HP-25C strange behaviour



#16

Hello Everybody,

I'm considering buying an HP25C with some minor (I hope) problems, I recall the topic was already discussed, and I did some digging, but couldn't find anything. Please forgive if the question has already been answered.

There are two known issues, the first is that key numer 1 doesn't allow any input, but I hope it's just a matter of cleaning the keyboard.

The second one looks more complicated. If key number 4 is pressed a number 6 will show up on the display and will be treated like that.

Does anyone has ever had this problem and know how it can be fixed ?

Thx for help, Alberto


#17

I had a similar problem with my HP-25C in which pressing the "4" key resulted in a "5" being entered. I also had a similar problem with a HP-33E. In both cases the problem was solved by chemically cleaning the keyboard by soaking in distilled vinegar, and then drying in my oven @ 150 degrees F (66 degrees C) for 3 hours. The keyboard on the HP-25C is heat-staked to the front of the calculator, so there is no direct access to the key contacts. You have to disassemble the calculator, so you can soak only the keyboard assembly. As to why it does this, I believe it's due to the one-piece film design of the keyboard snap buttons, where pushing one button can lead to actuation of a different button.

Hope this helps.

Good luck,
Michael

#18

Hello Alberto


Michael is absolutely correct, it should be a minor keyboard cleaning problem.

In the case of your #4 key, sometimes a conductive dust or battery corrosion precipitate can short the keyboard. What happens is two traces from two keys can connect and the logic input sent to the keyboard processor is recognized as the wrong key input.

In the case of your #1, either it is a non-conductive piece of debris or broken dome that creates the 'click' sound and creates the contact. The broken dome problem is not easily fixable but you can determine if the dome is broken by the sound and feel. They should be identical to the other working keys.

Another problem on the keyboard is that the keyboard processor is not processing the info due to a bad connection at the chip itself, again due to conductive debris or corrosion.

When cleaning the keyboard use water to dissolve any sugar residue such as spilt beer or wine or pop. Then rinse with Michael's vinegar solution. Then rinse with 90% alcohol. I personally use an ambient air drying box. That is, a shoe box with an hair dryer attached to one end as in the picture. The hair dryer is set to COLD causing only room temperature air to pass over the keyboard and PCB.

Shoe box:

[/pre]


In conclusion:


1. remove the PCB from the Keyboard
2. remove the LED panel from the PCB
3. soak both the PCB and the Keyboard in warm (not hot) dilute
water.
4. rinse
5. soak both PCB and Keyboard in dilute vinegar
6. rinse
7a. dry the units (I prefer ambient air temperature as my
controls are not that accurate and there is actual airflow
penetrating the units cracks and crevaces)
7b. while drying your keyboard do not wipe the front dry. Dab it
dry with a soft cloth. The Woodstock silk screening is
notorious for comming off. This is due to the Clear Laqueur
finish decomposing from UV rays over the last 30 years.
8. assemble and test.

As a last resort, but very safe, I use a citrus based adhesive remover found at your local electronics shop. I have some HP 15C that have been working perfectly (Really, perfectly) with respect to the keyboards after using the above method and finishing it with the citrus adhesive remover. It is safe for the silkscreening and internal keyboard components.

On the reverse side of the keyboard you will see holes corresponding with the centre of the keys. A quick spray of the cirtrus oil in the offending keys hole will invariably solve the contact, and key bouncing problems.


This link to an old post might be helpful.


#19

Thanks for all of you for you hints, I had a similar problem with one of my HP-25 and I have learned a lesson ! I had used a contact cleaner sparay, a 90% alcool spray and water to rinse.

As a result, most of the yellow labels simply faded away and
the structure itself has become gray.

What do you exactly mean with "distilled vinegar", is this the same we use as a salad dressing ? I'm very interested in finding a good product for soaking the keyboard without problems.

Thx for help, Alberto

P.S. I'd like to include some pics, how I'm supposed to do that ?


#20

The local store should have some white vinegar. I dilute this 50%.

Cheers


#21

Has the vinegar some cleaning properties ? Do you think it will dissolve and remove the corroded parts ?


#22

White distilled vinegar is clean from impuities and is usually reduced with water to 5% acidity. It is primarily used for cleaning purposes. For example, you can pass it through a coffee maker to clean coffee residue and remove odors. It is very good at removing copper (green) corrosion, as it reacts chemically to disolve the corrosion without damaging the underlying circuit traces.

#23

Vinegar is slightly acidic. The battery precipitate is a base. I am not talking about corrosion, but the powdery precipitate that outgassing of the batteries can cause to the circuits not directly exposed to leakage.

Water will not dissolve the precipitate which has it's own electrically conductive properties. Vinegar will. The down side is a severly corroded trace inside or under a IC may dissolve to the point of no contact. This would of course be undetectable without a logic probe.

Suffice to say that I have so far (45 calculator restorations) never had a calculator come out of the procedure worse then it went in. I have successfully resurrected many keyboards with this technique.

Cheers, Geoff

#24

Quote:
P.S. I'd like to include some pics, how I'm supposed to do that ?

Pictures are included by wrapping their location on the web as follows:

[img:http://www.picturelocation.com/mypicture.jpg]

The above requires you to have somewhere on the web to host your picture. Or you can set up a guest file here at the MoHPC, upload your pictures, then do as above.

...


#25

And here we go with the picture (not for the faint at heart)


#26

Unfortunately the white bloom is the Clear laqueur finish breaking down. This is repairable on the "classics" that allow the keyboard to come apart. The keys drop out and the calculator can be re-laqueued with success.

Unfortunately the 'woodstocks' do not allow for keyboard dis-assembly.

Cheers, Geoff


#27

I've had great success fixing this with a rag treated with the automotive vinyl and plastic restorer "Armor All". I've rubbed it on keyboards as bad as this and restored them to nearly new appearance. The treated calcs have been on display for well over five years without apparent problems. You can keep rubbing it on until it dissolves the marred finish, and I've not had this remove any painted key labels. Give it a try!*

*Best results have been obtained by sitting in bed, watching t.v. and polishing the calculator as a prized treasure. LEEEEDDD's!!!


#28

Thank you very much, that's absolutely worth a try, as far as for the yellow labels no hope I can restore them, right ?


#29

I've never tried, but am wondering if simply printing the key labels from a good scan onto adhesive paper with a color laser printer and then cutting them out might work. I've had good luck doing this to produce missing HP calculator faceplates. They look almost as good as the originals.


#30

Thank you so much for the polishing hint, I have used a teflon based product, and this is the result (to be finished yet, but wow ...), I didn't have considered the idea of using car polishing products for HP keyboards ....

As for the letters, I would think of Letraset, if size, font and colors
are available, I'll check it out and let you know.

Thank everybody for the help ! Alberto


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