Samsung un32eh5000f No backlight.

[ossroy]

Re: Samsung un32eh5000f No backlight.

When you unhook the cable between the main board and the power supply board then plug the TV into the outlet, that will put the power supply and backlights driver circuits into self-test mode (it the board has pull-up resistors), so if the power supply, LED driver circuits, and the LED strings are OK then the backlights should come on and stay on as long as the TV is plugged in.
At this point you need to report the Voltage on the power supply connector that goes to the main board and also at the LED connector (LED connector in place).

OK. Mainboard cable disconnected, then TV plugged in to wall. Through two of the holes in the upper center of the metal back of the screen, I can see some backlight shining through, but not through any of the many other holes except one in lower right corner. When I look at the front of the screen, it is all black. As far as voltages:

Pin Label Voltage

1 B13 13 VDC
3 B13 13 VDC
5 Vamp 13 VDC
7 Vamp 13 VDC
9 GND 0 V
11 B5V 5.2 VDC
13 B5V 5.2 VDC

Pin Label Voltage

2 PWM 3 VDC
4 B13 13 VDC
6 BLU 3 VDC
8 GND 0 V
10 GND 0 V
12 A5V 5.2 VDC
14 PS 3.5 VDC

And the backlight connector is:

Pin Label Voltage
1 D+ 140.5
2 5- 30.2
3 NC
4 5+ 30.2
5 D- 0.1

Now the interesting part... When I accidentally shorted pins 1 and 2 on the backlight connector, the whole screen lit up! Looks like the backlights are all functioning! I stood the TV up so I could see the front and took a chance shorting those pins again, and sure enough, the whole screen lit up. Sure wish I had a schematic of this thing so I could know what I'm doing.

So what do you think is going on? This is only my second flat panel repair and I have no concept of how these things work, like I did with the old CRT sets. My hunch is there is an open on the power supply board.

-Roy

[budm]

Re: Samsung un32eh5000f No backlight.

You have about the similar problem as OP, bad LED's, read post 1 ~ 7.
The LED's are connected in series to form one big LED string for this LCD panel.

[budm]

Re: Samsung un32eh5000f No backlight.

BTW, this your power supply board, right?
BN44-00665A
https://www.shopjimmy.com/samsung-bn...led-board.htm#
You should also verify that the LED power supply filter cap CM874 is still good.

[ossroy]

Re: Samsung un32eh5000f No backlight.

You have about the similar problem as OP, bad LED's, read post 1 ~ 7.
The LED's are connected in series to form one big LED string for this LCD panel.

Then how did the entire panel light up when I shorted pins 1 and 2?

[ossroy]

Re: Samsung un32eh5000f No backlight.

BTW, this your power supply board, right?
BN44-00665A
https://www.shopjimmy.com/samsung-bn...led-board.htm#
You should also verify that the LED power supply filter cap CM874 is still good.

Yes, that's my board. I'll check that cap.

[budm]

Re: Samsung un32eh5000f No backlight.

Then how did the entire panel light up when I shorted pins 1 and 2?

Pin Label Voltage
1 D+ 140.5: This the Anode pin of the #1 LED string.
2 5- 30.2: This is the Cathode pin of the #1 LED string.
3 NC
4 5+ 30.2: This is the Anode pin of the #2 LED string which is connected to the Cathode pin of the #1 LED string.
5 D- 0.1: This is the Cathode pin of the #2 LED string which goes to ground via PWM controlled MOSFET (Control element to maintain constant current) and the current sensing resistor to maintain constant current.
As you can see, the Vdrops of the #1 LED string is very high (140.50V -30.2V = 110.30V) which indicate high resistance compare to the #2 LED string.
https://assets.shopjimmy.com/media/c...00665a-top.jpg
If look at the LED power supply chart, it shows 91V @450ma. You are getting 140.50V because the load is light indicating problem in the LED string some where.
Since I do not know what your LCD model number is, this what I have from shopjimmy for the two LED strings: 12 LEDs per string.
https://www.shopjimmy.com/samsung-cy...s-2-strips.htm

[ossroy]

Re: Samsung un32eh5000f No backlight.

Had a chance to work on it Saturday for a while (I had my kids this weekend so I couldn't spend too much time away from them) and found something interesting after taking the panel apart. After the two LED strips were exposed, I disconnected main from power boards, plugged in power and lower bank lit dimly. Swapped top with bottom and BOTH strips lit dimly. The last LED in what was the upper strip did not light. I swapped them back and again they all lit dimly, except the last one of the top strip which is burnt out. I assumed when LEDs went dead they were open, but apparently this one was passing some current. I removed the lens and when I jabbed my probe tip into the bad LED, thus shorting it, both strips came up to full brightness and my DMM read 96v.

I wish I could figure out how to detach and jump out the LED, but I see no way to unsolder it. Can anyone here tell me how to do that? It would save me money and get the dining room table cleared off before my girlfriend visits this weekend.

[budm]

Re: Samsung un32eh5000f No backlight.

In the long run you will be better off replacing the LED strips.
Do you have hot air station and any other tools for SMD repair?
Here are example of tools:
https://www.shopjimmy.com/shopjimmy-...rk-station.htm
https://www.shopjimmy.com/yihua-862d...t-air-tool.htm

[ossroy]

Re: Samsung un32eh5000f No backlight.

I've never done any SMD work. I think I'll try my heat gun to slide it off and tack a resistor in it's place. The LED is in the upper right corner where it's loss will hardly be missed. The TV has already been replaced by a larger Smart TV and this one will now become a second monitor for my PC.

[budm]

Re: Samsung un32eh5000f No backlight.

The LED's are driven by constant current source so put resistor in place because the Vdrops on the resistor will vary with the setting of the backlights level, you can use 5 1N4007 in series to simulate Vf of the LED.
If the backlights is set to fixed value then you can calculate the value of the resistor based on known Vf of 3V and known current (300mA is typ.), so 10 Ohms 2W flameproof may be OK.
You may be able to get away with bypassing the bad LED but you must set the backlight level (not brightness) to as low as possible.

[ossroy]

Re: Samsung un32eh5000f No backlight.

I calculated 18 ohms, had a 2W, 22 ohm resistor in my junk box, so I used that and the backlights lit up wonderfully. I was able to get the bad LED off with a heat gun. Now to put it all back together and hope I still have a picture. Thanks for your help.

-Roy