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HP Forum Archive 19

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HP48GX: lost tactile responce in F keys
Message #1 Posted by Harold A Climer on 22 Apr 2009, 5:22 p.m.

I have a HP48GX (and not the oldest one I purchased) on which the top row of keys no longer give tactile feedback when pressed. What causes this? Is there a fix without having to tear open the calculator? Were calculators manufactured during a certain time period more prone to this problem? I have three HP48GXs. Two were made in the USA and one in Indonesia

Harold A Climer Dept. Of Physics Geology, and Astronomy U.T, Chattanooga Rm. 406A Engineering, Math & Computer Science Building 615 McCallie Ave. Chattanooga TN 37403 Harold-Cimer@utc.edu

      
A few questions to ask. Then we can talk about it as invasive key fixes are difficult on the 48's
Message #2 Posted by Geoff Quickfall on 22 Apr 2009, 7:57 p.m.,
in response to message #1 by Harold A Climer

Harold, hello 

1.  Do the keys still function
2.  The second row to the top, do these keys have the same tactile
    response as the lower rows of keys.  That is, does the tactile
    response increase the farther away from the f keys one travels?
3.  spilt anything on it lately?
4.  using a dropper, place some 99% alcohol on the offending keys,
    work the key with succesive pushes (many 20 or 30 clicks each  
    key.  Allow to dry.
5.  did the keys respond to the cleaning?
6.  was the calculator dropped?

Cheers, Geoff

            
Re: A few questions to ask. Then we can talk about it as invasive key fixes are difficult on the 48
Message #3 Posted by Harold A Climer on 22 Apr 2009, 10:00 p.m.,
in response to message #2 by Geoff Quickfall

1. Do the keys still function? Yes but sometimes one is not sure if they took or not.

2. The second row to the top, do these keys have the same tactile response as the lower rows of keys? Yes In fact the F1 key still has a bit of the tactile feel left. 2A.That is, does the tactile response increase the farther away from the f keys one travels? After the first row they are all about the same: maybe a bit better as you go towards the bottom, but is hard to tell. 3. spilt anything on it lately? No. 4. using a dropper, place some 99% alcohol on the offending keys, work the key with successive pushes (many 20 or 30 clicks each key. Allow to dry. Will try.

5. did the keys respond to the cleaning?

6. was the calculator dropped? No.

                  
Re: A few questions to ask. Then we can talk about it as invasive key fixes are difficult on the 48
Message #4 Posted by Geoff Quickfall on 22 Apr 2009, 11:02 p.m.,
in response to message #3 by Harold A Climer

There are three ways to lose the 'feel' of the keys. Especially ones used as frequently as the F keys on a 48 that has dedicated F functions; i.e. the VAR menu with assigned folders and variables

Firstly:

The construction of the keyboard includes a domed contact. As the key is pressed into the apex of the dome, the dome deforms toward the keyboard contact completing the circuit and giving the resounding click.

The setup is similar to the following HP 45 keyboard with the exception that the raised portion is domed not rectangular ( I currently don't have a picture of the keyboard layout for a 48):

With consecutive use this system will fail over time.

Secondly:

The calculator is held together by plastic rivets. Actually plastic inserts into plastic tubes which are then glued under pressure. The upper rivets, depending how well they were formed at construction may give. Especially under duress such as a drop or extensive use. As the rivets loosen, the distance the key has to move to engage the 'dome' increases and the deformation of the dome may have a reduced click to it.

Thirdly:

The sticky residue problem. This will entail the cleaning of the keyboard, non-envasively. This can be done by removing the batteries, battery hatch and memory cards as well as the memory card hatch.

Conclusion:

Without seeing your calculator and assuming it is not the sticky residue problem or rivet problem; it would seem that it metal fatigue of the dome contact system.

Not much can be done for that other than 'Fix that Calc'. If the cleaning does not improve the feel, try the following:

1.  Place the calc on a flat surface.
2.  Gently apply finger pressure at the edges of the calculator
    corresponding to the row with the F keys
3.  while applying pressure, press the keys with your free hand
    and see if the keys respond normally.

This would indicate that the rivets may be at fault. Both the dome fatigue and rivet would require surgery unfortunately.

Good luck!

Geoff

                        
Re: A few questions to ask. Then we can talk about it as invasive key fixes are difficult on the 48
Message #5 Posted by Jeff O. on 23 Apr 2009, 7:32 a.m.,
in response to message #4 by Geoff Quickfall

Quote:
it would seem that it metal fatigue of the dome contact system.

I was under the impression that the HP48 uses plastic domes with some sort of conductive spot on the underside. I've never had one apart, so I cannot say for sure.

...

                              
HP 48 keyboard design
Message #6 Posted by Didier Lachieze on 23 Apr 2009, 9:15 a.m.,
in response to message #5 by Jeff O.

About the HP 48 keyboard design, here is an extract from the "Hardware Design of the HP 48SX Scientific Expandable Calculator" article in the June 1991 HP Journal, page 29:

Quote:
The tactile feel of the keys is a result of painstaking design, testing, and quality assurance efforts. The 0.004- inch-thick Mylar dome sheet contains 49 details, which provide the snap feel as each key is depressed. Each key has an actuator which presses against the top center of the dome. As the dome is pressed it deflects at its base into a dome spacer recess. At this stage the force is building linearly with deflection. As the force builds to the trip point the dome buckles, causing a sudden drop in force. Momentum and the resulting imbalance of force between the finger pushing on the key and the key no longer pushing back causes the inside of the dome to impact firmly against the keyboard contacts. The inside of the dome is printed with a pad of conductive carbon graphite. Upon switch closure the dome pad shorts the interdigitated carbon graphite contact fingers of the keyboard. Below each key switch is another recess created by a 0.003-inch-thick chassis spacer. This recess allows the keyboard to wrap around the depressed dome, resulting in a much more reliable area contact instead of the point contact of a dome against a flat plane.

Edited: 23 Apr 2009, 9:26 a.m.

                              
HP48 keyboard
Message #7 Posted by RETH on 23 Apr 2009, 9:38 a.m.,
in response to message #5 by Jeff O.

I just pulled one apart (and cut one of my fingers :) ) - 4 layers, top one made of clear plastic with stamped domes partly filled with dark conductive material, next one made of slightly thinner clear plastic with holes below the domes, third one - thin white plastic painted circuit board with lines on both sides and the fourth - made of paper to insulate the third from the bottom. I took picture but I'm not sure how to post them here. I can't believe it looks so simple and cheap and still worked perfectly after all these years!

                              
Thanks for the description..
Message #8 Posted by Geoff Quickfall on 23 Apr 2009, 11:59 a.m.,
in response to message #5 by Jeff O.

My intent was to show the deformation style. The mylar dome is a variant of the HP 45 click response in that it deforms a convex structure which is subject to fatigue.

If you have pictures, post them. Not that one can fix a cracked mylar dome, but it would be interesting.

Cheers, Geoff

                                    
HP48SX Keyboard
Message #9 Posted by reth on 23 Apr 2009, 7:17 p.m.,
in response to message #8 by Geoff Quickfall



BTW once you see those the keyboard and the calculator are certainly dead :>)

Edited: 23 Apr 2009, 7:31 p.m.

                                          
Re: HP48SX Keyboard
Message #10 Posted by Geoff Quickfall on 23 Apr 2009, 8:00 p.m.,
in response to message #9 by reth

Thanks Reth,

I guess you sliced the heads off all the plastic rivets? Wow!

I haven't had any completely dead ones to get that far yet.

Cheers, Geoff

                                                
Re: HP48SX Keyboard
Message #11 Posted by bill platt on 23 Apr 2009, 8:15 p.m.,
in response to message #10 by Geoff Quickfall

It looks like there was some blunt force trauma involved before that...:-)

                                                      
Re: HP48SX Keyboard
Message #12 Posted by reth on 23 Apr 2009, 9:50 p.m.,
in response to message #11 by bill platt

You bet! Took me 15 minutes and some brutal force :)
Cheers

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