How to open/unscrew a Model M?
The shell of the Model M is held together by four hex nut screws. Three are recessed. You need a nut driver to open it. The proper size is 7/32" and it should have a slim barrel. You can find the correct driver in a Sears.
The proper name of the product is Craftsman 7/32 in. Easy-To-Read Socket, 6 pt. Deep, 1/4 in. drive, model #45815.
This is what it looks like:
Look here for the Sears product page:
http://www.sears.com/shc/s/p_10153_12605_00945815000P?keyword=craftsman+7%2F32
You will also need a 1/4 in. spinner to attach it to. Craftsman 6 in. Spinner Handle, 1/4 in. Drive is what you need. Sears carries it too here: http://www.sears.com/shc/s/p_10153_12605_00943393000P
Whenever I have a need to open a Model M, usually because of something I spilt, I could never seem to find my driver. This meant an quick trip to Sears to buy another. If you don't have an urgent need to open your keyboard and would like a one piece solution, see here:
http://www.clickykeyboards.com/index.cfm/fa/items.main/parentcat/11298/subcatid/0/id/105901
The Model M will not work with my PS/2 port
If your Model M does not work reliably or at all, your PS/2 port may be to blame. The Model M is a device from the mid-to-late 80s, and the keyboard control board and LEDs inside the machine draw a lot more current than a throw-away, el-cheapo modern keyboard use. It requires 275mA, whereas a modern cheapo keyboard may only need 1/10 of that. Some PS/2 ports just cannot provide the current for a Model M to run reliably or at all. I have never encountered this particular problem myself, but according to the link below, it can be found in many motherboards.
There are two solutions to this problem, both are listed on this page:
http://web.archive.org/web/20071014064100/http://www.geocities.com/jszybowski/keyboard/
(This site went down with the rest of Geocities, thank goodness for the Web Archive).
If the site becomes unavailable in the future, then the answer is that you need to solder a 4.7k resistor from the Clock line to the +5v line and one 4.7K resistor from the Data line to the +5v line. For those Model M's with the detachable keyboard cable, here is the pinout for the SDL jack on the back of the keyboard:
Your other option is to buy a PS/2 to USB adapter, which brings me to my next section:
Adapters
Before USB, there were two standard ways to connect an IBM PC compatible keyboard to a computer. Earlier keyboards used a 5-pin AT plug. Generally, you often found the AT-keyboard plug on everything from an IBM PC/XT/AT to an OEM motherboard with the AT form factor. Later keyboards used the PS/2 port, as did the IBM PS/2 computers, many other pre-built computer models and motherboards based on the ATX and successor form factor. While the form may be different, electrically the 5 and 6 pin connectors are compatible. All that is needed is a pin converter to plug onto the end of your keyboard's connector.
The Model M keyboard was first released when IBM was still producing XTs and ATs in 1986. Since these machines used the AT connector, the (black) cable was issued with an AT plug on the end. When IBM began manufacturing their PS/2 computers, they discontinued the XT and AT computers. Model M keyboards came with grey cables with a PS/2 plug on the end. The AT and PS/2 cable are fully interchangeable from the keyboard connector. However, you can also use an adapter.
USB and AT & PS/2 keyboard interfaces are not really compatible, especially with the Model Ms. You need a PS/2 to USB converter, and they are not all the same. I would recommend this one:
It has been verified to work with an Model M with a variety of USB systems and supported operating systems. If your motherboard supports USB keyboards, you should be able to use it even in MS-DOS. One issue: my keyboard will not respond if you select the "Restart in MS-DOS Mode" in Windows 98SE.
Resources
Purchasing
I have already linked to http://www.clickykeyboards.com/ twice on this page, and the site is the most important site on the web about the Model M. First, it is a great visual catalog of the various Model Ms IBM released. Second, you may be able to find a Model M to buy, but you will pay top dollar for it. They have parts to repair or replace a missing or faulty component on that Model M you purchased on eBay. The guy who runs the site is pretty cool and has helped me in the past on more than one occasion.
If you want to buy a used Model M, eBay is the best place to find one. There always seems to be plenty for sale. Here are some tips:
1. Read the auction listing carefully for information about the product's condition.
2. Make sure there are no missing keycaps or keyboard stands and all the LEDs work. Ask if the auction is unclear.
3. Get a look at the label to determine the date of manufacture.
4. Be prepared to clean the keyboard once you receive it, but test it first.
Last, we come to Unicomp. I have previously made my views known about its product here. I did not care for their USB 104 Customizer, but perhaps their PS/2 Models are more solidly constructed. They also have quite a few options. If you do not want to deal with the a used eBay model, this company is your only option. They were featured in an interview on National Public Radio, which is worth a listen:
http://www.npr.org/templates/story/story.php?storyId=100076874
Technical
If you need technical information about the Model M, look at these pages:
http://www.mcamafia.de/pdf/pdfref.htm - Reference Manual for the Keyboard
http://www.mcamafia.de/mcapage0/keyboard.htm - Keyboard Maintenance
http://www.ibmmuseum.com/OhlandL/keyboard/Keyboard.html - Miscellaneous Tech Info
http://mykeyboard.co.uk/keyswitches/ - Keyboard Switch Info
Upgrading
http://zevv.nl/play/misc/ibm-usb/ - Instructions on how to fit a USB converter inside a Model M
http://www.pigdog.org/model_m_retrofit.html - Instructions on converting from QWERTY to DVORAK
Enthusiasts
http://geekhack.org/ - Forum for enthusiasts of clicky keyboards of all makes and models.
One last word. There are lots of reviews of the Model M around, almost all of them highly positive. A Google search should inform you that there are a large number of converted fans of the Model M.
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