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HP Forum Archive 19

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HP-45, strange key response
Message #1 Posted by Thomas Radtke on 16 Jan 2010, 9:12 a.m.

I really don't want to open my 45 again (did that twice to successfully clean a key contact and the switch contacts), so I ask here if I can safly ignore this problem: One of the keys shows a strange behaviour. When only slightly pressed, just a fraction of a degree, there is a clicking noise with high frequency. Further pressing it down gets to the normal key click and the machine responds as expected. So, there are two pressure points on the rolldown button.

I can almost not feel the first click, and it woudn't be disturbing if it only would not be of notable audibility.

My question: Will I get a problem with this key due to what ever causes that click? If so, can I do something about it?

Thanks!

      
Re: HP-45, strange key response
Message #2 Posted by Vieira, Luiz C. (Brazil) on 16 Jan 2010, 10:02 a.m.,
in response to message #1 by Thomas Radtke

Hi, Thomas;

I have an HP15C with an opposed problem: two keys have no sound, no click at all, but they work pretty well.

You see, with the Spices, HP41 and Voyagers, the keyboard has a rubber sheet between the keys and the keyboard. In this HP15C, the two keys made a small hole in the rubber sheet right under them, mostly because the keys have just a small 'flat tip' that punctuate the metal cap (dome?).

The earlier series of calculators had a plastic sheet between the keys and keyboard, and I'd guess that in your case, the plastic is probably being briefly dragged over the metal contact (hence the hi-pitch noise, like a 'squeak',I guess) before the metal contact is actually pushed down.

Can you try to identify if this is what happens?

Cheers.

Luiz (Brazil)

            
Re: HP-45, strange key response
Message #3 Posted by Thomas Radtke on 16 Jan 2010, 11:18 a.m.,
in response to message #2 by Vieira, Luiz C. (Brazil)

Hi Luiz,

thanks for your answer! It feels and sounds very much like a stiff piece of plastic the key rubs along, or a part of the metal dome. It is very constant.

From opening it before, I know that the domes (or 'bridges') do look heavily used. Don't remember if there was a protecting sheet of plastic between keys and domes.

I'll probably hope for the best and leave the 45 as it is, as long as someone points out that the key must be repaired. The back lable already has some slight marks from removing it before.

Thomas

                  
Re: HP-45, strange key response
Message #4 Posted by Randy on 16 Jan 2010, 11:46 a.m.,
in response to message #3 by Thomas Radtke

The obvious:

  • Crud on the back or front of the plastic keytop
  • Something sticky inside the hole where the keytop sits in the case top

The not so obvious:

  • Small stress fractures developing in the metal flex strip. Really not much you can do until it breaks completely

There should be a thin piece of clear plastic sheet between the keytops and the keyboard assembly. The sheet is held in place with the plastic frame that sits atop the pc board. Two small projections hold the frame in place by engaging though the board and into the back plate of the calculator. The sheet has been called the "sneeze guard" in some old posts.

FWIW, the next time you open it, don't replace the two screws under the label. The case will never know the difference and it makes re-entry a non-decision.

                        
Re: HP-45, strange key response
Message #5 Posted by Thomas Radtke on 16 Jan 2010, 12:38 p.m.,
in response to message #4 by Randy

Thank you very much for your answer, Randy!

Quote:
The not so obvious:
  • Small stress fractures developing in the metal flex strip. Really not much you can do until it breaks completely

That sounds more like it, I fear.

Quote:
FWIW, the next time you open it, don't replace the two screws under the label. The case will never know the difference and it makes re-entry a non-decision.
Great idea!

Thomas

      
Re: HP-45, strange key response
Message #6 Posted by Thomas Radtke on 18 Jan 2010, 5:30 a.m.,
in response to message #1 by Thomas Radtke

I did it, opened it again.

Pushing the dome is like pushing any other dome. They are equal. The key is also clean, no obvious problems.

Putting everything together again, the click is still there. Mysterious.

            
Re: HP-45, strange key response
Message #7 Posted by Randy on 18 Jan 2010, 9:06 a.m.,
in response to message #6 by Thomas Radtke

Those spot welds that attach the strip - have they blown through the top or are they flush? I cannot tell based on the photo... they look questionable. If one has given way, it will allow extra flexing of the strip and might be the cause of the click.

HP actually replaced complete strips during service, sometimes the welds where not as good as the production work. The fact that the faces have so much pitting is a sign of heavy use, perhaps it is on it's second set of key strips which would explain the welding.

                  
Re: HP-45, strange key response
Message #8 Posted by Thomas Radtke on 18 Jan 2010, 9:20 a.m.,
in response to message #7 by Randy

Quote:
Those spot welds that attach the strip - have they blown through the top or are they flush?
I could inspect them with a microscope. With my bare eyes (and finges) I can't decide.

Would you do anything about it? Would a drop of superglue do the job?

                        
Re: HP-45, strange key response
Message #9 Posted by Randy on 18 Jan 2010, 9:56 a.m.,
in response to message #8 by Thomas Radtke

Press lightly on the strip and see if it flexes at the welds. If it pops up around that area, that's the source of the click.

I've had no luck with glue, mixed success with soldering.


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