Trivia, Model M & F 

 Model M ; Buckling Spring Technology and membrane switching system 
  Cut model of a Buckling spring key
  Spring and platter 
  Replacing bad springs
  Remove  springs
  Iside 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 )
 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 
    Miodel F
    Buckling spring torsional  snap actuator;                                           No.4,188,611,   Oct. 3, 1978
    Model 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.
Keyboard Research 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 allowed me to use  his pitcures.  

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 ) ,  the spring hits actuator and generates clicking sound and at the same time a platter ( or hammer, whatever ) at the end of the spring hits and press membrane sheets Membrane sheets are covered by black rubber sheet .  You can't see  membrane sheets in above pictures.      

Spring and Platter
Coil springs and hammers used in model M2 ( including 42H0468 ) are identical with those used in Model M. 
Some models have black platters while the others have white half-translucent  hammers.   
Those coils and platters can be interchangeable among  all of model M ( of course except rubber spring 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 platter  (hammer) of Model M and Japanese A01

       L; Model M                  R; Japanese A01   

            L; Model M,           R; Japanese A01


       L; Model M,            R; Japanese A01


                       Bottom view

                        Upper view


  Upstanding post on the upper surface of a platter.   


Is there any rule setting a spring onto 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.

Replacing bad springs
By what reason, a spring within a barrel got loosened or bent,  Spring got loosen or streched. Any way to replace to new spring ?

Basically, no.  But if you really want to do, there is a way.   Please do it by your own risk.
I learned following procedure from Mousefan, the Authority of  Switch.  When you refer this method, This should be also noted accordingly.

 1. Prepare good springs ( probably you can get them from Unicomp. or take them from defunct
    donor boards.)

 2. Make your own tool for surgery using a bamboo stick  ( See picture below )
    When you make a similar rod, please make a distinctive edge where you reduce diameter of
    your bamboo stick.   One in the picture is just an example to show the method.
    If the length of small diameter zone is too short, you'll widen and loosen your new spring.  

 3. Pull out damaged spring.

 4. Set a good spring at the end of a rod you made in step 2.

 5. Set the rod together with spring into the barrel, then place the spring onto the post ( tiny projection) on 
     a platter.

 6. Push down the rod and turn ( rotate ) it slowly giving pressure continuously.
     You may think direction of rotating a rod should be counter-clock wise according to M spring,  but
      I recommend  you to turn your rod in counter clock wise.
      Diameter of a projection is wider then that of M spring in order to hold a spring tight.  If  the    
      post on a platter which is made of plastic material happen to block the cut-end of a coiled spring,
      and you still rotate the rod with pressure,  your spring may be loosen or stretched out.  
      But so far,  both direction probably may work.      Do as you like.

Step 2.  Make a rod like this 

Step 4.  Set a replacing spring


Step 5.  Insert the rod and place spring on the post

Step 6.  Press down the rod and  rotate it

Check if your spring is set properly or not by pulling it up gently.


How to remove springs securely from a donor board

Go to a drug store and get tweezers. Select thinner one. Grind inner edges and make them in round shape.  

         upper: normal      lower: grinded

left: normal                right: grinded

Insert your tweezers into the barrel.
If you can't push it down to the post of a platter.
then grind and adjust width of your tweezers


      Catch tight the end of the spring and pull it up. 
    If the spring is set tightly and you can't pull it up,
                         then joggle it.


Here comes the sun

no comment needed against my fingers

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 
ID 380X N-1797
Date 47-1983,
Mod C-1,
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 ? 

Glass fiber PCB is attached on the curved lower steel plate. PCB itself is not  flat but is also in a wide curve.

Looking from the bottom side of the lower steel plate

Left side; Comparator?    
A large DIP module might be a controller.  It 's marked as "8310354 8332,  PI P A 998741, L3374976, Intel "

Back View of a controller PCB



Some kind of coating covers surface of the main PCB including every terminals of signal line ( large  square parts ) .
Look carefully a close look of terminals. You can identify no cupper surface is exposed on the main PCB.


Wow!  Such a big plater ( blade, hammer lever , I don't know  how IBM named it.  )
The lever is made of carbonized plastic. 
When the key press downward and the plater meets terminals on the PCB,  capacitance between the terminals are increased and then a switch turns ON.  I don't know detailed mechanism.

Every actuators can be removed from metal base plate ( upper plate ). 

Do not pull out a space bar from this keyboard ( I mean any kind of model F keyboard.  XT 83 key keyboard and AT 84 key  keyboard are both included.)   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...... 

Front view of  P/N 5640991

  Keys except text/data keys may be placed wrong position. 
   Forgot to take  pictures before I pulled out all the keys  :-)   


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

 K/B  P/N


 U2 P/N





















 Incompatible with Wintel PC








 Incompatible with Wintel PC




 Space Saver M




 Totally different from other PCB. 

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 ). 

U1/U3 :  2392161     U2 : 1389986  

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.  )

U1/U3 :  2392161     U2 : 1389986,  CP1 & CP2 : B9HC0114 102M x8

Two brown capacitor packs are soldered below J4 .


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

U1/U3  2392161,     U2 1386802,  CP1/CP2 ; Solder pads

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.

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 ( revived from internet archives. Original page is now missing ).  
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 )

U1/U3 :  2392161     U2 : 1393295
Connector at J2 is missing,  instead there is J8 for film cable connector for LED block   

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



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 )

 Bottom left     : Clock , to  the Yellow cable
 Bottom center : GND , to the White cable
 Bottom right    :  DATA, to the Red cable
 Upper right      :  +5V , to the Black cable

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


U2 : 1390538,
CP1/CP2 : Solder pads are there.
J2 connector for LED block is missing
U2 : Absent ( outline only )
Pulled from Space Saver Model M P/N 1319472, Date 15JUN87.


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

Mr. John Szybowski reported 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.  Unfortunately his web page is no longer exist.  However the page was salvaged from internet archive and now stored at  9595 - Ardent tools of Capitalism by Louis F Ohland..  If you can't reach the page, Click HERE 

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