iPhone 3gs backlight only works when brightness turned right down.

Hi,
I have an iphone 3gs with a backlight issue. The backlight only works with the brightness turned right down. As soon as i turn up the brightness.
The backlight goes out. I have tried the logic board in another phone. Also tried a new LCD. And i have also changed the 6R8 backlight coil.
I have also tried a hard reset, As well as a full restore, So i do not think that this is a software problem.
There are no missing parts or capacitors on the logic board. I have not done any further testing.
So if anyone has had the same problem and solved this issue please advise.

the same here

It seems like to tried everything

Some people say this problem is related to humidity or water damage.

6) Use Vacuum Cleaner

You can also use vacuum cleaner to suck extra water out of iPhone. If you have Air blower that can also be used at low temperature setting to dry it. Any kind of Air blower or dryer can be used, basically water will vaporize when air blows through the iPhone, remember Air and water can reach in tiny holes which we can’t even see.

7) Freeze Your iPhone

Put the iPhone in the fridge. It sounds bizarre, but the cool air will draw out any moisture safely. Make sure to place it on a towel first and do not put it in the freezer or the LCD screen will be damaged. Take it out every ten minutes to check on it until it is dry.

8 ) Pull Your iPhone Apart

If you’re technologically savvy, as a last resort you can pull apart your iPhone and dry and clean each piece individually and put it back together again. You have nothing to lose. The warranty is probably already null from the water damage and some iPhone users have done this before and had their iPhone working again. You may find that you have to replace a few pieces, but this is still cheaper than a complete replacement or having the Apple company repair the Iphone for you.

9) Put Rice With The iPhone

Towel dry it and put your water damaged iPhone into a dry zip lock bag filled with white rice and leave it somewhere warm and dry for twenty four hours. The rice will draw out the moisture without damaging your phone.

Although your first instinct with a water damaged iPhone is to scream and then buy a replacement one, you might not have to. Although there is no guaranteed way to repair a water damaged iPhone, the above tips may work for you and save you a lot of time and heartache.