LCD Monitor Backlight

[Menticol]

Hello guys!

I know this may be a repeated thread, but actually I'm writing using the damaged screen, no time to search!!!

The backlight turns off ramdomly, and works again if the monitor is shut off and then turned on. Howeverthe time it remains on is also random, varing from minutes to seconds (like now)

What can be wrong in the bakclight PSU? I'll be wating your answers using the sound of "new inbox message"

 

[HiTech]

Often the LCD Backlit Inverter board overheats -- more specifically, the switching transistor used in the inverter section. Look for telltale signs like brown/burnt components or a bulging electrolytic cap. Sounds as if its not too late if you can attach a heatsink and thermal paste to the device(s). Tricky part is finding a small yet effective heatsink. Before doing all that use some Freeze spray on suspected components when it flickers off... if it comes back on it's definitely a thermal sensitive components needing temperature management.

Below is a sample of a typical backlit CCFL inverter board. Note the small surface mount power transistor. They will get very warm to hot, enough to char the circuit board and foil traces near it. I have fixed two such boards by attaching a low-profile but long heatsink and apply good thermal paste. The LCD displays are still working perfectly.
**broken link removed**

 

[Menticol]

First of all, Thank you so much Hitech, I really appreciate the fast answer and your detailed instructions.

Unfortunetly, the transistor temperature is near ambient, and doesn't change before or during the "blackout"

My TFT monitor is an old Compaq 1501, bought in 2003. I prefer not to buy another one, due lack of money, and because it's adapted to a custom case.

I see in your picture the PCB with two transformers. Mine only has one, and in the other side (where the second transformer would be placed) there is a set of SMT IC's.

I have identified two sets of high voltage wires coming out from the PCB, each one connected to its respective lamp. Turning on one lamp and shutting off the other doesn't prevent the blackout to occur on the operating lamp.

Since I readed your message the screen didn't turn off, but it will eventually
Please, Do you have any other suggestion?

 

[Nigel Goodwin]

A common problem is the tuning capacitors (the brown ones in the picture above), or the inverter transformer - these commonly go high resistance, and it can be intermittent.

 

[user_88]

Do you see any large electrolytic capacitors ?.... they are known to age and become out of tolerance .... Looks like that would be the black cylindrical object in the center of the photo above. ..... Should be easy enough to remove and replace with a good part.

 

[picbits]

Another problem seems to be the electrolytic capcitors on the inverter power supply.

I've just got a 19" Proview monitor back up and running with a virtually identical problem to yours by replacing all the electrolytics on the inverter board.

Cost me around £3 in parts in the end. I'm still waiting for the dreaded black screen but it seems to have worked for the past few hours.

 

[Menticol]

Thank you Nigel, User_88 and Picbits by your suggestions!! The only capacitor I could replace seems ok, the others are SMT and unfortunetly, far beyond my equipment / knowedge.

As a funny note, the blackout didn't ocurred again since I wrote the last message. Its like the strange noises on the cars, that dissapear when the mechanic is on board, but return when he leaves.

 

[Boncuk]

I'd also suggest to make a "knock" test, we used to do in times of electronic valves equipped radios and TVs.

Use something (not too heavy) in the range of a sledge hammer and knock the board first, then knock part by part.

Don't do it hasty, since you would likely overlook a detail in circuit behaviour.

The very different time periods of malfunction points towards a badly soldered part.

Boncuk