first step on all LT is to remove the main battery. off bottom with latches.
then the LT runs on AC power , endlessly and no dim screens.
any LT thinks its running on battery only, it goes to dim mode.
NO OS stated, by you, XP./vista,w7/8/10?
lines, are not a good sign, is this a 2008 laptop with the infamous
solder-balls from H3LL, Nvidia failures. here lets look that up.
seems yes, year 2007 stamped clear as day in the service manual (Free) at Hp.com. (aka, lead-free *****,etc)
so do this, run on AC only.
then, twist the bottom case then the screen case.
UN-nuturally, if lines change (add or subtract) it's a bad GPU or screen.
gently, do that, takes only gentle forces and is common failure.
test 3 is push the J key harder, like 5 lb ham on back of hand hard. screen changes? this is to get stress on the GPU, my be a waste of time, no 2 lapstops the same on lay out. even different by sub models, same name and number.
since i cant see your lines, and make great advice, above is wild guess on what you saw not me.
so,, if lines means, its only DIM ? then that is another answer
lets do DIM only.......
battery out main, ok? leave it out, so AC is the only powser
so the I Have full power logic runs it at full brightness.
and the brightness button actually works.
ever heard of CCFL. Back lamps.
the all die, everyone, for sure this old. 7 years old.
if used allot for sure. it's not LED black lamps here, understand that, the back lamps that last for ever are not HERE.
they (CCFL) go dim, go yellow(all do) and go intermittent or dead.
some even strobe,(flicker just before the END)
all do. every one. after about 20k hours used(adds up easy)
10 hr/day x 5.5 year is that, yours is 7 .
99% tubes bad. stats.
1% inverter bad, or both.
the inverter takes, low voltage and makes about 500vdc
for the tubes to run.
if you had photos of BAD, answers get x10 more easy.