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What are you waiting for?
So basically, the key to fixing this is a new 72-pin connector. let's take a look in the Nintendo and I'll explain a bit further what's going on.
So this is the inside of the cartridge slot. You can see the pins down there. When Nintendo made the NES in the mid 1980's, the system was built cheap to make sure it could be affordable. Unfortunately, this means that the pins are little more than bent metal that contacts the cartridge by friction. After years of jamming cartridges into the slot like an addict needing a fresh fix, the pins have bent. Dust has gathered on them and gummed them up, making them an ineffective connector. By blowing on the cartridge, you could sometimes get them to work by making the cart a bit cleaner. By licking the connectors (ew), you could get it to work by making a somewhat better connection. Over time these solutions don't work though because the slot either gets too dirty or the pins rust from having a barrage of moisture thrown at them. Your only solution? A 72-pin connector, fresh off the nearest internets.
You can find these in a lot of different places. I've seen lots of them on eBay go for as little as 5 dollars (click here), eStarland has some for sale for 9 dollars http://www.estarland.com/index.asp?page=NintendoNES&cat=&product=16956&q=...I got mine from The Exchange in Squirrel Hill, Pittsburgh. The price was $15. The price is definitely right.
The first step to replacing the pin connector is turning the NES over and removing the six screws holding the base together. Don't worry about the silver screws near the controller port. Once you get the six of them out, pull the bottom of the case. The case will come apart pretty easily. Set the six screws aside and don't lose them.
The second step is removing the silver shield that covers the cartridge slot and logic board. If your NES is facing forward like the one below, there are two screws on the left, two in the rear, and three on the right. Once you get these out, the metal will lift off, leaving the cartridge slot exposed.
After you get to the cart port, you'll need to remove the screws that hold it in place. Once again, if the slot is forward, there is one on the bottom left, one on the bottom right, and two on either side of the back. Be careful...there are two longer screws that go into the back of the port. Don't mix these up with the others!
After you take all of these out, the cartridge port that holds the cartridge will be loose. To get it off of the board will require you to push the spring down and lift out/up at a 15 angle. The port will come off if you are persistent. Just jiggle it a bit and it will come free. You cn see how I did this below.
The next step is the one you've waited for -- remove the old cartridge slot and replace it! To get the old one off, you'll have to lift the logic board up a bit in the back. If you are nervous about flexing the board a little to get this off, you can remove a few more screws that are over near the power and RF adapter, but you shouldn't have to do so. Just lift lightly with your right hand and pull with your left as I am in the picture below. The connector is being held on by friction so it might be a bit stiff.
Once the connector is off, you'll see something that looks eerily similar to an ISA connector with ten extra pins and have horrible flashbacks to 1993. I know I did.
And this is where I bust out my beautiful friend, Mr. 200-proof. Swap the logic board's connector with a bit of alcohol, cleaning it thoroughly. Make sure to get the pins on the bottom too!
After you clean the logic board's pins, the new connector needs to be put on. Take it out of the plastic bag and look at the pins. They should all be even and there should not be any crud in them. To replace the connector, do the same procedure as before when lifting the board. Pull the corner up with one hand and slide the connector on with the other hand. In the pictures below, you can see me lifting lightly on the corner of the board to get the connector on. Also, make sure the connector is on right. The screw holes should line up with the ones on the board.
After putting the new connector on, slide the cartridge port back over the connector.
When you go to screw the port down, make sure you don't over tighten the screws. If you do, there will be a noticeable "bow" to the front of the slot and when you try to push a cartridge down in the slot it wont stay down. Now would be a good time to try a cartridge in the slot to see if it will line up with the pins correctly and stay in the slot when you push it down. In the picture below, you can see what happens when you overtighten the screws too much. See how there's a big bend in the front of the cartridge slot? That's not good.
Once your cartridge port is back in place, put the screws back which hold the metallic shield in place.
Finally, put the lid back on the NES, turn it over, and screw the six case screws back into the deck.
And now your Nintendo should work like new!
Before you shove any new games into the cart slot, clean them thoroughly. I know the back of the cartridge says not to clean the connectors with alcohol, but I've never had any problems with doing that. I just use some high-proof alcohol to ease in evaporation, some q-tips and some elbow grease. If you don't clean your cartridges after you do this though, you probably wont see much of a difference!
I really hope this helps you out. I love my oldschool Nintendo and still play it when I have free time. Ever since Nintendo's patents expired, there's been a flood of imitation Nintendo's hitting the market, but to me there's nothing better than playing on the real deal. Enjoy!
DarkNymph