Hello, A couple years ago, the father of my gf gave me this Cassette player, seems to work ok, the vumeters got light, and...

I have no previous experience in Macs, but all points the battery. Seems the battery it's somewhere bad with the cells. Inside the battery are some smaller batteries, and when one of them is wrong, it discharges very fast, and it's not providing the electricity the computer needs. The sensor detects that the bad cell is charged, and says its at 31%, but it will drain quick and it will be dead fast. You can search some software about the health of the battery (maybe can check each cell and tell which is bad)

Check if the power supply is dead. You can always pick another travel adaptor. As far as I know, laptops as the voltage is given by the power supply, don't use any fuse. It's quite difficult to diagnose smd problems specially on laptops, because the components are pretty close one to the other, but you can check if any part is burnt by having a look to it.

Check if the lens is clean or scratched. If is still blurry, should be a matter of the camera itself, or the brightness settings on the video maybe...

PAINT: sand it down from 400 to 600 to 800(wet) grit sandpaper, prime it with quality spray primer, sand it again with 1200(wet) grit, paint it with the desired spray color, and apply a clear coat. Tips and tricks: While painting keep always like a span away from the surface. Apply light fast-moving coats, to prevent dripping, as it happens when the paint stays flowing in the same point for too long. Before painting the car, try before in a cardboard to "feel" the strokes and not dripping the paint. To avoid any damage on the plastic, use water based spray paint if you can. Usually all brands are high pressure paint, for a begginer, you can try the Montana brand if you can, is very easy to use, specially the '94' kind, the matt colours, and they also have water-based too... I don't know if there is water based spray clear coat, if not, I always reccomend using acrylic, because the solvents of the "normal" clear coat sprys sometimes react with the paint and mess the whole work, specially when working with...

The black bar appears when you turn it on with no cartridge inside. Try it with a game, you can tell any of your friends, make sure anybody will have a game lying around their bedroom inside a drawer. The battery light, don't worry, is to tell you it's on. You can have a blank screen because a game glitch as if it were turn off and you run out of batteries in no time. When the batteries are dead, the light is off, just doesn't turn on (a little trick is, if starts to flick the light, it's time to buy another pack of AA batts. The speaker drains the battery like !&&* too.)

I never dissassembled one, but, you can try to rotate the hinges (if you can) 180 degrees, maybe is an issue of just tightening the screws a little more. Also could be the new shell, specially if its a cheapo one, are poor quality build, and most of the time will wobble and cause the problem, or if it's used, could be broken as well...

Youtube has plenty of nerf mods to shoot darts with more power. Usually work with springs, try to make the spring carry more force by stretching it a little bit. Don't overdo it, as it won't keep the original shape. Maybe try with other springs...

Try with Recuva free software, it worked to me a couple times when i had to find an erased file, are the same guys who use CCleaner to fix registry issues on windows, also cleans innecessary files, freeing lots of space. https://www.piriform.com/recuva