|Column Tag:||The Electrical Mac
By Richard Clark, Mission Viejo, CA
The Incredible Dying Keyboard
I have a confession to make - I dont own a Mac Plus. What I do have is an ancient 512Ke which began life as an early 128K. Its had two power supplies (analog boards to those in the repair business), two keyboard cables, a new 512K mother board, and an Incredible Dying Keyboard. A different key dies about once a month now; theres no warning, the key just stops working.
The Incredibly Frustrating Repairman
Fixing a broken key is supposed to be simple for the repairman - you just open up the keyboard, remove the old switch, and solder in a new one. Its a five minute job, and that includes waiting for the soldering iron to warm up. Theres only one problem: the rest of us cant get the special keyboard switches that Apple uses. After a run-in with one particular dealer who refused both to sell individual switches and to fix the keyboard (he wanted to sell a new keyboard for $130), I decided to open up the keyboard and see what I could do.
Fixing Dead Keyboards is Easy (and cheap)
I learned that, with some limited soldering and mechanical skills, most keyswitch problems can be fixed. You remove the switch, make an adjustment or two, and replace it in the keyboard. Its easy and works almost all of the time. So far, Ive managed to repair 11 out of 12 dead switches myself (and the one failure was because I tried a shortcut which damaged the switch beyond repair.)
Notice that these repair techniques only work in the case where the switch is the culprit. Also, I havent tried these with one of the new ADB keyboards, but Apple seems to be using the same switch supplier, so this should work. Also, these techniques may not apply to locking keyswitches (like the Caps Lock). If the entire keyboard is dead, then I cant help you (except to say take a good look at your keyboard cable because if you stretch the cable too much you can pull the connections apart inside of the clear connectors.) If the whole keyboard is dead, and it isnt the cable, then youll need a new keyboard since Apple doesnt supply the spare parts for fixing the keyboard electronics to anybody.
Now that weve gotten the disclaimers out of the way, well discuss what youll need. First, youll need some basic soldering equipment: a soldering iron, solder (use only rosin-core solder, not acid-core) , and some sort of solder remover. The vacuum-type removers seem to work better than solder wick for keyboard work. A small can of flux remover (sold in Radio Shack as cleaner/degreaser) is optional but helps tidy up the keyboard after youre done.
Youll also need some basic tools and supplies: a medium-sized Phillips screwdriver (about 1/4), two or three small straight-blade screwdrivers (about 1/8 across), a pair of needle-nose pliers, and a pair of scissors or an X-Acto knife. Youll need two small pieces (about 3 each) of wire, stripped at both ends. And, depending on the problem youre fixing, youll need either a small tube of cement or some self-adhesive labels.
Finally, youll need to know how to solder and unsolder on single-sided printed circuit boards. Anybody whos built a Heathkit or two will understand how to do this. If youve never handled a soldering iron before, then get a friend to help.
The Secret Life of Keyswitches
OK, if youve gotten this far, then youre interested in fixing keyboards. So lets talk about how keyboards work. Looking at Figure 1, you can see that a keyswitch consists of five major parts: the upper and lower case sections, the stem, a spring, and a contact assembly. The contact assembly consists of a plastic frame containing two metal pieces shaped in such a way that when you press on the front of the assembly, the metal pieces come together and close the switch. When you press down on a key, the stem travels downward, resisted by the spring, and presses on a small metal tab sticking out of the front of the contact assembly. This compresses the assembly and closes the switch. When you remove your finger, the spring pushes the stem back up which removes the pressure from the contacts and opens the circuit again. The keyswitches are wired to an encoder which turns the switch closure into a signal for the Mac.
So, how does a keyboard break? Well, theres 2 common problems: if the switch just quits working (or works only if you press the key hard and/or wiggle it around), then the contact assembly is wearing out. Or, if the key seems to be floating - that is, the keycap is a little higher than all of the others and is putting out multiple characters on each press - then the upper and lower case assemblies have come apart and arent guiding the stem properly. Both of these problems are easily fixed.
Fixing Keyboards: Step by Step
So, heres the step-by-step procedure for repairing a Mac keyboard:
1. Organize your tools and plug in the soldering iron. Detach any cables connected to the keyboard.
2. While the soldering iron is warming up, turn the keyboard over and remove the 5 phillips-head screws holding the bottom of the keyboard case on. Remove the bottom casing and set it aside, then lift out the keyboard assembly. Set the top casing aside with the bottom casing.
3. Using a flat-blade screwdriver, pry up the key cap from the broken key and any surrounding key caps that might get in your way.
4. Turning the keyboard over, locate the 2 pins coming out of the broken switch. (I typically circle them with a pencil after I find them, just so I dont have to search for them several times.) Unsolder these connections, being careful not to overheat the connection and lift the foil up off of the board.
5. Using a pair of needle-nose pliers, gently move the unsoldered pins to be certain that they are completely free from the sides of the hole.
6. Turn the keyboard back over, so that the key caps are on top. Referring to Figure 2, press in on the 2 tabs at the top and the bottom of the switch. While you are pressing these tabs inwards, lift out the switch. Please resist the temptation to use pliers on the stem of the switch - you could break or damage the stem beyond repair. Having a second person comes in handy here!
7. Now that you have the switch out, you can begin the repair. If the problem is a floating keyswitch, then just glue the top and bottom halves of the switch together (making sure that all of the pieces are inside, of course), and skip the next step. If the problem is the more common dead switch type, then do the following:
a. Using a pair of small screwdrivers (X-Acto #2 knife blades also work well), and looking at Figure 3, insert the blade of the screwdrivers between the gray tabs and the black upper body. Press the blade all the way down to the bottom of the case on both sides and the two halves will pop apart. Pull the case halves apart, being careful not to lose the small spring.
b. Stack 2 or three layers of self-adhesive paper labels together (I use a spare disk label), and cut it to about 3/8 square - a little smaller than the back of the contact assembly. Then, referring to figure 4, stick this paper shim to the back of the contact assembly. When a keyswitch dies, its usually because the contact assembly has nothing to press on behind it. The labels provide the necessary backing.
c. Carefully re-assemble the switch. Because of the labels, it may take a little pressure to get the top and bottom casings back together. Dont forget to install the spring!
8. Now its time to test your work. Using the 2 pieces of wire, solder one end of each to each of the pins on the back of the switch. Solder the end of one wire to one of the pads on the keyboard where the switch was soldered, and solder the other wire to the other pad. Now plug in your keyboard, turn on your Macintosh, and test out a few keys, including the one you just modified. If the modified key fails, then take it apart and check your work. Re-assemble it again and try again.
9. Unsolder the wires from the keyboard and keyswitches. Then check that the pins are sticking straight out from the back of the switch and straighten them if necessary. Next, put the switch back into the hole in the keyboard where you removed it. Make sure that the pins in the switch enter the holes in the circuit board, and press the key in until the gray tabs snap into place.
10. Re-solder the switch into place and (optionally) spray a little flux remover on the connection just to clean things up. If you use cleaner, be careful not to get any inside the switch!
11. Re-assemble the keyboard, and youre done!
The whole process should only take about a half hour the first time you do it, and about 10 minutes each time after. As I noted before, I have 11 keys on my keyboard modified this way (fixed with a stack of paper labels), and the earliest repairs have been working perfectly for a year now.
Unlike most MacTutor articles, you wont be getting a program listing with this one! So I hope that knowing how to fix your own keyboards will satisfy both you and David Smith. Happy Hacking!