Front-panel push button repair

[Earendil]

I've got a pretty old instrument recently (~1990). I've noticed one of its push buttons doesn't work ("fast"). I've opened the instrumented and it turned out that push button springs are badly corroded. The "fast" button's spring become so thin that it doesn't work anymore.

Any idea how to fix these? I think I will need new springs. At least it seems easy to install these springs. They are just pushed in place. So the question is can I buy these somewhere? Or should I try to cut them from some sheet of metal? And what kind of metal?

[wn1fju]

These metal leaf springs have been discussed numerous times on eevblog, Yahoo HP newsgroups, etc.  Google around.  A recent EDN magazine article states: 

"The return spring is an elegantly simple steel leaf measuring 0.588 in. long by 0.140 in. wide by 0.002 in. thick."

and

"Correspondents in Yahoo’s HP-Agilent group offered a number of replacement suggestions, including trimming springs from stainless-steel shim stock, beryllium-copper EMI grounding fingers, and theft-prevention tags found in DVD packages."


In a worst case scenario, you could find a cheap, non-functional, vintage HP piece on eBay and steal the springs out of it.  I've done that by buying an
HP 3437A voltmeter for a few bucks.  Not only do you get the springs, but they didn't heat-stake down the red switch housings.  So you get some spares
of those too.

[Earendil]

Shim steel doesn't work according to the Yahoo forum. These things are made from a very springy material.
So in the end I bought 0.05mm spring steel strips: http://www.ebay.com/itm/291409809877

[Robomeds]

I'm not sure where to find it but Nitinol strip would work nicely. 

[don.r]

Shim steel doesn't work according to the Yahoo forum. These things are made from a very springy material.
So in the end I bought 0.05mm spring steel strips: http://www.ebay.com/itm/291409809877

Does anyone know if these work? I am considering buying a roll of 1095 Hard Carbon and selling 3" X 0.588" strips (makes about 21 springs). If these ebay ones work, this would be a better option for me.

[Stray Electron]

  Those look like they're in a piece of HP gear.  The problem with the HP stuff is that those springs are in long strips that cover the entire row of switches and they are spot welded to the circuit board in two places on each side of each switch.  We always called them "buckle strips" because of the way each bump would buckle when pressed and make that nice snap action key press.  Buckle strips make for good key clicks but they can be a bear to repair. I've cleaned the contact surfaces by cutting narrow strip from an old 80 column punch card and wetting it with alcohol and then inserting it under the raised "bump" of the switch. Then hold the switch down lightly and pull the paper card through it to clean the contacts.   To repair them I have ripped an old strip off of a scrap piece of HP gear and cut a section just big enough for one switch.  Cut the bump off of the old switch and leave just a narrow edge of the old material on each side of the bad switch. Then solder the salvaged portion to the edges with low temperature solder.  Usually the old switches can be simply cleaned and I've only ever had to repair a few of them but it can be done.  I believe that they're made of some kind of Berylllium Copper to get the right stiffness, springyness, electrical conductivity and corrosion resistance.  Beryllium isn't something that you really should be cutting or grinding on!   But sometimes you do what you have to do.

   I've never tried it but some of the HP stuff like the HP-41 calculators used round domes of springy material for the key clickers (aka oil-can principle).  One dome per key and they were held on by covering the entire keyboard with a large piece of tape that held each dome in position.  You MIGHT be able to rob one of those domes out of a HP-41 and use it to replace the "bump" in your buckle strip.

[don.r]

  Those look like they're in a piece of HP gear.  The problem with the HP stuff is that those springs are in long strips that cover the entire row of switches and they are spot welded to the circuit board in two places on each side of each switch.  We always called them "buckle strips" because of the way each bump would buckle when pressed and make that nice snap action key press.  Buckle strips make for good key clicks but they can be a bear to repair. I've cleaned the contact surfaces by cutting narrow strip from an old 80 column punch card and wetting it with alcohol and then inserting it under the raised "bump" of the switch. Then hold the switch down lightly and pull the paper card through it to clean the contacts.   To repair them I have ripped an old strip off of a scrap piece of HP gear and cut a section just big enough for one switch.  Cut the bump off of the old switch and leave just a narrow edge of the old material on each side of the bad switch. Then solder the salvaged portion to the edges with low temperature solder.  Usually the old switches can be simply cleaned and I've only ever had to repair a few of them but it can be done.  I believe that they're made of some kind of Berylllium Copper to get the right stiffness, springyness, electrical conductivity and corrosion resistance.  Beryllium isn't something that you really should be cutting or grinding on!   But sometimes you do what you have to do.

   I've never tried it but some of the HP stuff like the HP-41 calculators used round domes of springy material for the key clickers (aka oil-can principle).  One dome per key and they were held on by covering the entire keyboard with a large piece of tape that held each dome in position.  You MIGHT be able to rob one of those domes out of a HP-41 and use it to replace the "bump" in your buckle strip.

You might be thinking of a different switch as the buckle spring in these is purely mechanical. The contacts are made elsewhere in the switch and have gold plating. All we need here is "springiness" and apparently HP used 1095 steel and I think the ebay ones are heavy on the manganese. Still if it works, it works.

[shakalnokturn]

FWIW I've replaced some of those with strips of mylar cut out of floppy disks. You get a softer and less clicky feel to the key but it does work well.
Mylar doesn't rust...