HP-25 keyboard

[Brad Barton]

I've been lurking here for several years now, and have enjoyed the discussions on all aspects of HP calculators. However now that I've acquired some older models, I find myself in need of assistance and have finally registered.

Among my new problem-children is an HP-25. I fussed with the contacts for a while, and sent off for some new 1.2V NiCads and repaired the battery box. I finally was able to get it working, but it's not very reliable.

I must admit up front that in my excitement at my new toy, I was dumb enough to plug the unit in to the charger without a battery in place. However, I seem to have gotten away with it, because when I put my voltmeter on the charger terminals it only registers about 1.8 VAC. So hopefully a bad charger has saved the 25 from my own stupidity, and preserved the ACT chip from destruction.

The problem now is that the display acts erratically. It always displays 0.00 when I start it up, but once I enter a number the decimal points act in the opposite manner that they should. The decimal that is supposed to be illuminated is off, and all the others are on.

Also, the keyboard does not register the number 1, and also has a bit of trouble seeing a "divide" command. Plus most of the time when I press 4, I get a 6.

I'm hoping that the keyboard problems can be fixed, but there's not much information here on the dis-assembly of the Woodstock keyboards. I did read a recent thread where the author used a combination of vinegar, water then alcohol to cure a non-registering key.

Can anyone tell me if a problematic keyboard could be the source of the strange display problem? Further, are there any links you could point me to that show how to safely disassemble a Woodstock keyboard?

There appear to be at least two melted plastic studs at the upper right and upper left of the back side of the keypad that would need to be carefully drilled out. Is there a preferred method of doing this so that proper re-assembly is assured?

[Dale Reed]

Brad,

The decimal point inversion is the HP-25's way of telling you that the battery voltage is low.

As for the cleaning of the keyboard contacts, I think others here will be much more helpful than I could.

Dale

[Brad Barton]

Dale,

Thanks for the quick reply. If low voltage is the culprit, then maybe I need to do a bit more cleaning of the contacts. Probably wouldn't hurt to try inserting a few layers of aluminum foil to help things along.

I'll give those items a shot, but still not sure how to get that keyboard apart.

Thanks again,
Brad

[Harald]

Hi Brad,

you can run the calculator on normal 1.5V alkaline batteries as well. That should get rid of the low battery indication.

Disassembling the keyboard is not easy. You have to cut off the heads of the heat-stakes that hold the PCB in place. That is the easy part. Reassembling is the tricky bit. I have had success with a fine tipped soldering iron, carefully melting the heat stakes through the hole in the PCB. By pushing the soldering iron through the hole, you will get some material coming out the top. This won't be as good as new, but it works. Should you ever have to do this again, you are in trouble though, as there won't be enough material left.

Cheers,
Harald

[HAL9000]

Quote:

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You have to cut off the heads of the heat-stakes that hold the PCB in place

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Not sure if I read it somewhere (maybe Gerson Barbosa mentioned) or I just thought of, instead of cutting the heads of completely try to trim around (enough to be able to clear the hole) so you will have more material to work with. I have not done it, not sure if possible or how well it will work, just a thought. Good luck.

[Brad Barton]

It does indeed run quite a bit better with 1.5V alkaline batteries. Also, the keys that weren't registering are beginning to come around with a bit of exercise.

Since it appears that taking the keyboard apart is a bit of a pain, and the keys are beginning to register, I think I'll leave the keyboard alone for now.

I may try the vinegar/water/IPA trick as a first resort if needed.

Thanks everyone for the suggestions. With your help, I've got a somewhat functional HP-25. How cool!