Things you'll need include Q-Tips, isopropyl alcohol (I'm using 91%), a phillips-head screwdriver and a toothbrush. Depending on what's going on inside the Advantage, you may also need some vinegar. I used some apple cider vinegar.
Oh, and you may want a Hazmat suit and check that you're up to date on all your shots. You never can be too careful when cleaning game controllers.
Step 1: Open It Up
You can see in the picture that the B button knob is very rusty. Not a big surprise.
I used needle-nose pliers to unscrew the nuts. There are washers as well. Set them aside. If they're rusty like mine, we'll come back to them soon.
Unscrew the ball knob on the joystick.
Next, flip the Advantage over. You'll see two screws — one centered at the top, the other at the bottom. There are actually 6 screws total. The other 4 are underneath the rubber pads. I just used a pocket knife to peel them off. They were disgusting as well. Anyway, unscrew them all and set the screws aside.
Now hold the Advantage shut, flip it over and set it down. You should be able to pull the top off.
The inside of mine looks fairly bad. There's a large rust spot on the base and there's grime or something all over the inside. You can see grime and rust on the potentiometers.
Someone obviously spilled something on this. Let's clean it up.
Step 2: Remove Boards
There's also a funky little bracket attached to the main board. Remove that and keep it separate as well.
You should be able to pull both boards off now. Get a look at them. Mine were gross. Not that that's remotely surprising at this point.
Step 3: Cleaning
Depending on how bad they are, you may want to let them soak for a long time.
If you've got any rusty non-electronic parts, break out that vinegar. I soaked the nuts and washers overnight.
Go to town with the alcohol and q-tips. A surface layer of the PCB actually came off, unsurprisingly around the A & B buttons. That shouldn't harm anything — the damage was already done by the Coke or whatever was spilled on it.
This all took me quite a while. Everything was disgusting. I went over some key points with a multimeter to make sure that no large connections were completely destroyed. I wasn't very methodical; I just wanted to see if things seemed ok, and they did.
I had to pay particular attention to the turbo button potentiometers. I may need to replace them; I haven't decided yet. I actually needed to spray some WD-40 on the pot for the B button to loosen it up.
When you've decided that the soaking parts have had enough, rinse them off and go over them with a toothbrush.
Step 4: Drying and Further Cleaning
I also sanded as much rust off the base as I could manage.
Now let everything dry. If you're soaking parts in vinegar, just let everything sit overnight.
Step 5: Reassemble
Now just reassemble everything. The only potentially tricky part is the buttons. Just place them in their holes in the controller top. Put any rubber contact pads in there too. Check that the Player 1/2 switch is placed appropriately and just put the board on top and screw it in. It should be fine.
Get all the internals in order, close and screw it shut and put the joystick knob back on. Then put the pot nuts and washers back on. The knobs should still be somewhat tight.
Now test it out. Mine worked a lot better, though there is still some weirdness when using turbo with the A button. I may eventually replace the pots.
Hopefully you started with one that was in better shape than mine and you now have a like-new NES Advantage. Now go dominate your enemies.