Dell 2709W

[eidam655]

Hi everyone,

I have a used Dell 2709Wb on my desk that works... Sometimes. There's two symptoms which I'm hoping have a common cause.

Issue #1 is that occasionally, when being turned off for a while, the monitor does not turn on. The power button does not react (nothing lights up), replugging the power cable does nothing. After being unplugged for a few hours, the first insertion still does nothing, but plugging it in the socket again but reversed brings it to life.

Issue #2 is that when working occasionally after an hour or two the screen switches itself to some kind of diagnostic mode, where no buttons work and it cycles through colored screens (red, green, blue, gradients).

After disassembly and visual inspection I found nothing to be out of ordinary. No flux residue, no blown up components, browned spots, leaks, even all the capacitors look alright.

When measuring, the only oddity I found on the high power side was that the voltage drop from + to - on the bridge rectifier (D10XB60H) was only one voltage drop (0.55V) and not two.

On the "logic board" i found a "step down switching regulator," part nr L4978, that always receives 23.5V, which leads me to believe that the power supply is OK.

When the monitor does not work, the readouts on the IC's pin read as follows:
1 - GND
2 - SS_INH - 0V
3 - OSC - 23.4V
4 - Vout - 0V
5 - Vcc - 23.5V
6 - BOOT - 0V
7 - COMP - 0V
8 - FB - 0V

When it does work (and outputs a picture), it's like this:
1 - GND
2 - SS_INH - 12.6V
3 - OSC - 2.8V
4 - Vout - 12.35V
5 - Vcc - 23.5V
6 - BOOT - 20V
7 - COMP - 0.5V
8 - FB - 3.3V

Unfortunately I have no idea what to check next, where to look for failure. I will be grateful for any pointers and if more information was needed, I'll gladly measure some more stuff. And what's more, I'm at loss as to how i would go about diagnosing the color screen switching.

[lotas]

Re: Dell 2709W

Catch the moment when the monitor does not turn on and at this moment start heating this regulator with a hairdryer or a soldering iron tip, if the monitor starts up when its temperature changes, then feel free to change it to a new one.

[Per Hansson]

Re: Dell 2709W

When measuring, the only oddity I found on the high power side was that the voltage drop from + to - on the bridge rectifier (D10XB60H) was only one voltage drop (0.55V) and not two.

It might be because the bridge rectifier is in circuit and the readings are affected by something else.
But the fact that it helps to reverse the mains plug also points to that you could have some intermittent failure in the bridge rectifier.
If you have another one from some ATX power supply for example to put in temporarily and see if it improves things that might be a good test.
Also interesting to measure the voltage on the main filtering capacitor before and after that is done:
Because if one diode in the full wave bridge rectifier is broken then it will be acting as a half wave rectifier.
And since the capacitance if fixed it should result in a larger voltage drop with a given load.

[eidam655]

Re: Dell 2709W

some progress:

The voltage across the big capacitor's legs is always 309V when plugged in, on or off.
The heating of the IC did nothing.

However, I now have a completely new issue - the monitor turns on but only displays colored stripes (pixel column by pixel column) and the buttons on the panel are unresponsive.

[eidam655]

Re: Dell 2709W

(oh no, can I not edit my previous posts?)

Here's a photo of the lines. I have the monitor laying face down on the table, so the photos are also funky.

[eidam655]

Re: Dell 2709W

And more progress from today morning. TL;DR I was poking around semi-randomly in hope of finding something.

I measured some more voltages and studied the pinout of the L4978.

IC701 / L4978:
At pin 2, for SS_INH it says "A logic signal (active low) disables the device (sleep mode operation)." So if I understand this correctly if there's no (sufficient) voltage at this pin, the IC is kept off? The next question then is where do I get voltage from? )
I mean, what I can see is that Pin 2 is connected to capacitor C709 and to the Collector of a transistor Q701. Its Emitter is grounded IIRC and the Base is connected to a R712 1kOhm - R711 10kOhm - C711 capacitor. On the 1kOhm I get 3.2V, but on the Base there's just 0.7V which then falls to 0.03V which is then on Pin 2 as well. I just don't know what to do with this information.

Change in display:
Since the lines from yesterday showed up out of the blue, but the backlight was working and all the 24V rails were present, I tried to turn the monitor on without the wide connector plugged in (the one that I guess is sending the display signal on the panel itself). I measured some voltages, some were at 0, a lot were at 3.2V, some at 5V, but some were on 0.04V or 0.2V.
However when I plugged this connector back in I got immediately a diffrent picture - the screen started cycling the various colors and gradients from post #1. The power button was still unresponsive, but I guess it's a progress?

I also found this older thread about the same monitor with a lot of information. It looks like the logic chip itself might need a reflow in my case as well, as it seems to me that that's where the monitor is failing.

A sidenote: is there a way to check if the proximity power buttons work as intended? When unplugged, where would I check for voltage changes to see if the buttons work?