Model M ; Buckling Spring Technology and membrane switching system 
          Cut model of a Buckling spring key
            Springs and hammers
            Inside Model M 
            Repairing Model M ( reuse inner assembly )  

     Model F ; Buckling Spring Technology and Capacitive switching system 
            A  sample of capacitive switch keyboard .  p/n 5640991 
            Variations of  Control PCB
            Pin out of SDL connector and PS/2 plug ( and AT DIN plug )
            Pin out of J7 Terminal keyboard ( 5 pin header )
            Stabilizing Rod  ( added 2009/08/03 ) 
           Translucent Key Caps ( From Korean web site,  United Keyboard Mania  by zoOoz )
                                                 English translation,    Japanese translation 
           Model M can't be recognized when connected to some modern mother board. 
           How to reduce Buckling sound

     U.S. Patents relating buckling spring technology     
       Catastrophically buckling compression column switch  and actuator;   No.3,699,296,   Oct.17,1972
       Pushbutton switch assembly with pivotable conductive ...           ;       No.3,731,030,   May.1,1973 
       F Buckling spring torsional  snap actuator;                                        No.4,188,611,   Oct. 3, 1978
      M Buckling switch actuator for low force membrane contact switch ;    No.4,584,317,    Jul, 9.1985.   

      IBM document introducing model M 
           Development of a membrane switch-type full-travel tactile keyboard

Buckling Spring Technology and membrane contacts.
Please check Buckling Spring section of Qwerters Clinic maintained by Mr. K. Tanaka//.    You  can see
everything about Buckling spring mechanism there.   This page explains IBM Japan 5576-A01 keyboard but the buckling mechanism is basically same with 101 key enhanced Model M. except the fact that A101 uses individual key modules.  

For Japanese readers. KEN-JIN web site maintained by IT-san  is also recommended to understand buckling spring technology and its brief history. 

Buckling spring key; Cut model 
Great thanks to  Mr. K. Tanaka// ( The owner of famous web site " Qwerters Clinic ") who offered wonderful pictures.  

Inner structure of a key top is designed to let the spring bend toward designated direction only inside a key unit.   When a spring completely bends ( buckles ) ,  it will hit actuator and generates clicking sound and at the same time a platter ( or hammer ) at the end of the spring will hit and press membrane sheets  (  membrane sheets are covered by a black rubber sheet .  You can't see  membrane sheets in above pictures.  )

Springs and hammers
Coil springs and hammers used in model M2 ( including 42H0468 ) are identical with those used in Model M. 
Some models have black hammers while the others have white half-translucent  hammers.   
Those coils and hammers can be interchangeable among  all of model M ( of course except rubber spring type M  ).
Japanese readers are advised that coil springs in 5576-A01/003 are different from those used in Model M.   They are shorter than Model M springs. 

Pictures below show deference of an actuating rocker (hammer) of Model M and Japanese A01

Is there any rule to set a spring to a hammer ?
As far as I can see, every springs set on each hammers with a starting point of coil spring locates within 180 to 270 degrees.  Don't know if  there is any reason for it.   If I set a spring in different position,  buckling error occurs frequently.

How to reduce Buckling sound
If you feel your M is too loud when you type and you want to reduce buckling sound without changing your M to a cheapo membrane keyboard .
Use silicon grease.  Pull out key assembly and paste a little dose of silicon grease around the wall of barrel and buckling springs.  You can control sound level with the volume of grease.  
Spray type silicone grease is also easy to use.
............And actually IBM provided such a variant of Model M.          

Model F ; Buckling spring technology  with capacitive switches
Model F was introduced together with the IBM PC.  Key action is US patented IBM Buckling technology and the switching system is so called capacitive contacts switch. Capacitive switch does not require any physical contacts between two terminals.  Specially designed electrical components detects changing of capacitance between terminals and at some point of capacitance, switch turns on.  Model F uses  plastic made platters ( hammer lever ? ) to change capacitance of each switches,  When a platter hits main PCB, capacitance between two terminals on the PCB placed below a platter increases and immediately a switch  turns  ON.   Capacitive switches made by other manufacturers such as Keytronic, BTC and TOPRE ( a Japanese manufacturer which still makes very expensive keyboards with capacitive switches ) do not generate clicky sounds.    Only IBM's model F with buckling system makes louder clicking sounds when platters hit main PCB directly.  Those who like IBM model M may like IBM model F but those who does not like model M because of its buckling sounds definitely dislike model F .    Because Model F is much louder than Model M when we type.     I like model F so much.  Model F  AT 84 key keyboard is the one you can use with usual PCs without any modification. You just need an  AT to PS/2 cable to use the keyboard.

English explanation about Capacitive switch is available in this web page.
The author misunderstand about mechanical switch because the author introduced buckling mechanism of model F as  an example of mechanical switch.   As far as Capacitive switch concerns,  the explanation seems to be correct..

Please examine U.S. Patent  No.4,188,611 for further detailed information.    

Let's see inner layout/parts of p/n 5640991 
5640991
ID 380X N-1797
Date 47-1983,
Mod C-1,  ( I think "Mod." doesn't mean "Model".      "Modification" may be?   
Made in U.S.A. by IBM , Armonk. NY. USA
Description :  One piece made key top, Buckling Spring Technology,  15 pin D-sub connector.   Data  processing keyboard ? 

Do not pull out a space bar from this keyboard,  XT 83 key keyboard  and AT 84 key  keyboard.   Stabilizing bar is set underneath the upper metal plate and you can't place it properly unless you disassemble the keyboard .
That's why I can provide above pictures.  I knew that, but I just forgot about it when I pulled out  keys  to make them clean...... 

Variations of Control PCB
Also check Model M keyboard section  of  Ardent Tool web site  maintained by Louis Orland.

1389989  K23-3^*1 EC528561 1087,   dimension 170mm x 50mm 
Pulled from Model M P/N 1391401, 13MAR87 made .
# 1087 following to EC number probably indicates mfg week/year ( 10th week of 1987 ). 

There is a 4 pin header J2 at the right side of the PCB to which 4 yellow wires from LED block are connected ( not a film printed cable ).

P/N 1390120 sold with the last PC/AT has the same control PCB.

^*1 The number begins in K-prefix varies among models.   For example, 
    the number of a control PCB of model 1390120 in my hand is " K26-1". 

Signal assignment of connector J1 

1392993 K26-1^*1  EC 528682  3787,   dimension 170mm x 50mm
Pulled from Model M 1391401 manufactured in 22SEP1987.
Almost same with 1389989 above.  There are 2 capacitor packs at the position of CP1 and CP2 below J4.    Except these two register packs,  This PCB seems almost same with 1389989.   (  I'm not so sure whether  "CP" means  capacitor packs or not.  )

1351523 K26-3^*1  EC528682 39/87,   dimension 170mm x 50mm

Looks like almost same with 1389989 1392993 except missing 2 brown capacitor packs below J4.  There are solder pads  at the position CP1 and  CP2 .  Main control chip is different from above 2 PCBs.  

This PCB is used in Gray color model M keyboard with a black square logo plate , Part number 1388032,  ID 1001377, Date 02OCT87, Plt J3
The keyboard doesn't work with a usual PC ( I mean Wintel PC not a genuine PS/2,  don't know if if works with a PS/2 ) . I don't know if those missing CP1/CP2 blocks have some effects on this issue.  Or if the difference of controller chip P/N causes the problem...   When  I swap the control PCB to a 1392993,  the keyboard works properly.

2004.06.27
Examined DC level of data/clock line  and found each lines are something around 2.1V while others keyboards are around 4.8V.  Then  I tried data/clock line modification adding 2x  4.7K ohm registers according to IBM PS/2 Keyboard modification page .  
Result ; No luck.. Three LEDs flashed repeatedly and PS/55E gave 301 error again.   
Conclusion : Model M  P/N 1388032 with a controller PCB P/N 1351523 might be a proprietary keyboard for some special system.   Most possibly  Keyboard controller is not compatible with usual PCs.  (  Or mine has a defective controller PCB ) 

1393291  KF530-3,  EC 528693  4488
Made in 44th week 1988?  (  Mfg date of the keyboard is 25OCT88 )

Seems to be a popular type of control PCB used in earlier type of Model M between 1988 and 1990 .
Main difference is connection between a control PCB and LED block. Each PCBs are connected via film cable instead of wires with 4 pin headers. 
Variations of this PCB in smaller dimension can be seen in Louis's page. 

1398011 

PCB is pulled from a Model M  P/N 59G7980 by Lexmark with drain holes, blue logo on a gray round plate, manufactured 02FEB1993.
Film cable for LED block is combined with matrix lines and connected to J3 on the PCB together with key matrix lines.   This PCB is made in HongKong.

1387210  K26-3, EX528635  0987, possibly made in HongKong

Pulled from 101key terminal keyboard ( not sure )  Model M  P/N 1390680, MFG date 02MAR87.
J7: 6pin connector ( Uses 4 pins only , 5 pins are ready ) for keyboard cable.
J6: Unknown , shortened to PCB GND with a metal alloy.
U2: IBM 1387213  ( HD6805V1NO1P )

At the end of the cable, it has a  strange AT like sideway plug.  You can see a picture of the same plug and the pin out can be seen in this page .

Pin out of connector J7,  5 pin header  ( on the controller PCB )

Keyboard p/n 1390680 has very funny characters at the position of Numeric  Keys.  Numeric keys are available at the upper row of text/data keys ( below Function keys ) .
This Control PCB does not work on with Windows PC.

1394523  EC. 528735

Pulled from 122 key terminal keyboard P/N 1394099 , MFG date 05-05-97,  manufactured in UK by IBM ( not Lexmark ).

Cable connector is marked as J1．
J5/12 pin header connector  is left open. ( no cables are connected ).
U2 P/n is 1394068  (  EF6805U3P-BE512 ).

Not sure about a plug at the cable end ( the cable was cut at the keyboard side . I got the keyboard for  "parts" ) .    I assume the plug might be either RJ11 or RJ45.

1390536  EC528682,  K23-3, 5287

Stabilizing Rod
In addition to Space Bar,  early models of 1391401 have  stabilizing rod beneath both  "Enter"  and  "+"  key at numpad section.    This design was already seen in 1390121 and 1390130 sold together with late XT and AT computers respectively,  one year earlier than appearance of 1391401 and PS/2 computers .  

Seems like IBM changed  design at some point in 1989 

Model M can't be recognized when connected to some modern mother board

It is reported by Mr. John Szybowski that model M can't be used properly when it is connected to some newer mother board with faster processor.  This problem can be solved according to John's method.
Go here ;   IBM PS/2 keyboard modification page( stored at web.archive.org) 
.