string of diodes connected in series with a constant forward voltage drop. Most brands including LG and RCA will detect undervoltage and shutdown the backlight with even one LED shorted. Samsungs do not.

Depending on the circuit, the negative end of the string may be connected to a FET and a sense resistor so typically will not be at ground. The FET is switched with PWM modulation to dim the backlights.

[R_J]

Re: Samsung UN50F6300AF No Backlights

It looks like one string connects to pin4 (+ supply) goes into the panel, and comes out on pin3, which connects to pin10, goes into the panel and comes out pin8 (-return)

The second string goes into the panel on pin6 (+supply) comes out on pin5 and connects to pin12, goes back into the panel and comes out on pin11 (-return).
Pins 15 & 16 are current monitor pins and I don't know where they connect to the strings in the panel.
There are also 4 0Ω resistors on the distribution strip in the panel

[budm]

Re: Samsung UN50F6300AF No Backlights

OK, still at it. Still confused. Please see my little drawing. That is how I think this is how it works, in my mind. if that is wrong let me know.

I took the measurements from the back of the board using the pin locations shown by budm. i don't understand this, but this is what i got. All measurements were taken with black on chassis and red on pins tuning power on each measurement.

Pin 12 - jumps up to about 350 V then drops to nothing.
Pin 10 - same as 12
Pin 5 - jumps to 400 V then climbs to 438V and remains steady.
Pin 3 - same as 5.

The backlights did fire up once for a few seconds when I was probing the CNL802 connector, but only once. Thanks again.

I do not understand your readings, in your picture you show having DMM connected but the LED connector not in place.
The reading showing pin 12/10, 5/3 to be the same, are those reading with LED connector in place?
I need to see the reading with LED connector in place.
My pictures on post 23 is the exact match to your board? Unless my picture does not match your board may explain why the readings do not make any sense.
You do have LED tester, correct?

[budm]

Re: Samsung UN50F6300AF No Backlights

I do not understand your readings, in your picture you show having DMM connected but the LED connector not in place.
The reading showing pin 12/10, 5/3 to be the same, are those reading with LED connector in place?
I need to see the reading with LED connector in place.
My pictures on post 23 is the exact match to your board? Unless my picture does not match your board may explain why the readings do not make any sense.
You do have LED tester, correct?

BTW, did you check the resistance of diodes D9151C, D9251C in both direction with one leg lifted off the board?

[R_J]

Re: Samsung UN50F6300AF No Backlights

I made a slight mistake in post #29 regarding IF1 & IF2, it seems IF2 (pin15) goes to ground & IF1 (16) is the only (current sense?) line from the panel. (if thats what the IF is ?)

[MEB]

Re: Samsung UN50F6300AF No Backlights

Sorry for the sporadic lateness of my replies. I have been working extended and crazy hours. I appreciate your guys promptness. I am again at work, but will get back to you on your test requests.

budm - I haven't but will check the resistance of diodes D9151C, D9251C in both direction with one leg lifted off the board. I will check my original readings again on CNL 802, and yes I have an LED tester. I have used it to check strings when they are out of the panel, never from the PSU because I can't tell what is going on when they light up through the holes in the metal. At that time, I do not understand what the drop in voltage or lack thereof on the tester is telling me either. I will get back to you as soon as I can. Thanks.

[MEB]

Re: Samsung UN50F6300AF No Backlights

Ok, I re-checked the measurements of CNL 802 with the led cable connected, black on chassis and powering up on every pin.
Same as before:
Pins 11 and 12 - jumps up to > 350V then drops rapidly to 0.
Pins 5 and 3 - jumps to > 400V then ramps up to a steady 437V

I forgot I loaned my soldering iron to my daughter so I will take the board to work and lift those diodes and report back.

[nomoresonys]

Re: Samsung UN50F6300AF No Backlights

"Pins 5 and 3 - jumps to > 400V then ramps up to a steady 437V" That's bad, just read the thread, bad string or connection, verified by what was it 5 or 6 members.

[R_J]

Re: Samsung UN50F6300AF No Backlights

The voltage is boosted, similar to a pfc circuit, I suspect his meter has poor high frequency rejection and is giving a false reading

[MEB]

Re: Samsung UN50F6300AF No Backlights

R_J I have enclosed a picture of my meter. Should that be OK?

budm - D9151C = 264.9 black on cathode
0L red on cathode
D9251C = 265.1 black on cathode
0L red on cathode.

I started this thread because I wanted to know how to determine the state of the LEDs from the PS. It looks like we may have to put that on hold and use the LED tester. You guys can't tell me I told you so, because that wasn't on my initial agenda. but go ahead anyway. I am hoping now to use the results from the LEDs to help explain what I got at the PS.

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.

Ok, if we go with the tester, mine is a DVL 300 (or something like that) Super Led Tester with 3 red numeral LEDs. I have only used this as a power source to test strips removed from the panel. neilc6 says "A shorted LED will cause an undervoltage situation and an open LED will cause an overvoltage situation. The voltage drop should be constant going through a string of diodes connected in series with a constant forward voltage drop."
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.

Anyway, SJ is having a sale, so it is a good time to buy LED strings!

Thanks to all of you.

[neilc6]

Re: Samsung UN50F6300AF No Backlights

Your LED tester (if it reads 255V with no load) is displaying the maximum voltage it can produce. So a 255V reading will indicate an open circuit and 0V a short circuit. LEDs used in TVs are diodes and will typically drop 2.7V (Vf) if forward biased regardless of current. Reverse biased they will not pass current and wont light up. Some models use LEDs that are 6V (seen on some LG models). If you test one LED, it should measure 2.7V. When LEDs are wired in series, the Vf are additive. So 10 LEDs in series should measure 27V. If one is shorted, it will measure 9 x 2.7 = 24.3V. If you measure 5.4V on a string of 10 LEDs, then you can assume 8 are shorted. This undervoltage is the condition that will trigger the backlight to shut down in non Samsung TVs. If there is an open you will need to check every LED on the strip to find the open one. Failing LEDs can be slow to turn on, flicker, or read higher/lower voltage.