Dell S2715Ht LED backlight

[leggot]

Hi all,

I have a dell S2715ht, it has an led strip backlight and has an odd problem, the led strip turns on for about a second before turning off. The monitor power led stays on and shining a flashlight into the screen reveals a good picture.

I was able to crack the monitor open and test the voltages going to the led strip, the led strip lighted momentary so I only got readings at that one given instant.

The led/lcd panel is an LG LM270WF7. I would be satisfied using an external power source to power the LED backlight, independent of the monitor; however I haven't been able to find any datasheets for the monitor or the panel. If anyone has experience with the wiring of LED backlights, help would be much appreciated.

Green (GND?)
Yellow (~15V)
Orange (19-20v)
Red (19-20v)
Brown (~15v)
Black (~15v)

[budm]

Re: Dell S2715Ht LED backlight

Can we see the whole pictures that show how all the boards and the LCD PANEL are connected together?

[leggot]

Re: Dell S2715Ht LED backlight

[budm]

Re: Dell S2715Ht LED backlight

It looks like there are 4 set of LED strings (I see 4 current sensing resistors (LED's Cathode return): 1R00 (R372~375)), that 16-PIN IC next to the LED connector should be the LED driver IC, you can look at the number on top to get the spec sheet of the IC.
L702 inductor looks to be the LED+ Voltage for the LED common Anode strings, see which pin of the connector has direct connection to L702.
Also check to see how R372~375 connected to each pin of the connector. I see that one end of R372 go to pin with GREEN wire and another end of the resistor goes to the pin of the IC, so what DCV do you have on that GREEN wire?
GB98AGN??
I cannot find the spec sheet for it.

[leggot]

Re: Dell S2715Ht LED backlight

Hi,

Thanks for the quick reply.

I noted the following were connected:
R732 - Green to pin 1
R733 - Black to pin 9
R734 - Brown to pin 11
R735 - Black to pin 12
L702 - Red and Orange (tied)

The IC says GB98AGN CF7NIN 1448A2 at I701. I couldn't find an easily accessible datasheet, I emailed a place; we will see if they respond.

According to this russian forum, the hp elite display e221c has the same chip. This might give us some insight to the led driver of the dell, but again I can't seem to find a service manual.

When I measured the green wire voltage, (black probe to black wire, red probe to green wire) it was about negative 15 volts. This indicated to me that green would be the reference voltage, thus I measured everything in relativity to green.

[budm]

Re: Dell S2715Ht LED backlight

Red and Orange is the supply Voltage to the LED Anode.
The other four wires are the Cathode return wires for each LED strings (4 strings) and all those 4 wires should show the same reading if there is no problem with the LEDs.
R732 - Green to pin 1
R733 - Black to pin 9
R734 - Brown to pin 11
R735 - Black to pin 12
L702 - Red and Orange (tied)

Please verify the color.

[leggot]

Re: Dell S2715Ht LED backlight

I apologize, here are the corrections

R732 - Green to pin 1
R733 - Yellow to pin 9
R734 - Brown to pin 11
R735 - Black to pin 12
L702 - Red and Orange (tied)

[Davi.p]

Re: Dell S2715Ht LED backlight

Budm: "It looks like there are 4 set of LED strings"
I think you mean there are 1 string with 4 series of leds...
in fact i have count 29 leds from the picture, is not entire, suppose ther are 32 leds, divided by 4 are 8 led/series=20v/8=2,5v per led. Now what you have to do is to grab somewere a small battery 3v an with some probes supply each led, if you find the one faulty you have to: 1. search for a replacemet strip OR 2. search for a compatible led and buy some.. (replacement is not easy)

[leggot]

Re: Dell S2715Ht LED backlight

Actually, there are 60 leds, it is rather difficult to take a picture of the entire panel. In the momentary flash, it appears as if all the leds light up, however it is difficult to verify all 60 leds do flash.

[Davi.p]

Re: Dell S2715Ht LED backlight

20v/15led=1,3v per led, seems a bit low, but if you can supply 1,5v by a battery to each led you see if them lights up, if not you can watch which pin of the IC goes to a small cap. and after to GND (screw hole), if you find only 1 of theese, short circ. the cap to gnd and try to power up. You could see all led stay lit apart the broken segment, then you can measure V on each led and then apply that to each led of the broken segment to test.

[leggot]

Re: Dell S2715Ht LED backlight

20v/15led=1,3v per led, seems a bit low, but if you can supply 1,5v by a battery to each led you see if them lights up, if not you can watch which pin of the IC goes to a small cap. and after to GND (screw hole), if you find only 1 of theese, short circ. the cap to gnd and try to power up. You could see all led stay lit apart the broken segment, then you can measure V on each led and then apply that to each led of the broken segment to test.

From what I can tell, all the LEDs do work, the entire strip flashes momentarily when the unit is first turned on. This leads me to believe this is an issue with the circuitry that controls the power going to the LEDs.

I am not opposed to by passing this circuit and simply applying voltage to the LEDs independent of the circuitry; however it would be nice to be able to dim the monitor.

I am not sure if this answers your answer, I'm not sure if this proves that all the LED strips are functioning correctly.

[Davi.p]

Re: Dell S2715Ht LED backlight

I dont really know about leds but as other semicondutctor components, it can go short at a certain temperature or current, so what seems good to you, it can be not. Apart this leds failure from what i have read many times is the big part of led B.L. problems. You can also test the PS, disconnect green wire by sliding it out (use a sharp tip, needle..) then make a jumper between another return to the place of green, i don't know if it works, never tried, in another way you know that the IC needs to sense a certain voltage range on a resistance, i don't think are those 1ohm resistances, them are probably fuses, them are internal to the ic, we don't have the datasheet so we can't supply that voltage to hijack the ic, but i remember of a Vestel chassis i have studied, i suppose is about from 1 to 3v not more.

[Davi.p]

Re: Dell S2715Ht LED backlight

GB98A is made by O2micro: https://cdn.badcaps-static.com/pdfs/...6f8b3d4b96.pdf
at page 21 a very ugly schematic, no datasheet..

[budm]

Re: Dell S2715Ht LED backlight

Right now the boost converter is not working,
You have 4 set of LED strings, 15 per string, you are only getting 19V.
Monitor OFF and disconnect LED connector, black probe on the chassis, red probe on L702 then turn on the monitor to see how high it jumps up to.

[leggot]

Re: Dell S2715Ht LED backlight

L702 jumps to about 60v and returns back down to 19-20v

[budm]

Re: Dell S2715Ht LED backlight

OK, so the boosted Voltage did startup without the load (LED connector not in place).
From post 1:
Green (GND?)
Yellow (~15V)
Orange (19-20v)
Red (19-20v)
Brown (~15v)
Black (~15v)

So what is the actual Voltage on that Green wire since now we know that Green wire is not GND but it is one of the 4 LED string.

[leggot]

Re: Dell S2715Ht LED backlight

Ok, comparing the voltages to the chassis ground:
All the voltages decline after spiking at the voltages listed below.

Green - 12v (this is what the multimeter says, I think it spikes higher, but it is difficult to tell as the multimeter 'thinks' for a second then displays the voltage)
Yellow - 20v
Orange - 60v
Red - 60v
Brown - 20v
Black - 20v

Also, one string of leds stays on for about a second longer than the rest - possibly corresponding to the green wire?

[budm]

Re: Dell S2715Ht LED backlight

Something wrong in that LED string circuit, it is either the IC or the LED string. One way to find out is to swap the Green wire with other LED wire to see if the problem will follow.

[leggot]

Re: Dell S2715Ht LED backlight

First I disconnected the green wire - this led to 3/4 of the led string to function as intended.

Upon swapping the green wire with the black wire, the same 3/4 of the string functioned; however now the string tied to the green wire doesn't turn on when it was reattached. Possibly the green wire string broken when it was attached to the black wire.

Is there a way to test each LED individually? It would be acceptable to swap that one with an LED that is removed from the end of the string - that position would just have a jumper wire on it.

[budm]

Re: Dell S2715Ht LED backlight

You can test each LED as long as you can get to the solder pads for the LED, and if your meter can provide enough Vf and current to turn on the LED.