CCFL backlight failure modes

[ivan747]

Hi everyone,

I’m renting a place with furniture, including an LCD TV from 2012.

I got quite a good deal with the landlord so I’m trying to keep a good relationship and not to be picky. At the same time, I’m responsible for repairing or replacing anything that breaks during my stay.

On day #3 I turned the TV on for the first time and I found that half the screen was dim, so the first thing I thought was a burned out CCFL tube on the backlight. Was about to tell the landlord the next day. Then 40 minutes later the tube slowly started to brighten up and now it’s at mostly the same brightness as the other tube, also the same tone of color. Found it curious, because it’s now because it turned on so gradually… it didn’t seem like an intermittent connection to me. It’s probable that the TV has sat unused for 6 months. I did move it around a bit with the usual care one has for LCD panels.

Im both curious from the technical side and also for protecting my wallet and landlord relationship. Has anyone seen something like that happen before? Do you think it’s going to fail in the next 6-12 months?

[ivan747]

Note: after 6 hours cooling down I just powered it on and the backlights still work, from a cold start.

[floobydust]

Probably will fail in the next 6-12 months... usually due to bad electrolytic capacitors on the TV power supply board, for the backlight power. You can turn up the backlight intensity and see what happens. A weak CCFL tube will just be dim or flicker, and bad HV wiring to the tube that arcs will make noise/EMI and trip a safety circuit and shutdown the backlight.
Check the TV make/model and see what issues they have.
It's not your fault if the landlord's TV is defective, it's probably his old one that started misbehaving and he put it there and bought himself a new one. They are like that.

[ivan747]

Nice to know! Yeah, I’m sure some of the nicer furniture is the landlord’s old stuff. I don’t hear any buzzing so I’m hoping it’s the caps, that’s always a satisfying fix for me. Anyway, took a picture to cover myself. Thanks for the explanation!

[benj38]

This does not sound like a CCFL tube problem. I have never encountered, nor heard of, any such failure mode of fluorescent tubes, and I have had my fair share.

As @floobydust commented, this looks much more like a power supply problem with main culprits being either an electrolytic capacitor (perhaps partially reformed after few hours of power following its long un-powered period) or an HV transformer with leaky impregnation that absorbed humidity which got baked off after a few hours of  being powered on.

[simo.fora]

Dear Gents

I have an instrument LDC backlight with a bit of strange behaviour.

Once the driver is powered the light is on for ~2s and then the driver shuts off(the driver T5001 stops IC-ing). This behaviour is repeatable.
The driver behaves the same if there is no output connected.

I suspected the driver of being the culprit, but if I connect a 100kΩ resistor to the output, the driver operates and does not shut down. You can even smell the resistor doing its part.

Did you encounter a failure mode of the CCFL that would lead the driver to shut down like that (possibly due to low power consumption )?

Thanks for any input

[floobydust]

Some CCFL backlight power supply ICs do a self-test on power up. If after a few seconds tube current is too low or too high, maybe an arc is happening on the wiring etc., shut off.
Check your electrolytic capacitors on that incoming rail, or maybe your tubes are old and not drawing enough current and considered an open circuit.