I know this question has puzzled many people over the decades, and although I figured this out a while ago (like the monkey with the stick and anthill), but since I had to do it again today, I figured it might be a good time to document the process. First of all you will need some tools:
1. A small punch
2. A hammer
3. Locking pliers
4. A very long screwdriver
And lastly you will need a locked
Zero-Halliburton case like this one I bought at the swap meet today.
If your case has a combination lock, you may attempt a
brute force attack first, but this is a sure-fire way. Step 1: Take the punch and hammer, and carefully tap the long pin from the center of the piano hinge.
Step 2: Once you have 2" of pin protruding from the other side, get the pliers and grab the pin. Pull the entire pin out.
Step 3: Now that the case is open from the back, pry the two halves apart slightly and using the very long screwdriver, remove the screws holding the lock on. Please notice the screwdriver in the photo, it is touching the WRONG screw, if you unscrew this, the handle will come off, you need the undo the screws on the lock itself.
Step 4: At this point, the lock should pop off and the case will come apart, some jiggling, cursing or cajoling may be required. With the lock removed reassemble the case by putting the two sides back together and replacing the long pin back into the piano hinge.
Step 5: This may be a bit tricky, but now you need to reset the combination. Peek inside the lock at the tumblers, as you turn the tumblers, the little wheel next to the number wheel will have a flat spot. You need to turn each dial until the flat spot faces the back of the case. Once you figure that out, flip the little lever on the back of the lock and reset the combination to whatever you desire (optimally 000 so you don't forget).
Step 6: Screw the lock back on. Now you have fully functional case.
Caveat Emptor: Do not attempt to do this to anyone else's case but your own. You may damage the case if you are not careful. You may need other tools not listed above, perhaps a bigger hammer or a different screwdriver. Some swearing may be required depending on the size of the hammer and the velocity with which you hit your thumb. This procedure works best on a empty case, if you have equipment inside the case, you may have trouble getting at the back side of the lock. Good luck.