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HP-25C bad keyboard -- repairable?
11-07-2018, 04:39 PM
Post: #1
HP-25C bad keyboard -- repairable?
My HP-25C died many years ago but I've kept it around. I have a vague recollection of it developing a bad key and that teenaged me took it apart and the darn thing never worked again.

Anyway, I recently found this forum and discovered the possibility of breathing new life into my old calculator, so I dug it out and opened it up to see what condition it was in, particularly the keyboard.

Turns out it's worse than I remembered. I found a half dozen keys that make intermittent or no contact when pressed.

Is the keyboard repairable? If so, what's the process?
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11-07-2018, 11:24 PM
Post: #2
RE: HP-25C bad keyboard -- repairable?
(11-07-2018 04:39 PM)mpark Wrote:  My HP-25C died many years ago but I've kept it around. I have a vague recollection of it developing a bad key and that teenaged me took it apart and the darn thing never worked again.

Anyway, I recently found this forum and discovered the possibility of breathing new life into my old calculator, so I dug it out and opened it up to see what condition it was in, particularly the keyboard.

Turns out it's worse than I remembered. I found a half dozen keys that make intermittent or no contact when pressed.

Is the keyboard repairable? If so, what's the process?

Depends what is wrong.

Broken keys, broken spring contacts, dirty contacts, corroded and possibly broken PCB tracks from battery leakage (blue stuff on tracks) or faulty chips.

Corrosion is fairly easy to spot, and a careful cleaning is a good place to start.

The chips are sensitive to static electricity, so care needs to be taken with circuit board handling to avoid damage.

cheers

Tony
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11-08-2018, 03:09 AM
Post: #3
RE: HP-25C bad keyboard -- repairable?
(11-07-2018 11:24 PM)teenix Wrote:  
(11-07-2018 04:39 PM)mpark Wrote:  ...

Depends what is wrong.

Broken keys, broken spring contacts, dirty contacts, corroded and possibly broken PCB tracks from battery leakage (blue stuff on tracks) or faulty chips.

Corrosion is fairly easy to spot, and a careful cleaning is a good place to start.

The chips are sensitive to static electricity, so care needs to be taken with circuit board handling to avoid damage.

cheers

Tony

Thanks.

At this point I'm focusing on the keyboard itself. To be a little clearer, I used my multimeter to check continuity through the switches. I don't know why these seven keys are dead or unreliable, but I imagine if there's any hope of repairing them, I'll have to somehow separate the keyboard PCB from the keys and top plate.

Or maybe it just needs some compressed air to blow out some dust? Are the contacts sealed?

Any hints on how to undo the dozens of plastic rivets apparently holding the keyboard assembly together?
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