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Acer AL2416W backlight shutting off after .5-2 seconds

This is a discussion on Acer AL2416W backlight shutting off after .5-2 seconds within the Other Hardware Support forums, part of the Tech Support Forum category. I just took a broken Acer AL2416W monitor from one of my friends who was going to throw it away,

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Old 03-16-2011, 08:07 PM   #1
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I just took a broken Acer AL2416W monitor from one of my friends who was going to throw it away, because I figured it would be a shame to waste it. I've tested it pretty thoroughly every way I can think of beyond taking components off of the circuit boards. Here's what I found out:

The "Acer" logo appears on the screen for about half a second and then disappears, regardless of if there is any video input.

The flashlight test succeeds; the backlight shuts off not the lcd panel itself. When plugged into a computer I can just barely see when I drag windows from one of my working monitors onto this monitor's screen.

There are 6 outputs from the inverter board powering the backlight. The fewer of these connectors that are attached, the longer the screen stays lit, up to around 2 seconds.

Turning the monitor on and off again constantly for about 5 minutes (it seems like this time increases every time I use this strategy) will cause it to work until it is turned off for a significant length of time. I can turn the monitor off or let it go into sleep mode for more than 5 minutes and when I turn it back on or wake it up it will still be working properly.

All voltages coming into the inverter board from the power supply are normal when compared to the datasheet for the monitor; they do not change when the backlight shuts itself off after a second.

There are 10 green capacitors on the inverter board. 8 of them are in two lines of 4. these 8 and one of the other ones show 24 volts on the positive side until the monitor goes into sleep mode. The 10th one shows 12 volts.

There are 2 larger blue capacitors. The both show 24 volts on the positive side until the monitor goes into sleep mode. (I have no way to test if the capacitors themselves are working without removing them from the board, which I'm not prepared to do until I know it's absolutely necessary, I just use them as useful reference points within the circuit to tell where the problem might be. None of the capacitors are bulging or leaking.)

There are 12 transformers, two for each output to the backlight. Each transformer has 4 inputs. Two of these inputs display a heavily distorted square wave for the duration that the backlight stays on, then they drop to 0 volts. The square wave is not constant, it morphs slightly over the course of its life, possibly responding to the heating up of the components.

I assume the problem is not the transformers because otherwise the voltage inputs to them would read consistently until the monitor changed state to either off or sleep, but I may be wrong.

There are three surface-mounted things labeled "F" with an M printed on them, which I assume are fuses of some kind, two of which seem to line up with the symmetry of the two bays of 4 green capacitors, 1 blue capacitor, and 2 transistors. One, however, shows a distorted square wave for the duration that the backlight is on, at which point it levels to a constant voltage, while the other always displays a constant voltage.

The controlling IC has two interesting pins, one shows a triangular wave, the other shows a square wave, both waves only persist for as long as the backlight is lit then they prematurely cease.

I've tried to pack everything I know into this description, does anyone have any ideas as to what the fundamental problem might be?
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Old 04-06-2011, 05:34 PM   #2
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Also, pins 3 and 5 on what seems to be the controlling IC show a square wave for the duration of the screen's activity, but this square wave disappears when the backlight dies a second later. pins 3 and 5's waves ultimately end up at the AC power input pins of the transformers. Every other transformer is controlled by one of the pins (3 or 5), the others are controlled by the other. pin 3's square wave ends up crisp and defined when it reaches the transformers, whereas the wave from pin 5 turns into a heavily distorted remnant of the original wave by the time it gets to the transformer (I've attached pictures of these two waves).

on further inspection, the waves coming directly out of the chip seem identical but they are not, the one from pin 5 has a glitch at the beginning of it
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