keyboard repairing

Saturday, May 2, 2009

The Incredible Dying Keyboard

I have a confession to make - I don’t own a Mac Plus. What I do have is an
ancient 512Ke which began life as an early 128K. It’s 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; there’s 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. It’s a five
minute job, and that includes waiting for the soldering iron to warm up. There’s
only one problem: “the rest of us” can’t 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. It’s easy and works almost all of the time.
So far, I’ve 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 haven’t 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 can’t 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 isn’t the cable, then you’ll need a new keyboard since
Apple doesn’t supply the spare parts for fixing the keyboard electronics to

Now that we’ve gotten the disclaimers out of the way, we’ll discuss what you’ll
need. First, you’ll 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 you’re

You’ll 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. You’ll need two small pieces (about 3“ each) of wire, stripped at both
ends. And, depending on the problem you’re fixing, you’ll need either a small
tube of cement or some self-adhesive labels.

Finally, you’ll need to know how to solder and unsolder on single-sided printed
circuit boards. Anybody who’s built a Heathkit or two will understand how to do
this. If you’ve never handled a soldering iron before, then get a friend to

The Secret Life of Keyswitches

OK, if you’ve gotten this far, then you’re interested in fixing keyboards. So
let’s 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, there’s 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 aren’t guiding the stem properly. Both of these
problems are easily fixed.

Fixing Keyboards: Step by Step

So, here’s 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 don’t 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

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, it’s 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. Don’t forget to
install the spring!

8. Now it’s 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 you’re 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 won’t 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!


Use The Below Code To Use Emoticons In your Comments

:)) ;)) ;;) :D ;) :p :(( :) :( :X =(( :-o :-/ :-* :| 8-} :)] ~x( :-t b-( :-L x( =))

Post a Comment

Related Posts with Thumbnails