Monitor dark only when display PC, normal TV

My Samsung is dark only when display PC, watching TV, Video is normal. Color are OK - not lines, no dot no pink or color subdues.

I dissemble the monitor and found 1 CCFL out of 4 is completely dead, there's another 1 (that paired with the dead one) is on and off and that one is on when in TV/Video mode and off in PC mode. So, I went ahead and changed all the CCFL bulbs and they come on nicely - all4 of them in every mode but the monitor is still dark in PC mode not the others.

Not sure now what's the problems

Please advise.
Prasert