I was planning on replacing the 72-pin connector in an NES, since my NES has been doing the red square of death tango for a while now, but after busting it open I found that the contacts where in good condition, so I opted to give them a good cleaning.
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Start by unplugging your NES, then remove the top cover.
After you remove the shielding covering the cartridge slot, you will see the 72-pin connector mocking you.
Get a small brush and plenty of alcohol.
Soak the brush bristles, they have to be almost dripping.
Get any disposable game from your collection (In my case, that was “Defender of the Crown” by ULTRA) and clean it’s contacts with the brush, once you are certain they are clean, soak then with the brush, don’t dry them afterwards.
Brush and soak the 72-pin connector, get ready to insert the cartridge from before.
Slide the cartridge in and out then giggle it around a little while pressing down on the connector end (yes, I know how that sounded).
Slide out the cartridge and clean it’s contacts thoroughly, then repeat the last 5 steps util you’re satisfied. Once you’re done, dry the cartridge contacts and leave it standing to make sure any stray alcohol finds it’s way out of it.
After letting the 72-pin connector dry, test out the NES, the usual pressing of the RESET button after powering on may still apply, but you should have a working system. Finally cover it up and get a beer from the fridge.
If you have some thin cleaning wipes, you can stick one between the cartridge and the NES connector, it really helps with stuck on dirt.