Faulty monitor, capacitor help

[tinpanalley]

I've got a faulty Asus PA238Q. Unfortunately, I needed a monitor so I had to buy another. But I've been encouraged to use it as a project to learn how to replace capacitors because it has been suggested that that is probably the problem with the display. So, my question is can someone help me with this, help me identify what to look for and what would need to be done?

Thanks for any help!

[Hackentosher]

What makes you think it's a cap? What are the problems you're experiencing?

[tinpanalley]

21 minutes ago, Hackentosher said:

What makes you think it's a cap? What are the problems you're experiencing?

As I said, it has been suggested to me that that is probably the problem. Asus PA238Q.

Signal leaving GPU fine, other displays work, cables all fine, display powers on, but then image doesn't appear.

[Hackentosher]

1 minute ago, tinpanalley said:

As I said, it has been suggested to me that that is probably the problem. Asus PA238Q.

Signal leaving GPU fine, other displays work, cables all fine, display powers on, but then image doesn't appear.

I mean that could be just about anything. Typically when a cap goes bad, it's somewhere in the power stage which would prevent the monitor from powering on. It sounds like you don't have any evidence on it being a cap. I mean if you just want to practice recapping things, you can't break it any more than it already is. If you want to actually diagnose it, you're going to need to open it up because no one can magically tell you what's wrong with it when all you provide is "it was working now it don't." Anecdotes and common problems are good things to go on, but again it could be anything.

 

Because it powers on but there's no image, my bet is on either the display connector, or maybe one of the display driver chips. Probably not worth fixing with how much time it'll likely take to properly diagnose the problem. In addition, do you have the tools and experience to find and solve such a problem? Ie: soldering iron, multimeter, flux, solder wick, solder sucker, etc.

[Unimportant]

11 hours ago, tinpanalley said:

display powers on, but then image doesn't appear.

Does it power on and come out of sleep when a video signal is applied? (Usually indicated by some status LED).

If so, try shining into the screen with a strong flashlight and see if you can see a faint image. If you can then the backlight does not work, which is a common problem for LCD monitors in general. Cause of that could still be lots of things.

 

If you're absolutely convinced it is a capacitor, you'll need a ESR meter and a capacitance meter, and test suspect capacitors for both their capacitance and ESR - failure mode can be one or the other, or both. Failed capacitors need not be bulging.

[mariushm]

It could always be a capacitor, but in this case probably not.

 

More likely, one or several leds that form the backlight are dead (either shorted or bond wires inside the led are broken so the led doesn't light up) which makes the backlight controller detect an imbalance and stop the backlight from working, which means you get nothing on screen.

You could open the lcd display and check the voltage going to backlight using a multimeter ... most likely there's gonna be 2 or 3 outputs, most likely 2 , one for each edge of the lcd display ... and each output powers a chain of leds installed in series (so you have something like 40-100v depending on how many 3v/6v are chained together across the width of the panel).

You can put the meter probes on output and turn on the lcd display and the MAX feature of the meter  will show you the peak voltage ... you should have same voltage on both outputs give or take 0.5v-1v ... if there's a bigger difference, led chain is faulty.

You can also test each led individually with a meter in diode mode, but may have to scrape the traces near the led to put the probes on the actual circuit board traces. Some multimeter don't have the forward voltage high enough to actually test 6v leds, in which case you can get a 9v battery, a diode and a resistor and test ...  negative of led -- [- 9v battery +] - [ + =>| - diode ] --> resistor (100-1000 ohm ) ---> to positive of led