There are many reasons to pull the trem with strings on for doing everything from regular maintenance to major surgery. Anything from chasing tuning instability, basic cleaning of the trem, changing the studs, inspect or file the knife edges, tighten the arm holder, or altogether change the trem. [[To change the trem just unlock all the strings and remove them. Reinsert them in the new trem making sure they're in the right saddles and not crossed! Helps if you eyeball the intonation on the new trem before installing, hopefully the old one is intonated and eyeball the distances right from that one [90% of the used guitars I buy have never had the factory non-intonation changed]]. I also do all my neck work on new guitars by pulling the trem, because I'm not restringing it, I'm just making some adjustments. Adding a neck shim is almost mandatory and there's no better way to get access to the bare fretboard than to just pull all the strings to the side. When I'm installing a neck shim I'll polish the frets and then oil the board while the neck is off the guitar. [If I'm not adding a shim and therefore not removing the neck, I'll mask off the pickups with tape before getting any steel wool even close to them. [To pull the tape stick another piece on top of it or all the wool fibers sitting on it will pull straight to the pickups when you pull it off.]] In essence, a very easy way to work on a double locking guitar in many circumstances and for many reasons.
Everybody is always so paranoid the first time they pull a trem. I'll demonstrate it here on a $5000 DNA just to put you at ease. This is the only sane way to remove a trem, with the strings attached. If you ever tried to install a trem without the strings on it, don't. The guitar should be at full pitch with the nut pads locked down, as it would be to play. **WARNING** Do not attempt to pull a Double Edge piezo bridge!! It is held in by a bundle of 6 fragile wires that are double soldered through the circuit board. You will require some soldering "skills". You can pull a piezo bridge just enough to change studs, or more precariously, file the knives.
First step, remove the trem cover. Really need a pic?!?! Well, OK.
I'll use a fingernail on a corner of the cover to lift it out. You can also just turn the guitar upside down and it'll fall out. "Some" will be in very tight and will require some finagling, beating around the edges of the cover to break it free from the clear if it's stuck, tweaking micro-screwdrivers in the holes to try and lift it, etc. I had one the clearcoat had actually wrapped around the top of the cover and was not coming out without breaking the cover out of it. It was on JP's Swiss Camo Custom Shop 7 string, I wasn't going to press my luck, the trem angle was "close enough" and it was obvious JP or his tech had never tried to remove it either!!
Remove the block lock [that locks the springs to the block]
Now you can pull the springs. I'll use a pair of needle nose pliers [careful not to gouge the inside of the cavity with the nose of the pliers!!] and remove each outer spring.
Springs have a tendency to just *fly* so I'll protect the body [the guitar's and mine:)] by covering the cavity with my free hand. These are springs and fairly stiff ones at that. Don't be a wimp and expect it to yank the pliers as they pop out. Let it, don't try to fight it. [the pliers are laying on the body so I could use the camera, they would be at a steeper angle, safely away from the clearcoat]
When you have just the center spring left you want to support the tail of the trem, where the fine tuners are, from underneath while you pull the last spring. I always work in my lap so I'll support the trem by having it sit on my leg. You can do this with the guitar face down on carpet, a towel, "padded" workbench, etc. All you're doing is supporting the trem so it doesn't fall out.
And here it is with all springs pulled.
Now turn the guitar over. I'll reach under the body and slip my fingers under the tail of the trem, apply a little pressure, and turn it over.
Here's how my hand would look supporting it underneath.
At this point it will just sit there, weighted against the string tension.
Time to remove it. Grasp it at the tail and pull out and away from the posts. The high strings are the only ones that will have any tension left in them so you're really just pulling the treble side of the trem. Pull it back and off the posts, then out of the cavity.
The only thing that makes it slightly tricky is that the trem block will touch the edge of the rout under the trem, on a guitar like this I'm very careful. Putting it back in is where you want to watch this even more.
The empty cavity, you can now do whatever you needed to do ;)
Reinstalling the trem is the exact reverse of the steps you just followed. Slip the trem into the rout watching the trem block and that front edge of the cavity as shown 2 pics ago. Pull the treble side hard enough to slip it over the posts and back into the V's on the posts. If it scrapes against the posts it's just fine, do not worry. Now, make perfectly sure the knife edges are in the V's!! [I have installed trems in a hurry and had one knife out of the V, springs back on, flip it over, and wonder what happened to the action?!?! Start from step one, remove springs and reset the knives in the post V's.] With the knives in the V's place your hand as shown above to support the tail of the trem while you turn the guitar back over, making sure the tail of the trem is supported when you take your hand away. This is where a knife edge will slip out of a V so keeping pressure on the tail will keep it in place.
After you've turned it over it's time to put the springs back on, starting with the *center* spring. Putting springs on is a little tougher than taking them off, they will require *force* to get the spring tang back in the hole. I'll push the pliers from behind with my thumb to add the required force, making sure i'm pushing down on the body hard enough to keep it secure. There is ample force required and you can easily move the guitar while pushing. Make absolutely *sure* the eye of the spring is *firmly* seated around the claw hook before you force the spring open. Work carefully, grasp the spring tightly with the pliers, keep the body secure, and push with even controlled force to open the spring, again, making sure the eye of the spring is firmly seated around the hook on the claw. The center spring is already installed, you can just barely see the spring I'm putting on under my thumb, which I'm using to force the spring open. Again, careful not to gouge the inside of the cavity with the nose of the pliers. I'm actually grasping the spring deeper in the nose than it looks, the tip of the nose is in shadow.
You will notice that as you force the spring open the tang will rotate counterclockwise as the spring opens. The tang will then be at an angle when you get it to the hole in the block. No matter, get the tip of the tang started in the hole, "then" push it perpendicular and wiggle it down into the hole till it's well started. Then with my thumb pushing down on the top of the pliers/tang to help keep it in place, carefully work the nose of the pliers out. Pull the nose out a bit, push tang further in the hole, repeat, till the spring is fully seated. Make sure it is fully seated before you remove your thumb.
If you have a tang that is at a severe angle already [the tang already has a back angle bent into it that makes it tougher to jump out of the block, this is not the angle I mean. I'm talking about if the eyelet is parallel to the ground the tang can be pointing 30 degrees either clockwise or counterclockwise] as many of the new guitars I've been working on, it's going to be much more important that the pliers are at a "cross angle" to the tang so that the tang won't slip through the pliers. Hopefully you'll get 2 tangs that point in one direction and one tang that points in the other to make pliers slip-age a non issue if you "think" about which spot to install each spring, but sometimes this means putting a spring on with the pliers working dangerously close to the body contrary to how I show it below. If the tang slips through the pliers your momentum can carry you halfway across the back of the guitar grinding your paint with the pliers the whole way. Look twice, analyze 3 times, think 4 times, install once!!
I will install the center and far spring as I have shown, but, the near spring I will install with the pliers facing the other way, of course pushing with my thumb, I just can't push and take the shot. [from the previous paragraph and in a perfect world, the center and far springs would have a tang pointing toward me, and the tang on this near spring would be pointing away from me. If you put needle-nose on the tang and think for a second you'll understand this] Someday I do intend to buy one of those "spring tools" just to try it out, but sometimes we get stuck in our ways and resist change. This page is all about how you can work on your guitar with tools found in any tool box and not so much how to stock a full time techs toolbox.
Why? Good reason. You need to keep the pliers at a low angle, remembering the spring tang will twist as you open the spring, you want to keep the pliers as perpendicular to the tang as possible. If the tang ever slips through the pliers your momentum will end up gouging the pliers across the back of the guitar. To keep the pliers at this low angle you get the pliers dangerously close to the edge of the trem cavity, and as you're applying force in an awkward position, might not notice you just cracked the clearcoat until, well, you just cracked the clearcoat!! Trust me, I learned this after I put a dink on the edge of a trem cavity, of a Donnie. Lessons like that you don't forget, you just wish you had learned them on something cheaper!!! See?
That's why you put that near spring on in the seemingly awkward position shown above. You lefty guys will have to figure your own way of doing this safely, I've never tried working lefty so I don't have any advice ;o}
Aside from the springs reacting to being closed and reopened, the guitar should be very very close to in tune just as it was when you pulled the trem. Make sure it's at pitch and double check the trem angle before proceeding. Adjust if necessary, but rarely is it needed if it was correct when you began. Let it sit for 1/2 hour if the springs were out a long time before you start playing with adjusting the trem angle, this will give them time to resettle. If you have replaced the trem with another one expect the saddles/intonation to be set differently on the new trem [good trick is to eyeball the saddles into approximate position to the trem you're removing]. Adjust everything before proceeding.
Make sure to remember to put the block lock back on. You do "not" want this so tight you bend the bar, it will bend very easily. You want the springs seated and locked down, but the lock only has to be "secure". These lock screws will also strip the block holes easily if you cross the threads, make sure the screw is perpendicular and carefully thread it in. Once the trem cover is back on the job is done.