I suggest you get some expert help to fix the board before you destroy it beyond repair - if it isn't already too late.

Keep in mind that many repairers will NOT even accept your board for repair if they know you have been "tinkering" with it.

Why not stick to mechanical repairs and adjustments for now until you have more knowledge of pinball systems.

You will only learn how NOT to do it by trashing boards. I suggest starting with simple things first not diving in to complex electronic repairs straight up when you barely know how to hold a soldering iron.

Just a caution on contacts. They are gold or silver flashed in solid state machines and should NEVER be sanded.

Use a non abrasive cloth to wipe the contacts.

You are likely to have further problems with the drop target contacts so I would look to get some replacements.