HP-25 keyboard problem (4 enters 6)



This is my first post to this forum, but i've been lurking here for a while. A friend just gave me a HP-25, in great cosmetic shape. It's really the cutest calculator i've ever seen, and i'd love to get it back into service.

Unfortunately, in the years it's been stored, the battery had corroded inside the calculator creating lots of gray gunk. I scraped as much of the battery gunk as i could from the battery and the calculator, and got the battery to charge. The calculator powers up and works. However, there is a problem: when i press the digit "4", a 6 gets entered instead (the "6" key also enters a 6). Once in a while, i can get it to enter a 4, but it reverts to 6 again.

I'm having a similar problem with the "1" key: it works fine most of the time, but every once in a while, it will enter a 3 instead of 1. All other keys seem to work correctly.

My current theory is that perhaps some battery gunk has become lodged under the keyboard, but is this possible? Or it could be a short.

Other than avoiding calculations that contain the digit 4, is there anything i can do? So far i've tried shaking, tapping, coaxing and cursing, but none of these seem to have made any effect.




I agree the HP25 is a cute little calculator. Form-factor-wise, these Woodstock series calcs are really appealing to me, too. IMO, they were the best looking LED calcs ever, and mechanically rock solid.

Based on your explanation, I consider a short by some salt (caused by the battery leakage) being most probable. So, some careful cleaning may do it. Please check the archives of this nice museum for some disassembly and repair instructions of Woodstock calculators (at least one is in the articles section, too).

HTH, Walter

Edited: 3 Dec 2007, 1:02 p.m.


I doubt it's the keyboard.

Set the logic board in small jar or glass and immerse in vinegar up to the battery tabs. Let it stand for an hour or so. Rinse then soak in water for an hour. Shake the water off and place in warm area to dry for 24 hours. A small fan or airflow will speed things along. That should get rid of any crud under the IC's which could be the cause of the weird behavior.

A big caution with the 20 series units is that they can be fried electrically if you use the HP charger and the battery is bad or not making good contact. Most owners charge the batteries externally, never connecting the charger to a good unit. It is the only sure way to prevent irreversible and for all practical purposes, irreparable damage.

PS: Remove the LED array beforehand... it is not sealed.

Edited: 3 Dec 2007, 12:57 p.m.


Good to know. I got the external charger also (and an extra battery that i wasn't able to revive). I tried charging the bad battery in the calculator - i'm glad i didn't fry it.

I don't know when i'll have the time and courage to take it apart, but i'll get to it eventually. For now, i think i'll remove the battery, just in case.



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