Repairing HP plastic push buttons?

[nctnico]

I recently bought a piece of HP gear with the older plastic push buttons (the ones with the distinctive 'click' when you press them). Unfortunately the click mechanism on some buttons has been worn making the buttons a bit flaky. Is there some way to take the buttons apart en fix them (using donor parts)?

[wn1fju]

The buttons on HP pieces (I assume you are talking about around 1980 or so) contain little pieces of metal that flex
when the button is pressed.  This metal "snap" is what you hear and feel.  The little metal strips dig into the plastic
housing of the switch and the plastic shaft connected to the button and can VERY easily pop out.  This leaves the switch
functional but without any real springy feel except the bounce-back from the gold fingers on the bottom of the shaft.
Open up the front panel carefully and most likely the metal spring is lying around somewhere.  Do the disassembly over
a clean table so you can see them if they drop out of the front panel.  If you can find them, they can easily be put back
into position with a tweezers.  If you can't, rumor is that you can make them out of the right piece of metal stock.  I use
to know what metal to use, but can't remember now.  Google around.

[edpalmer42]

Here's a web site that talks about repairing these switches:

http://www.rbarrios.com/projects/HPSWITCH/

If the piece of metal is lost or broken, I've heard that it can be replaced with the metal strip that's inside the top stick-on theft prevention tag in this picture.  There are two strips inside.  The thicker, stiffer one apparently works.  I haven't tried it, but I've been collecting those tags for a future repair session.



Ed

[nctnico]

OK. That website is very informative! Google only found page after page after page about replacing keys on laptops.
Anyway, I did find such a metal strip in the apparatus but I threw it away thinking it was a stray bit of metal. 
I guess I have to get creative and check very carefully if there are no more stray metal strips!

[lowimpedance]

If you can obtain 'feeler gauge stock' locally that in the correct thickness will work  (its a spring steel). Look up Starrett for example :
http://www.starrett.com/metrology/product-detail/1-Precision-Measuring-Tools/11-Precision-Hand-Tools/1115-Fixed-Gage-Standards/111506-Thickness-Gages/S667A

Metal Engineering shops may have a few odd strips lying around etc.

[edpalmer42]

OK. That website is very informative! Google only found page after page after page about replacing keys on laptops.
Anyway, I did find such a metal strip in the apparatus but I threw it away thinking it was a stray bit of metal. 
I guess I have to get creative and check very carefully if there are no more stray metal strips!

Yeah, it could ruin your day if one of those metal pieces decided to dance around on the circuit board!   

Ed

[nctnico]

I went a slightly different route (it is an HP4274A BTW). I ordered some small springs (3.5mm outer diameter) and installed these under the non-lit buttons. This provided me with more than enough of the original leaf springs to refit the lit buttons. I would have liked to replace the leaf springs on the lit buttons as well but didn't want to take the chance to short the LED. I really hate how these buttons feel when you press them. I bought the HP4274A anyway because I get a really good deal for this 5 1/2 digit LCR meter.

[wn1fju]

For those who frequently contend with the old 1980's HP "click" switches, I've found a somewhat palatable alternative.  You can
replace the entire switch mechanism which are usually heat-staked into position and cannot be reinstalled once they are removed. 
The problem is that the last time I inquired, Agilent wanted something like $25+ for a new switch (who knows if they have any left now). 
BUT, I had an old defunct HP 3437A voltmeter lying around and the switches in them are NOT heat-staked (they are held down by brackets). 
I don't have a heat-staking tip for my soldering iron, but a broad tip will work if you are fast and precise.  The good news is that HP 3437A's
go on eBay often really cheap (like $25).  For basically the price of one new switch from Agilent, you can have 13 new ones!