The monitor is bright and has horizontal sweep lines?

@msapik your Macintosh is getting up there in age. I would suggest that you take a look at the capacitors on your video board as well as the power board. Your electrolytic capacitors should all be EOL so a recap will help. Check the boards for corrosion. There may be lifted/separated traces etc.

This computer uses a CRT and there are some potentially lethal voltages present. For now you really only want to remove the back housing and take a look at things. Take some good pictures and post those on here. That way we can try and help you identify the issues and to guide you. Always keep it unplugged and do not touch anything directly with your fingers. No, it is not like disarming explosives or anything, just use common sense. You are working around high voltage. Nothing is going to happen if you follow the basic rules :-)

Remove the cover and take some good clear focused and large enough images. Post those with your Question. That way we can see what you see and hopefully help you further.

Adding images to an existing question