HP48GX: lost tactile responce in F keys



#2

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


#3

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


#4

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.


#5

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


#6

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.

...


#7

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.

#8

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!

#9

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


#10



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


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


#11

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


#12

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


#13

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


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