Screen has no Backlight

[YellowMonkeyBoy]

Just had a HP G70 Laptop dropped in to me this afternoon.

After fiddling around with the machine I managed to get the screen to show a picture however it is completely dark.

The client was honest in saying it had recently been dropped (fell off the back of a sofa onto a hard tile floor) and you could see this from the way the screen casing was a little pried apart from the impact.

I have connected it to a second screen via RGB cable and this is fine and I diagnosed it with a broken LCD Inverter. If I pull over the 2 pin plug from the inverter there is no change in the display and doesn't ever act like the Back light is trying to kick in, i.e. getting no power?

Am I right in thinking its the Inverter?

They're a lot cheaper than changing the back-light/screen. Which does display fine just without a back light at all it seems.

Any Help would be appreciated on to whether this diagnosis is correct.

 

[FremontPC]

That drop could have boned the CCFL tube. My guess is you'd have to have a KV meter to check the output of the inverter. I know on CCFL TV's, the inverter will shut down if it sees a bad tube, don't know if laptop inverters do the same.

Gut tells me it's the tube.

 

[frederick]

It varies on laptops. I've seen what you are talking about fremont, where if one bad tube is defective, then the inverter supplies no power, but that is cause they are in series usually (cheaper than in parallel I guess). If they are in parallel, then one tube or half the screen would be backlit and the other not. Id replace the tubes, both, mainly out of integrity reasons. It was dropped. Check the inverter with a multimeter and make sure it didn't fry either. Also check the screen itself for dead pixels or other visible damage that could have been caused by impact

 

[Larry Sabo]

I highly recommend using a scrap screen whose CCFL lamp is known to be good for testing inverters, if you don't have a separate lamp tester. If it lights with the suspect inverter, there's your answer--it's the lamp. If it doesn't, it could be the inverter, the GPU or other components that supply the inverter. I'd try a new inverter first and if it doesn't do the trick, a new motherboard is likely required.

 

[FremontPC]

It drives an external monitor, so unlikely to be GPU.

 

[Larry Sabo]

Depends on the design. There are many that use Northbridge to drive the VGA (external monitor) but the GPU to drive the screen. If the GPU goes duff, you get a perfectly good image on the external monitor but crap on the screen. I have the schematic but need to study it more to verify just how independent they are (VGA, LCD). In any case, I know that in some designs, the VGA is independent of the GPU.

 

[FremontPC]

Good info, Larry! Thanks!