View Full Version : Samsung no backlights.


JAnoY
11-10-2014, 04:17 PM
I have a Samsung model UN46D6300SF that I'm having an issue with the picture going out. Or should I say the backlights keep going out. I still have sound, and when the picture goes out, I can shine a bright flashlight at the screen and see a picture. Just a very dark one. Since this set has LED backlights, I was wondering if it will act like the CCFL backlit sets. When 1 lamp goes out, the PS thinks there is a short and cuts power to all of them.

CoogarXR
11-11-2014, 08:38 AM
Yes, in my experience, the LED backlights are usually all-or-nothing (I have seen some Sony models that fail in halves though). However, I have never seen an actual LED failure. It's usually the LED driver board that is the issue. I never tried to repair the driver board, I usually just replaced it.

lnx64
11-11-2014, 11:36 AM
Being a Samsung, check the caps. I've had so many Samsungs with capacitor issues, that it's amazing Samsung refuses to reimburse me for my time because I chose to do it myself than pay their tech $400 per TV.

mstaton
11-11-2014, 11:45 AM
I agree, check the caps on the power supply board. Usually 3 bad ones but just replace them all

JAnoY
11-11-2014, 04:00 PM
I checked all the capacitor caps and none look remotely bulged. Even took a magnifying glass to get a better view of the bottoms of them (had one blow the bottom out on a computer monitor) Everything looks good. Turned the set on last night to check and the backlights are working once again. Left it on for 7 hours, turned it off overnight, working fine all day today. Sometimes the backlight will stay out for hours. Sometimes it only a few minutes. Just frustrating that the problem comes and goes.

dishdude
11-11-2014, 04:45 PM
Maybe try tapping on the board next time they go out? It might be a bad solder connection.

powerking
11-11-2014, 07:44 PM
You cannot just look for the bugling cap and/or electrolyte leaking physical symptoms of bad/failed caps. One needs a quality ESR meter to drill down as to whether or not a cap is indeed open/shorted/leaking/defective. A companion tester would be one of the old school 20-600VDC out of circuit leakage testers. Too many people these days are getting wrapped around the troubleshooting "axel" just looking for physical evidence of failures in this regard (IMHO); test do not guess is one of my favorite motto's.

Tom (PK)

rca2000
11-11-2014, 11:57 PM
LED BL's's DO go bad...guys. I have seen a good amount of them bad.

What usually happens is only one of the LED's opens...BUT usually there is only one or two strings..(except for the sets that use a "modulated BL", for "dynamic dimmng", these can have as many as twenty seperate LED strings all over the panel).

So--when an LED opens--it interrupts power to ALL of them. You can test this. Upon power on, you need to see if you are getting LED drive power to the BL. It should be at least 75 volts and as high as 250 volts--depending on the umber of LED's in the string and so on. If you see this voltage...and it then DROPS of, but you get NO BL..you probably have an OPEN string. There IS a way to prove this...but you MUST be fully aware of the BL driver ckt and ALSO be aware that is you make a mistake...you could BLOW the WHOLE PS !!.

Looking at you PS board--I was not able to tell if you have only one or two strings or not. I need to see the FOIL side--as that is where the LED driver transistors are. Normally...these are driven by the B+ I talked about...and it is on ONE side of the string. The OTHER side...goes to the dimming transistor. It then determines how close to ground it will brig the string.

If you are getting the drive to the string...THEN...you CAN "bypass" the driver transitor. But do NOT simply short the transistor to ground...it WILL blow the LED's !!

Take a 220 ohm 1 watt or so resistor and CAREFULLY..touch THIS to the side of the string that goes from the BL to the transistor, and the other end of the resistor to cold ground. Remember...the other side of the BL string goes to the B+ supply for the BL. WARNING !! Do NOT do this for more than a second or so--long enough to see if you get a BL flash on. The resistor WILL get hot--BTW. If you DO--the BL is probably ok--and you have a drive transistor or control ckt problem. If you do this and get NO BL and the voltage does NOT drop on the LED side of the resistor..or if there is NO voltage there...the bL is open somewhere.

CoogarXR
11-12-2014, 10:41 AM
LED BL's's DO go bad...guys. I have seen a good amount of them bad.

What usually happens is only one of the LED's opens...BUT usually there is only one or two strings..(except for the sets that use a "modulated BL", for "dynamic dimmng", these can have as many as twenty seperate LED strings all over the panel).

So--when an LED opens--it interrupts power to ALL of them. You can test this. Upon power on, you need to see if you are getting LED drive power to the BL. It should be at least 75 volts and as high as 250 volts--depending on the umber of LED's in the string and so on. If you see this voltage...and it then DROPS of, but you get NO BL..you probably have an OPEN string. There IS a way to prove this...but you MUST be fully aware of the BL driver ckt and ALSO be aware that is you make a mistake...you could BLOW the WHOLE PS !!.

Looking at you PS board--I was not able to tell if you have only one or two strings or not. I need to see the FOIL side--as that is where the LED driver transistors are. Normally...these are driven by the B+ I talked about...and it is on ONE side of the string. The OTHER side...goes to the dimming transistor. It then determines how close to ground it will brig the string.

If you are getting the drive to the string...THEN...you CAN "bypass" the driver transitor. But do NOT simply short the transistor to ground...it WILL blow the LED's !!

Take a 220 ohm 1 watt or so resistor and CAREFULLY..touch THIS to the side of the string that goes from the BL to the transistor, and the other end of the resistor to cold ground. Remember...the other side of the BL string goes to the B+ supply for the BL. WARNING !! Do NOT do this for more than a second or so--long enough to see if you get a BL flash on. The resistor WILL get hot--BTW. If you DO--the BL is probably ok--and you have a drive transistor or control ckt problem. If you do this and get NO BL and the voltage does NOT drop on the LED side of the resistor..or if there is NO voltage there...the bL is open somewhere.


Nice trick. I'll keep that in mind if I ever run into a tough LED problem.

JAnoY
11-25-2014, 12:28 AM
Ok, So the backlight finally go out again after working for several days. I took the screen out to see the backlights. It looks as though there is only 1 string of leds along the top of the frame with a plug at each end. I turned on the tv and got sound so I knew the set should be on and working, but part of the led string dimly blinked. About 1/3 of the string on the left side did nothing. I tried to unplug the left plug and the set did nothing. Power light blinked but the set wouldn't power up. Tried the right side plug, same thing. I had both sides plugged up, and still 2/3 of the leds dimly blinked. I tried to see which led was the last in the string that wasn't working at all, but they all poped on and I get blinded by the entire string shining brightly. Never had time to check voltages when the string was out, see exactly how many of the leds were not blinking, ...nothing. It's up and working fine. Thinking about just replacing the led string and keeping my fingers crossed. I don't want to try and bypass the driver transistor seeing how this set doesn't belong to me and I do not want to blow the PS or anything else. So where the heck do I look to buy a replacement led string?