LCD Backlight repair on a HP Pavillion N5350 laptop

[[email protected]]

My backlight just went out one day on my laptop. After browsing
around, I found that it was most likely the inverter chip. I found a
replacement one and put it in. The backlight worked great for about 1
day. During that time, I had taken the opportunity to fix a noisy CPU
fan and clean out the dust from the heatsink. By the time I had
everything back together, the backlight wouldn't come on again. I
assumed that the inverter had failed, and tried another one. Still no
backlight. Either I messed something up when I had it open, or I have
developed another problem.

I tried checking the voltage coming out of the High Voltage side of the
inverter, and it stays at just around 5VAC on my meter. I thought that
this meant that the inverter wasn't working properly, but I have since
read that the inverter will probably switch off if the light isn't
attached.

The input to the inverter is a 6 pin connector. I checked the
continuity of each line on the cable, and even took off the fiber
wrapping to verify the wires were not shorted, and everything looked
fine. It appears that two lines are +5V, two are ground, and there are
two other independant lines. One is at +2V and the other is at 0V. I
know that there is probably a line to tell the inverter to turn on, and
one to determine how bright to make the backlight. I am guessing that
the 0V is the actuation line, but I am hesitant to pull it up to +5
without being sure. I traced that line to a riser board and down to
the video card, but will have to dismantle most of the laptop to
extract the video card and trace it further. I can do this if needed.

I should point out that the LCD appears to function correctly, at
certain angles I can clearly make out the data on the screen, and it
looks fine.

I am a novice at electronics repair. I am a software engineer but am
making my way into electrical engineering. This will be my first
repair of this sort.

My tools are somewhat limited, I have a nice multimeter, and an old
scope.

In you guys' expert opinions, where should I go from here? I have a
hard time believing that I was shipped two defective inverters. Could
the first have blown the CCFL? How can I be certain without
dismantling the LCD unit? How likely is it that something on the video
card is going haywire when the LCD image is fine? I didn't disturb the
video card when messing with the cpu fan.


I will be glad to give more details, pictures, run tests etc. Just let
me know what you need.

Thanks,

Andrew

 

[Dave D]

My backlight just went out one day on my laptop. After browsing
around, I found that it was most likely the inverter chip. I found a
replacement one and put it in. The backlight worked great for about 1
day. During that time, I had taken the opportunity to fix a noisy CPU
fan and clean out the dust from the heatsink. By the time I had
everything back together, the backlight wouldn't come on again. I
assumed that the inverter had failed, and tried another one. Still no
backlight. Either I messed something up when I had it open, or I have
developed another problem.

Probably the lamp is on its last legs and the inverter is detecting this.

BTW- never measure the output of an unloaded CCFL inverter with an
electronic meter, you can easily fry it unless it is rated for the very high
offload voltages. An analogue meter should be safe though.

Dave

 

[[email protected]]

I never saw any indication of a failing CCFL. The light had been nice
and bright, no dimming, discoloration, or flickering. It has always
either been on or off. How can I make sure that it is the CCFL?

Thanks for the advice on the metering. My meter is rated at 1000VAC,
but I guess I need to have the lamp connected when I check it?

 

[Dave D]

I never saw any indication of a failing CCFL. The light had been nice
and bright, no dimming, discoloration, or flickering. It has always
either been on or off. How can I make sure that it is the CCFL?

By replacing it unfortunately!

Thanks for the advice on the metering. My meter is rated at 1000VAC,
but I guess I need to have the lamp connected when I check it?

Yes, but it won't help if it's shutting down straight away.

Dave

 

[Dave D]

Thanks for the advice on the metering. My meter is rated at 1000VAC,

Some CCFL inverters can generate several KV p-p offload.

Dave

 

[[email protected]]

Is there a standard pinout for the 6 pin connector for these boards?
If I was sure that the inverter was getting the correct input, I would
be more comfortable blaming the CCFL. I will start looking for a
replacement though.

Is it common for a ccfl to blow after replacing the inverter?

Thanks for your help.

 

[Dave D]

Is there a standard pinout for the 6 pin connector for these boards?
If I was sure that the inverter was getting the correct input, I would
be more comfortable blaming the CCFL. I will start looking for a
replacement though.

Is it common for a ccfl to blow after replacing the inverter?

Thanks for your help.

More to the point, it's likely a weak CCFL was the issue from the start, and
the inverter was shutting down- they do that by design. It's possible the
new inverter's shutdown detection threshhold was slightly different, giving
the illusion that all was well for a while. You may find the old inverter
works fine with a new CCFL, it's impossible to say for sure.

I can fix these things quite easily when they're in front of me, but it's
very hard to make accurate internet guesses.

Dave

 

[Michael Kennedy]

Some inverters burn up if they are powered without a load as well

- Mike

 

[[email protected]]

Ok. I took the CCFL out this weekend. What a pain. It does look like
it may have burned out. It looks like the connection on one side may
be burnt. I need to get some calipers and measure it and I will order
a replacement.

Thanks,

Andrew

 

[Dave D]

Ok. I took the CCFL out this weekend. What a pain.

Yes, often they can be a real swine to get out. Good luck with the repair.

Dave