DISCLAIMER: I'm not liable for any damage whatsoever when you follow this article. Do so at your own risk. It's a bit tricky, and it takes time to do it properly.
However, you probably should read this article and the linked articles before opening up your own keyboard!
So you got a model F but the spacebar is killing you. First of all a word of warning: you will need to completely disassemble your precious model F until you can access the "hammers" and springs. If it is your first time taking the keyboard assembly apart, read some tutorials and ask around when you are in doubt - these keyboards are sturdy, but you can have a hard time reassembling them if you hastily popped it open.
If you want pictures of the insides, check the threads made by Chloe and ch_123: Geekhack: IBM Model F PC/XT disassembly and Geekhack: Model F porn. (for the AT).
Flip the keyboard over and loosen up the screws. If you got a PC/XT keyboard, it's just two of them and the back plate should come clean off.
If you got a PC/AT one, the case is all made of plastic and will somewhat resist opening. Carefully pull the back shell upwards and push the black cable retainer down - it will come loose. Once the feet are pulled through, the back shell will still resist - slowly pull it upwards, it will creak and snap loose at some point.
Disconnect the cables to the main assembly. If you got the XT, simply open the connector legs and pull the blue connector out. If you got the AT, maybe take a picture of how the cables are wired up, then unscrew the ground connector and disconnect the two cables going into the main assembly. Remove the two screws at both sides of the keyboard and you should be able to take out the inner assembly.
Now we first need to find some props to let the assembly rest while we work on it. I used a stack of CDs, though four unused floppy drives or VHS tapes might work as well. Put the assembly keys-down and see if it can rest between the two stack with enough clearance - the keyboard should be suspended a few cms above your desk.
Then it's time for the real disassembly work: remove all the keys except the spacebar! If you have an XT, also remove the screw in the middle of the controller. It might be hidden under some foam, which you can either gently pull aside or remove completely if it started disintegrating due to age.
Take a good look at the long sides of the assembly - the front side has hooks whereas the back side has tabs latching into the fore-mentioned hooks. There is also one big tab with no hook - this one needs to be bent away from the back plate using pliers. Grab four to six clamps and place them near the hooks to compress the plates there and to keep the plates together once unhooked.
Check which way to slide the back plate. The AT keyboard should slide open without too much trouble when suspended on our props. The XT on the other hand needs some more convincing - I took a small piece of wood and let the front plate rest on it. A few taps on the side (now top) of the back plate with a hammer would then make it slide.
Once the hooks are unlatched, put the keyboard on your props with the springs toward the bottom. Take off the clamps and gently lift off the back plate - the controller should be attached to it. Be wary, the spacebar spring and hammer are still under tension, so they might come jumping out. Put both away, as we can finally take a look at the springs and hammers.
Modding the space bar stabilizer spring
Here is how my spacebar looks on the inside (the space bar side is obviously facing away from me). You can see why you can't safely remove the space bar: there's a spring / stabilizer on the inside! Take a close look at that spacebar. As it turns out, the spacebar doesn't use a simple stabilizer, rather it has a second long spring to push it back up (picture taken on another occasion) :
This large spring is the target of this mod.
To get it loose, your best option is to take a flat-headed screwdriver or other prying implement to release the spring from a notch in the black plastic (you can see where it is locked when you push the spacebar cap up and down). As soon as the spring is out of this notch, it will pop up allowing you to push the end out of the notch in the space bar keycap. Removing the complete spring should now be easy: carefully lift the free end until you can free the other end from the black plastic support and keycap.
There it is. You may have noticed that the spring is bent like a V when looked at from the top or the front. If you measure the distance from the middle to the ends when looking at it from the front, it measures 6mm. My spring was bent until this distance was only 3mm, giving a nice 60g of force for the spacebar. Here are the dimensions of what I believe to be an unmodified spring:
Now the trick: hold it on your thigh like I did in the picture below: keep the bend at the end horizontal when you hold it between your fingers and make sure the middle of the spring points down and away from you. Now push the ends straight down for about 3-5cm, the spring might curl a little around your leg.
Hold the spring in front of you and check that the spring is flatter but not completely flat. That should have bent the spring in such a way that it exerts less force.
Assembly trick: holding the spacebar hammer in place
When assembling your keyboard again you'll need to press the plates back together while keeping all hammers in place. If you read this article from the start, you'll know that you should remove all regular keycaps from the keyboard except the spacebar. This last hammer can be a bit of trouble, we can't remove the keycap so the spring is always under tension! You can try to squeeze the plates together and hope that the hammer will properly align with its barrel. If you are out of luck and trap the hammer between the back plate and the barrel, you might at best end up with a non-functional spacebar or in the worst case a broken hammer.
Seeing how I didn't want to take 5 retries to get the keyboard together, I devised the following trick:
- Get some thin fishing wir,e or some other strong wire with low-friction. Also grab a wide popsicle stick or a piece of cardboard. I used one meter, the exact length depends on how easily you can fasten the wire to the stick.
- Remove the spacebar keycap and its hammer (this should be easy as we haven't inserted the stabilizer yet).
- Thread a loop of wire through the spacebar barrel. Pull it until you can reach the top row of the keyboard.
- Put on the spacebar keycap and attach the stabilizer spring in the next section. Check that the two strands of wire are passing through the hollow part of the keycap!
- Now put the hammer's "heel" with the spring through the loop. (Make sure not to get the spring caught in the wire!)
- Carefully pull the strands away from the keyboard to tighten the loop. The hammer will be pulled close to the barrel until it snaps into place.
- Roll the ends of the strands on the popsicle stick or whatever you chose. Keep on rolling until you reach the front plate and give it one last tug to pull everything together. You need to make sure the spooled wire keeps the hammer spring compressed!
- Now assemble the keyboard as instructed below. The hammer should remain in its place allowing you to push the plates together.
- Once the plates are properly locked, turn the keyboard barrels-up and unroll the wire from the popsicle stick. Now gently start pulling one end and after a little while you should have removed the wire altogether. If it snags you might have to open up the plates again.
- Press the spacebar a few times. If it still clicks, you did everything properly.
Step 3: Step 4: Step 5: The first time you try this technique it might be as fiddly as just squishing the thing together. The main difference is that the hammer should remain in its place for as long as the wire keeps the spring compressed. If you just try to press the plates together without this trick, the spring might buckle when you put on the top plate and squash the hammer between the top plate and some other piece of plastic, forcing you to start all over again.
You can of course use something else than fishing wire. Another member, skcheng, used floss made from Teflon for an even better effect (it is after all one of the slickest solid materials).
If you followed the above trick halfway, you should have the spacebar attached with a loop of wire threaded to its hollow shaft.
Before you can put in the hammer and spring, you need to reseat the stabilizer spring. Make sure you don't turn the spring upside down. The middle should push down on the plastic.
I've found that the best way to reseat the spring is to just lay it on the plastic retainer like it should be attached, then to pivot it by lifting the middle. You can then put the ends into the slots on the spacebar keycap. Push the middle down again while you keep the ends in place with your index fingers. Now push one side into the ridges, then push the other.
Once the stabilizer is back in its spot you're set to complete the trick mentioned above.
Take the back plate with the controller board. Check that the PCB is properly attached (it can be removed from the back plate by sliding the special rivets through a hole in the PCB). Put the back plate on top of the front plate and check that the hooks align and fall through. In case of the XT, put some clamps on to squeeze the plates together and tap it vertically again to get the hooks to latch.
Bend the large tab back over the backplate. In case of an XT, reattach the controller using that single screw.
Now you can put the keycaps back on. I found that the best way is to put the assembly flat on your lap, barrels-up. Lift the spacebar side up. Because you now hold the keyboard at an angle, all the springs should tip over and hit the other side of the barrel. Pushing on the keycaps this way means the springs have the largest chance to properly snap into the keycap, otherwise your keys might not buckle half the time.
With all keycaps back on, you can put the assembly back into its case. In case of an XT, simply put the keyboard in it. For the AT, put the assembly back in its case when properly aligned and fasten the two screws. Reattach the cables.
The AT might pose some problems closing - I recommend to ignore the feet for a second and simply snap the front edge together at an angle. Squeeze it until it is properly closed, then gently push the back side down. Press the feet through the openings (make sure they are properly centered!) and put the cable retainer in its slot. Push it all together and fasten the 4 screws.
If all went well, you can now test your keyboard using the computer. I don't recommend doing it while the case is off, due to the capacitative nature any contact with a metallic or di-ellectric material might confuse the keyboard.
Whew. Luckily you don't have to do this every day! Good luck, and comment if you want to add anything.
Typed on my model F XT