Acer 5534 No Backlight (LED)

[lv128]

I'm trying to locate the fuse on an Acer 5534 montherboard that would feed the backlight. I've tried three new panels on the system, all of them turn on and get picture but with no backlight, so I'm certain the problem exists on the motherboard. Possibly the BL_ON switch to tell the board to turn on the backlight is at fault which I'm hoping is a fused circuit.

Would a transistor prevent the backlight from being triggered? If so how would I find which one on the motherboard? I reviewed a LVDS diagram and isolated a few possible pins, but I'm stuck at this point.

Flickr Photostream

What I've checked so far:
1. tracing the voltage from gnd on the LCD's board to the pins on actual LED strip = ~18V on three pins when tested against the main board GND, and this same GND to the rest of the board does not link up to the other 3 pins (combined) on the LED strip. Is the GND ever fused/switched? I always thought the voltage was the side that was fused/switched in all circuits.

2. checked everything that even remotely looked like a smc fuse but either they all give me a quick beep on cont. (usually indicating it's a cap) or a solid beep, I can't find anything that is definitely a fuse.

[alexanna]

Re: Acer 5534 No Backlight (LED)

It really doesn’t look like it’s too much different than an inverter set up in a CCFL type.
It looks like it’s called a converter, But it still uses a power, ground and a dim and a backlight on signal.
LCD parts explains how to wire up a universal, I would think yours should be similar
you should be able to check for those voltages.
http://www.lcdparts.net/howto/howtoXBConnect.aspx

[lv128]

Re: Acer 5534 No Backlight (LED)

I checked out that website before, looking for information on a different topic (about re-using LED strips pulled from broken LCD panels) but they're more geared towards selling a full replacement kit than powering one of my own LED strips.

Those pictures I have are of the motherboard in the laptop itself, not the LCD panel. The LCD panel has a controller of sorts built into the logic board on the back that is sending 18 volts to 3 of the 6 pins that would be connected to the actual LED strip.
And 3 other pins that are joined together (and 3 that are dead ends in the middle)
LED strip connector

The ones that should be ground are giving me infinite resistance when tested against a case ground and the other 3 are giving me 18v (when powered on) when tested against a case ground. But still nothing lights up on the strip. But since three different panels didn't light that's why I'm trying to find the problem at a motherboard level.

[lv128]

Re: Acer 5534 No Backlight (LED)

I guess the real question is, how can there be a traceable voltage to the pins, but no traceable ground?

[lv128]

Re: Acer 5534 No Backlight (LED)

(complying with advice from another, here's an inline picture)

[PlainBill]

Re: Acer 5534 No Backlight (LED)

I guess the real question is, how can there be a traceable voltage to the pins, but no traceable ground?

How familiar are you with transistors (I'm not being sarcastic, it allows me to tailor the answer to your experience level).

Here's the electronic tech version of my speculation. I've seen datasheets for controllers for LED backlights, but they gave very little detail. My speculation is that the LEDs are wired in series (wiring them in parallel results in nasty current hogging problems). The anode of the first in the string is tied to the positive output of the power supply. The cathode of the last in the string is pulled to ground by a transistor, with a low value between it's emitter and the negative output of the power supply.

This allows the controller to adjust the voltage across each string to maintain a constant current through each string. Because there is a transistor between the cathode of the LED and ground, your DMM will not show conductivity to ground.

PlainBill

[lv128]

Re: Acer 5534 No Backlight (LED)

I've got a fair grasp on electronics basics, but tbh, I'm better handling larger components than SMT stuff.
Ah, I think I understand why I can't find the ground then. Although unless the transistor is embedded into the LED, I'm not seeing any component.

Although, back on topic for this particular problem even though this may explain some of the issues I've been having on the other topic, I've tried a number of new LCDs on this laptop, and there is power reaching the LED strip, but no discernible ground, so if the ground is switched via transistor, the transistor must be located on the motherboard, and/or be burned out. Would this be a correct assumption instead of looking for a bad fuse (which is common on the Dell inspiron 1545)?

I see D45/D46 which isn't the usual marking for a transistor that I've seen, although they do look like them.
The Q31/Q32 would be some type of controller? and Q33 a transistor?

[PlainBill]

Re: Acer 5534 No Backlight (LED)

Component designators usually use a standard terminology. D is a diode, they are often found in three legged packages (two diodes). Q is always a transistor; U and IC are both used for integrated circuits. R is a resistor, C a capacitor, L an inductor, and T a transformer.

PlainBill

[lv128]

Re: Acer 5534 No Backlight (LED)

So would the transistor be the most likely fault? If so, is there a way to test in-circuit without removing it?

[PlainBill]

Re: Acer 5534 No Backlight (LED)

So would the transistor be the most likely fault? If so, is there a way to test in-circuit without removing it?

A transistor has three legs, number them 1, 2, 3 in any manner you choose. Set your DMM to the 200 ohms range. Measure the resistance 1-2, 1-3, 2-3. Orientation of the meter leads is not important. All should read open.
Now set your DMM to the diode test range (usually marked with a diode symbol). Orientation is important for this test; black lead goes to the first pin, red to the second. Measure 1-2, 1-3, 2-3, 2-1 3-1, 3-2. Four of these six readings should be over range, two should be about .5 volts.

PlainBill

[lv128]

Re: Acer 5534 No Backlight (LED)

With pin one being the lone pin on it's side (and also the common I believe) and the other ones flanking left and right on the other side, my results are:
1-2 - open, 2-3 open, but 1-3 starts at 170 ohm and slowly increases to open. is this a sign of a broken transistor?

As for the diode test, all of those combinations give me open circuit except for 3-2 which the dmm shows as a 3 digit number, millivolts I'm assuming starting at 480 then dropping and stabilizing around 365.
So if only one of them reports voltage, can I assume I need to replace that?

[PlainBill]

Re: Acer 5534 No Backlight (LED)

With pin one being the lone pin on it's side (and also the common I believe) and the other ones flanking left and right on the other side, my results are:
1-2 - open, 2-3 open, but 1-3 starts at 170 ohm and slowly increases to open. is this a sign of a broken transistor?

As for the diode test, all of those combinations give me open circuit except for 3-2 which the dmm shows as a 3 digit number, millivolts I'm assuming starting at 480 then dropping and stabilizing around 365.
So if only one of them reports voltage, can I assume I need to replace that?

Good work so far, now to interpret the results.

The resistance rising slowly indicates your DMM is charging a capacitor. The fact that it eventually shows open means the transistor passes that test.

The diode test may or may not be significant. The tests I described are for a bipolar transistor, (but will also work for a dual diode). Your results indicate the device is either a bad bipolar transistor or it may be a good FET. Transistors in SMD packages use a marking code - two to four letters and numbers that a manufacturer uses to uniquely identify one of it's products. Can you read the marking code on the package?

PlainBill

[lv128]

Re: Acer 5534 No Backlight (LED)

The transistor is marked as ADXV and then 86. The 86 is printed sideways.
I did a little poking around but was unable to find any good resources for matching the part number.

[PlainBill]

Re: Acer 5534 No Backlight (LED)

The transistor is marked as ADXV and then 86. The 86 is printed sideways.
I did a little poking around but was unable to find any good resources for matching the part number.

One of the best sites I've found is www.s-manuals.com. Unfortunately, I can't come up with a match there.

PlainBill

[lv128]

Re: Acer 5534 No Backlight (LED)

I'm stumped then, any other ideas of where to look, or how to power this backlight? even if the LCD backlight is on permanently whenever the computer is turned on?

[PlainBill]

Re: Acer 5534 No Backlight (LED)

I'm stumped then, any other ideas of where to look, or how to power this backlight? even if the LCD backlight is on permanently whenever the computer is turned on?

The service manual is available on Elektrotanya.

PlainBill

[lv128]

Re: Acer 5534 No Backlight (LED)

Thanks Bill, I've already look through that though, unfortunately, I'm in need of schematics instead of "replace motherboard" I've just moved on and turned the system into a headless unit for media center use, no sense in beating my head against a wall. Maybe if I encounter another one that I'm successful with I can revisit this one for a solution.
Thanks!

[PlainBill]

Re: Acer 5534 No Backlight (LED)

Thanks Bill, I've already look through that though, unfortunately, I'm in need of schematics instead of "replace motherboard" I've just moved on and turned the system into a headless unit for media center use, no sense in beating my head against a wall. Maybe if I encounter another one that I'm successful with I can revisit this one for a solution.
Thanks!

Probably a good idea. A main board replacement would fix it, but that is expensive. And troubleshooting notebook mainboards by proxy barely ranks ahead of a root canal in my list of desirable activities.

PlainBill