If your NES is showing a corrupted screen, a gray screen, or blinking blue, you may need a new 72 pin connector. The 72 pin connector is what the game cartridge connects to and over time the pins can become dirty, bent out of shape, or depressed too much to be able to read the game properly.
You can purchase a third party one for about $6, a refurbished OEM one for $10, or bend the pins yourself back into place using a safety pin, by using it to tug up each individual pin.
Phillips head screwdriver
Damp paper towel
Ready to replace or repair? Follow these ten steps.
Step 1: Unscrew and Remove Bottom Case
Turn the NES over. Remove the six screws from the case and lift off.
Step 2: Unscrew and Remove RF Shield
Remove the six/seven screws from the RF shield.
Step 3: Unscrew Cartridge Tray
Remove the six screws from the cartridge tray. The two gold screws will be slightly longer and will need to go back in the same location.
Step 4: Unscrew Motherboard Screws
Nearly forgot! There are two more to go. Remove the two screws from the bottom left corner that are holding the motherboard down.
Step 5: Remove Cartridge Tray
Slide the cartridge tray towards the top and lift off.
Step 6: Remove Pin Connector
Pull up gently on the motherboard while tugging off the 72 pin connector.
This is a good point to clean the inside of the NES with compressed air, and a damp paper towel to clean the plastic base. Mine was from a thrift store, so dirt, debris and a number of dead bugs were present. Lovely.
Step 7: Replace Pin Connector
Push the new 72 pin connector on. You may need to hold the opposite end of the motherboard to do this.
Step 8: Replace Cartridge Tray
Slide the cartridge tray back on.
!Important! If you look at the cartridge tray from the side, you will see a lip of plastic on the bottom. This will need to slide beneath the motherboard. If it is not beneath the motherboard and you replace the screws, the cartridge bay will sit higher on the motherboard, the cartridge bay will not depress and click, and the cover will take more effort to replace.
Guides that tell you not to screw the screws for the cartridge tray in all the way have not replaced the cartridge tray properly.
It can be helpful to depress the cartridge tray before putting it back on to ensure this happens.
Step 9: Repeat #4 to #1
Reverse the order the screws were removed in by repeating steps #4 to #1. When replacing the motherboard screws on step #4 it can be useful to have tweezers to drop the screws into place, or use a magnetic head screwdriver.