My screen goes black every time I turn it on

Question

I have a mac that the screen keeps going black. I’ve
searched the Internet and tried all the common solutions.  Details of what I’ve tried are below. It is a
MacBook, purchased probably around 2007. Sorry I can’t tell you more without turning
it on.
What I’m wondering is, first, if anyone has any suggestions
I haven’t tried. And second, do you think it would be worth it to take it in to
fix it or is it just an old computer and I’ll pay more to try to fix it then to
just buy a new computer. I do rather like my computer…
I turned it on, it would light up for a few seconds, and
then the screen would go black. Also, the little light at the bottom right
would also go black. Sometimes it seemed that the more I turned it on, the
longer the screen would stay lit. Sometimes it would just flash on, sometimes
it would make the start up sound and you’d see the apple before it went black.
It did appear that it was on, since the fan seemed to be on and it was running.
It just had no picture. Also, when I first noticed this problem, a few times it
did come on all the way. Later it didn’t come on all the way unless I did something
special (see below).
I went online and looked what other people were doing. I
tried:
Turn off the computer, disconnect the power cord. Let sit
for 5 minutes. Restarted the computer while holding shift key. This made no
difference.
I reset the NVRAM (or PRAM), as instructed here:  http://support.apple.com/en-us/HT204063.
I turned off the computer. Turned it on, held down Command, Option, P & R as
soon as I heard the startup sounds. Then lifted up when the startup sound
sounded again. This actually worked and it came on as normal. However, when I
turned it on again it did the same thing. Doing this again did no good.
Although I did hear the startup sound a second time. It will also flash on
again at the second start up sound.
Then I reset the SMC by unplugging it, removing the battery
and holding the power button for 10 seconds. This worked and I used my computer
for 30 minutes or so… before the screen went black. Doing this again did no
good.
I tried the Keypress Sequence detailed here: http://osxdaily.com/2014/11/22/fix-macbook-pro-booting-black-screen/
With the computer on, press the Power/OFF button once. Press “S.” Hold the Power
button until a hard shut down is forced. Wait 15 seconds and Power on. By the
sounds, it did appear it went to sleep and then had a hard shut down. This did
no good.
Next I tried, with the computer off and the cord plugged in,
I turned it on holding down Shift + Control + Option + Power for a few seconds.
Not sure what that did. The little light flashed, then there was a buzzing
sound, the screen flashed and went blank. And apparently went back to its old
standby.
I adjusted the brightness settings. Nope.

Answer 1

I believe that system has a hall sensor on it to detect when the lid is closed so it can put the system to sleep and/or disable the monitor so it can be used with an external monitor only. If the sensor is marginal or has a bad cable, it may intermittently cut the monitor on and off, and may likewise put the system to sleep. One way to check would be to plug in an external monitor and if the laptop's screen cuts off or doesn't show up, start using the trackpad and press on a non-typing key like the shift key. If the external display suddenly pops and becomes active while the laptop display remains black, it's probably a problem with the sensor or the cable/connection. You might be able to fool the sensor into kicking on and off by moving a magnet over it. It's located on the left had side of the base with the unit opened up, and right over the audio output plug.
I know of some people that put a small magnet right over the hall sensor's location so the laptop display turns off and they use the system with an external monitor, keyboard, and mouse. If fooling with magnets, make sure they're small and don't get them close to the hard drive because a strong magnet can wipe them out.
I don't know if this will help you much, but it might.