Diving into the world of CCFL backlight troubleshooting

[cvanc]

Hi all-

So one of my monitors lost the backlight and I'd like to fix it.  I've not done any CCFL repairs before.  I have not dug in to it yet and do not know if my failure resides in the lamps or the drive circuit.

In looking around the Internet I see a small variety of "testers" or "troubleshooters" available at reasonable prices and I wondered if any of you could verify these are useful tools?

Most of them look like simple freestanding power supplies - in other words, an external source of a known good drive signal to see if the bulbs are good or bad.  Some come with a variety of adapter cables, presumably to mate easily with the most common connector types.

I also wondered about using a HV probe on a DMM to measure the drive signal directly.  As I understand it the typical drive is in the high hundreds or low thousands of volts and some tens of KHz frequency - am I remotely correct about that?  So a HV probe should keep my DMM safe.

And how about just using a home-made pickup coil near the output transformer to 'sniff' and see if it's running at all?  Just a thought...

Any and all input is welcome, thanks.

[Agent24]

You can test the tubes with a basic CCFL tester, yes - easy to get on eBay and worth it.

The inverter can be tested with old CFL tubes from dead CFL lamps. That is probably the easiest way, no expensive equipment needed.


The hardest bit is actually replacing CCFL tubes. They are also cheap on eBay, but you must take care when replacing them. If you do end up needing to do this, buy a couple more than you need. You will very likely break at least one if you have never done this before.
Be very careful taking the LCD assembly apart. You can easily tear the LCD bond ribbons etc. Be sure you have enough clear desk space before you start.


Hopefully the inverter or its power supply is just faulty. They are much easier to fix. Often just blown transistor or transformer and not too hard.

[Kilo Tango]

One problem you might find is as the tubes get older they need a higher start up voltage and more current to get going. And one of the controller chip families, an example is OZ964, is intelligent in having open lamp and over voltage protection, so it may decide the lamps are faulty and shut down, even when they are still usable. I have a Dell 2410 that does this, the lamps flash once and that's it.

You could replace all the CCFL's in the hope of coaxing it to work again, or try modifying the controller turn off parameters, however another option is to replace the lot with LED strips and an appropriate controller. This has a lot of advantages, cheap, no high voltage, and it wont shut down if a LED goes out. You would need to use a fair number of LED strips to get the same quality of light output, however LED's shine forward so all the light produced is going in the right direction.

I used to use a Tosh backlight inverter as a quick method of driving CCFL's, just wire the shut down control on.

Ken

[Agent24]

LEDs are no better than CCFL for not tripping fault protection circuits, considering most LED backlights run multiple LEDs in series with around 80V DC or so (more efficient) and thus if one LED goes out (short OR open) then the LED driver will typically shut down as well.

IMO CCFL is more reliable as you only have 4 lamps (typical) and thus far less points of failure than a string of 30 LEDs, where it only takes one to fail and bring down the entire string. This is especially true when you consider the cheap LEDs stuffed in hot LCD assemblies. They will not last!

And, if you have CCFLs tripping the inverter protection then the lamps are bad. Just get over it and replace the lamps. Disabling inverter protection is only good for troubleshooting. You are creating a fire hazard otherwise.

[Kilo Tango]

Hmm, I was thinking about using the 12V LED strings you get and building an SMPSU converter to run off the local power supply volts. These LED strings seem to be quite reliable now.

My Dell monitor has 14 CCFL tubes, and replacing them all would be a pain.

Fire hazard .... hmm

Ken