HP 29C 1-2-3 keys not working



I'm the happy owner of a nice-looking 29C that's unusable because "1","2" and "3" keys don't respond.

I've tried to open it but, of course, since the keyboard PCB is stuck to the face bezel, I can't even check the status of this row of keys.

Does anyone know how to dismount the KB PCB from the chassis?

Thanks for help,


Does the mulitply key work?

With three keys in a row not working (and most likely the fourth as well), the problem is more likely to be the contact between the
keyboard PCB and the logic PCB. Check that the spring terminal isn't too compressed to make good contact.



You're right: the multiply key is out. What do you call the spring terminal?


Those are the pins that are soldered to the logic board, and press into the plated-through holes in the keyboard PCB. The portion that fits into the hole is bifurcated and springy so that it makes electrical contact. It's also possible that some contaminant has gotten onto the contact or into the plated-through hole.

If it's not a problem with the contact pin, the next likely cause is ESD damage to the ACT (processor chip). But the mechanical contact is the more likely failure mechanism.


How do you suggest that I access the area between keys and their PCB? I've noticed that they're kept together using melted plastic "sticks" (can't find the right term).

Is there a way to dismount that without making the calc unusable?


You shouldn't need to take the keyboard apart. There is no common failure mode that would be on the top side of the PCB and would affect the entire row of keys.


So... any diagnostic/advice?



There are 2 possible solutions

If there is some dirt:

1) Use an ultrasonic cleaner, rinse afterwards with clean (destilled) water, blow t dry with compressed air then let it dry fully over 1 day.

If it is a corrode contact:

Use contact spray (I normally use Contact 900) press the buttons many times, let it soak, and press again the buttons a number of times, try to wriggle the keyboard a bit etc.
Clean the keyboard again with the ultrasonic as described above.

Do NOT use the Ultrasonic to long or you will also remove the key text maybe.




As I've said, there is almost zero chance that the problem is in the keyboard itself, if it affects the entire row. It's highly likely (>90%) that it is the contacts between the keyboard and the logic board. One of the contacts is dirty or bent, or is not making contact with the plating of the hole it inserts into, or the hole is dirty.

You can test the keyboard with a multimeter, by probing the
holes for the appropriate matrix position. The information needed is on the HPCC schematics CD. I don't have my copy handy. The general approach is to set the multimeter to ohms (resistance). With the key not pressed, the resistance should be near infinite. With it pressed, it should be near zero (under 10 ohms).

Measure the contact resistance of a working switch first to verify that your test setup is working. Then if you can measure the correct resistances for the non-working keys in both the open and closed positions, that will tell you that the keyboard itself is OK, and that there is no reason to disassemble it further.

If the multimeter shows the keyboard is OK, then you need to check the inter-board contacts carefully.

I wouldn't do any ultrasonic cleaning or washing until checking with the multimeter.



Here is what I sent to Randy (FixThatCalc) to describe my problem:

None of the holes shows any sign of corrosion. If you except some cosmetic details, the functional part of the calc itself seems perfect... except the row of keys.

Now, when I try the calc outside of its chassis, simply sliding the battery pack at the right place, I notice that depending on where I touch the back solderings (on the left part of the PCB, when I face the keyboard), "2" and "3" often work, "1" and "x" being harder to "wake up".

Looking at the soldered pins and the ICs gives no clue of where the failure may reside.

Does this ring a bell to you? Especially touching the soldered part of the pins on the PCB?


That makes it sound like the problem is the pin contacts, as I've been saying all along.


Olivier, back in January, I sent you a photo so that we could identify which version of keyboard you had. I never heard back from you about this until a few days ago at which point I had forgotten about the earlier exchanges.

For the sake of those trying to help and to avoid duplication of effort, lets pick up that thread here publicly.

I've posted that photo originally sent below. I've added a small white circle and line pointing to the area that I think is the problem. I said I think it is the problem because I'm making an assumption about which version keyboard you have.

If memory serves me correctly, all 29C keyboards I've seen use snap domes formed on a plastic sheet, similar to the later 30 series keyboards. This is very different from the formed strips of the classic keyboards which were used on the early Woodstocks. The photo shows a 25 keyboard on the left with the flex strips, the keyboard on the right is a later model 29C keyboard with plastic domes. Notice that on the 25 keyboard you can see metal through the holes where the contacts are. There are no holes on the 29C keyboard due to the differences in construction.

The problem is most likely caused by a failure of the plastic heat stake(s) that hold the epoxy fiberglass part of the board against the flexing domed sheet. The heat stakes fail and the two halves loose connection(s). The area that I have circled is where the connection is made to the common of your keyboards problem row. The plate-thru hole connects to the tear-drop trace you can see through the board. The large circle is where the common of the plastic dome sheet connects to the circuit. It is only the pressure created by the surrounding heat stakes that hold them in contact. If the heat stakes fail, as is typical in that area since that is where most of the battery corrosion occurs, you're left with an intermittent keyboard.

Given the design of the molded part that has failed, there is little chance of a permanent repair. Any use of epoxies or the like will only result in frozen, non-moving keys. Small screws cannot be used as the surrounding plastic is only the diameter of the heat stake that has sheared off.

If that is indeed what has happened to your keyboard, I'm fresh out of ideas that would fix it.

Edited: 3 May 2007, 4:50 p.m.


You could verify Randy's hypotheis using the multimeter test approach I described earlier. That would tell you without a doubt whether the problem is the keyboard itself, or the interconnect between the keyboard and logic board.



First of all, accept my apologies for not having checked my mail... My iMac serves me as an info-warehouse but... I failed to drill th einfo out of it. Computers don't replace memory.

I've just checked the back of the keyboard PCB and, you're right: some heat stakes are missing, especially around keys 1,2,3.

Is there anything you can fix if I send you this one too (just to make my 19C less lonely...)? Or is a 29C keyboard a part that can be found with some patience?

Thanks and again, sorry for not checking my own archives :|


Hello Olivier,

I'll gladly send you a complete 29C keyboard (free).

Just drop me a mail.



Edited: 5 May 2007, 4:33 a.m.

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