You'll need three basic components:
1. A philips screwdriver
2. An X-acto knife or similar sharp, pointed blade.
3. A replacement NES 72-pin cartridge connector, available from eBay.
Make sure you get a new 72-pin connector, and not a refurb. They're available for about 7 to 10 bucks from a number of dealers.
First, a little background info. To prevent unlicensed games from being played, all front-loading NES consoles have a security lockout chip installed called the 10NES chip. It's designed to read a companion chip in every licensed NES game to verify its authenticity. The Blinking Red Light Of Death, as it's called is a condition whereby the 10NES chip can't authenticate the cartridge at power-on, so it resets the console and tries again every second until it can authenticate. This can happen even if the game is otherwise read properly, causing the title screen to show up briefly between resets. The most common reason why the 10NES chip doesn't receive the authentication signal is that the cartridge connecter inside the console has worn down to the point where it no longer makes a solid connection with the game. When that happens, the connector must be replaced.
Disclaimer time: Proceed at your own risk. There are no dangerous voltages inside the NES when it's unplugged, but the components inside can be damaged by static electricity once they're exposed. Be sure to ground yourself before opening the case, and don't go running around a carpeted floor holding the bare motherboard. It's also a good idea to make note of which screw came from where, when it comes time to reassemble your NES.
OK, let's begin:
Step 1: Flip the NES over and remove the six screws along the gray edge of the case. Don't worry about the two screws in the black part underneath the controller jacks.
Step 2: Flip the NES upright again and lift the top of the case off. Remove the five screws that hold the steel RF shield in place and lift it off. You can now see the black, spring-loaded cartridge holder, the cartridge connector and the NES motherboard.
|NES with top cover and RF shield removed|
Step 3: Remove the screws around the cartridge holder and around the RF/AV jacks near the upper-right corner of the NES. Unplug the larger blue connector and the two smaller green connectors on the right side of the motherboard by pulling them straight out. Don't wiggle them or pull on their wires.
Step 4: Lift the motherboard out of the case. Gently slide the cartridge holder away from the cartridge connector and set it aside. Now you're left with the NES motherboard and the cartridge connector.
|NES main board and cartridge connector|
Step 5: Pull the cartridge connector off the motherboard. If you need to, work it loose by rocking it back & forth as you pull.
Step 6: Slide the replacement cartridge slot onto the motherboard, facing the same direction as the old one.
At this point, I'd say 90% of your problems will be solved, and you can reassemble your NES. However, if you want to make damn sure that light never blinks at you again, read on.
Step 7: Flip the NES motherboard over so that the chips are facing you.
Step 8: Hold the motherboard with the cartridge connector and the RF/AV plugs facing down and locate the 10NES chip. It's a small 16-pin chip on the lower-right corner of the motherboard, immediately up & to the left of the metal RF box. The name Nintendo and the number 3193A should be printed on it.
|The 10NES chip above & to the left of the RF box. Cut the circled pin to disable.|
Step 9: On the bottom row of pins--the row of pins closest to the cartridge connector--count four pins in from the left or five pins in from the right. Sever that pin and only that pin with your knife to permanently disable the 10NES chip.
Step 10: Reassemble your NES and enjoy a blissful blinking light-less lifestyle. That pesky lockout chip will never bother you again, and your NES with its replacement cartridge connector, should play like it's brand new.