Backlight turns off after 2 seconds

I have an HP 2509b monitor that is experiencing backlight failure. After turning the monitor on, the backlight turns on and then off shortly thereafter. I have replaced the CFLs (thinking they may be bad) and checked a power supply mosfet and diode one YouTuber recommended replacing. I have also visually checked the capacitors; they all look fine. Yet still, the problem remains. What could be causing this issue, and how may I go about fixing it from here?

Thank you!

Edit:

Added photos of the power supply. The red wire was a fix of mine when a PCB trace came off with the MOSFET. (I removed it to test the MOSFET earlier in the repair).

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Hi,

When the backlight goes out how do you get it to come on again?

Is the monitor connected to a PC?

If so have you checked the power management features settings in the PC?

Does the green power light on the monitor also go out?

If not does it change to amber?

If you press the menu button does the menu stay visible on the screen?

When the backlight turns off and if the monitor is still powered on with a green light and you had a picture before it turned off can you still see the picture on the screen by shining a torch at an angle close to the screen to see if the picture is still detectable?

Sorry about all the questions just trying to narrow it down a bit.

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@jayeff,

Thank you for getting back with me. The backlight resets after power off. It is not currently connected to a PC, although I did try connecting it to my computer (with the same result: so signal for two seconds and then no backlight). I can't seem to change the input source without the backlight as it is too dark to see the input settings even with a flashlight. The power light is blue but occasionally turns amber. Also, I am unable to access the menu (at least the No Signal remains when I push the menu button).

Thank you again for your assistance!

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Can you post a picture of the capacitors on the power supply board? Your symptoms are very indicative of capacitor failure.

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Hi,

things to try.

Hold the menu button on the monitor for 10 seconds to see if you get a monitor 'locked' or unlocked' message appearing. if it says unlocked try changing your input source to a different input using the OSD if it stays on now.

The input source detection should be auto but if the above works try to select a different input source type and then if you have got a choice of video outputs types available, i.e HDMI, DVI vga etc.try connecting each one in turn to your monitor to see if it is a faulty input type and whether it is also that the auto detect isn't working.

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@jayeff Thank you! I'll try this as soon as I receive a few replacement parts... I messed up one of the SMD MOSFETs when I was trying to desolder it for testing (I don't have a rework station yet).

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I just replaced the MOSFETs and a diode in the power supply with no change. However, I am able to connect to a computer and display the image without the backlight. Holding the menu button down for 10 seconds results in an OSD lockout message.

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Hi, So the OSD lockout message appears "with"the backlight on then? If so then press the Menu button again for 10 secs to see if you get a unlock message.

Since you get the backlight coming on on power up and also with the locked message to me this proves the backlight circuit, but not the backlight "control"circuit with a signal provided i.e. when to switch it on or off.

I'm thinking that you have either a signal detect problem of some kind which even though you can "see" the video doesn't say to the processor to turn on the backlight or a firmware problem.

You say it comes on at power on. Since the signal is there as you know, it is not being detected and the backlight is switched off which sounds correct as this is what usually happens when signal is lost .although there is a signal missing message you did say the lamp changes which also indicates no signal. (again even though you know that it is there as you can see it albeit faintly because of the no backlight assistance)

Hope this makes some kind of sense.

It still could be a power problem though . Check all your different supply rail voltages if you can to see if they are there and that they are the correct value.

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The caps in the picture look good. Start testing the diodes and transistors (FET & BJT) and look for cold solder joints.

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Jay Developer will be eternally grateful.
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