Samsung UN32EH5000FXZA, No Backlight

[johnboy1313]

I picked this one up today. It powers on but has no backlight. A flashlight shows that there is a picture. I believe that I have one or more open LEDs. In standby, the main cap shows 169V across it but when powered up, it only shows 166V. I find this to be confusing. I expected to see ~390V at power up. In standby, CNL802 shows ~95V at pins 1 and 2. When powered on, I see ~180V at those pins and all marked voltages are present at CNM803 as well. With CNL802 unplugged and the meter in diode mode, I get O/L in every possible configuration on the plug. I suspect that this points to at least one open LED for what I suspect to be two LED strings.

Looks like I need to open the panel.

[tom66]

Re: Samsung UN32EH5000FXZA, No Backlight

This TV has no PFC booster, so the cap voltage will always be ~160V. You can tell this because there is no PFC inductor, diode, MOSFET, etc.

They can get away without PFC because LED TV uses very little power.

[CapLeaker]

Re: Samsung UN32EH5000FXZA, No Backlight

plus you can't test the LED string like that with a multimeter anyway. Only thing you can do with a multimeter there is to check if one of the wires is shorting out to the metal frame and each individual LED.

[budm]

Re: Samsung UN32EH5000FXZA, No Backlight

The LEDS stings looks to be connected in parallel, see bottom side connection.
Your meter is not going to put out Voltage to forward bias the LEDS to test the whole string.
What DC Voltage do you get at the LED connector?
You can put DCA meter in series with the wire to see if it shows any current draw.
https://www.badcaps.net/forum/showpo...4&postcount=95
https://www.badcaps.net/forum/showpo...&postcount=107

[johnboy1313]

Re: Samsung UN32EH5000FXZA, No Backlight

That makes sense. I got into the panel fairly easily. There is evidence of multiple LEDs being blown. It looks like there is soot at each end of both LED strips but the LEDs themselves don't look burnt like I've become accustomed to seeing. How do I test each LED? Do I need to pry the cover off each one and glue it back on when I've tested it?

[budm]

Re: Samsung UN32EH5000FXZA, No Backlight

I believe those LENS are snap-on.

[CapLeaker]

Re: Samsung UN32EH5000FXZA, No Backlight

yes, they are

[johnboy1313]

Re: Samsung UN32EH5000FXZA, No Backlight

It looks like these are glued on somehow. I broke the first one off and will have to super glue it back on. At any rate, the first LED shows .741V in diode mode and O/L the other way. I guess this means it is good???

If there are 12 LEDs on a strip and each strip is being fed ~180V, that means each LED needs 15V to light. Does that seem right?

[tom66]

Re: Samsung UN32EH5000FXZA, No Backlight

.741V is very low for an LED. When tested on a meter a good LED should glow slightly, if a reading is available.

How do multiple LEDs fail simultaneously? Design flaw?

[budm]

Re: Samsung UN32EH5000FXZA, No Backlight

White LED should have forward Voltage around 3~4V range (typical is around 3.3V).
0.741V looks real low to me.
12 LED x 3.3 = 39.6V+ V drops for current limiter.
http://www.maximintegrated.com/en/ap...ex.mvp/id/3070

[johnboy1313]

Re: Samsung UN32EH5000FXZA, No Backlight

It looks like I should be able to slip a sewing needle under each side of the lens to get to the LED mounting pads. This should give me something to probe without breaking each lens off. I have some known good LED strips from a different set here to see if they light when tested. They are 3.3V LEDs for certain though. Six LEDs on a strip, 20V from my bench top power supply will light the strip.

Back to my question above, does 15V each LED seem right?

[johnboy1313]

Re: Samsung UN32EH5000FXZA, No Backlight

White LED should have forward Voltage around 3~4V range (typical is around 3.3V).
0.741V looks real low to me.
12 LED x 3.3 = 39.6V+ V drops for current limiter.
http://www.maximintegrated.com/en/ap...ex.mvp/id/3070

The PSU is putting out roughly 180V to each strip when powered on. 95V in standby.

[budm]

Re: Samsung UN32EH5000FXZA, No Backlight

"Back to my question above, does 15V each LED seem right?" I do not understand what you mean by 15V each LED.
Just use 3V coin cell battery with 100 Ohms in series for testing these white LEDs.

[johnboy1313]

Re: Samsung UN32EH5000FXZA, No Backlight

180V from the PSU at connector CNL802 / 12 LEDs on a string = 15V each, or am I missing something?

[johnboy1313]

Re: Samsung UN32EH5000FXZA, No Backlight

3V doesn't light any LEDs. 4V only makes a handful of them barely glow. It's hard to tell they're even glowing.

[budm]

Re: Samsung UN32EH5000FXZA, No Backlight

180V from the PSU at connector CNL802 / 12 LEDs on a string = 15V each, or am I missing something?

The strings are connected in series.

[budm]

Re: Samsung UN32EH5000FXZA, No Backlight

3V doesn't light any LEDs. 4V only makes a handful of them barely glow. It's hard to tell they're even glowing.

That does not make any sense, are you limiting the power supply current o about 10mA, that should be more than enough the light up white LED quite bright.
Do you have any other white LED to try?

[johnboy1313]

Re: Samsung UN32EH5000FXZA, No Backlight

The strings are connected in series.

There's 24 LEDs total. 180V/24 LEDs is still 7.5V each. Maybe thats why the LEDs are blown.

[johnboy1313]

Re: Samsung UN32EH5000FXZA, No Backlight

I tested a good, known working LED on a different strip. I can light the strip of six LEDs with 20V at the connector. It shows .741 on my meter as well and does not light up when being tested with the meter.

[budm]

Re: Samsung UN32EH5000FXZA, No Backlight

What is the DC rating of the LED DRIVE boosted Voltage filter cap C9110?