The Museum of HP Calculators

HP Forum Archive 11

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HP-25
Message #1 Posted by Michel Beaulieu on 19 Mar 2003, 12:23 p.m.

I know it's a quite weel known problem but can someone tell me the "procedure" to get alive a beautiful but dead HP-25. Where to put my multimeter to point the problem?

Thnk you for answers!

MB

      
Places to start...
Message #2 Posted by Jeremy on 19 Mar 2003, 1:08 p.m.,
in response to message #1 by Michel Beaulieu

1. Check to make sure that the battery terminals are free of corrosion. If they have corrosion on them, get the big chunks off by chipping at it with a screwdriver. Once that is done, get the smaller chunks off by any means you can find that won't damage the terminals. You will have to take the calculator apart if the corrosion has reached inside the battery compartment.

2. The next thing I would check is the power switch. On the hp45 of mine that I just worked on, the power switch was amazingly dirty, but it still worked. When you take the chassis of the calculator apart, a little brass spring contact thingy should come loose. Clean that with rubbing alcohol and soft clean cloth until black junk stops coming off.

3. Look closely for any connections that might have been broken.

It is possible that someone tried to power it up with a charger and either no battery or a dead battery and that the damage might be pretty extensive.

Last, but certainly not least, check the page of this site dedicated to fixing classic HP calculators. Rather than just promote laziness and give you a good link, I will just tell you to go the the MoHPC main page, and look to the right. Under the heading 'Collecting and Using...' you will see a link for 'Repairs & Batteries'. Click that and find the option called 'How to Fix Classic Models' A lot of that information applies to fixing card readers, so it won't apply. But much of the information is pretty good!

Let us know how you make out.

-Jeremy

            
WOW, but...
Message #3 Posted by Michel Beaulieu on 19 Mar 2003, 5:52 p.m.,
in response to message #2 by Jeremy

Wow Thanks, it works! The switch is not working so i bypass it and i get it to work!

But now how can i "open" the keyboard to clean the switch? it seems to be "plastic soldering" in place like my HP-41. I never try to open my 41 keyboard but i need to open this 25 : how to do it safely?

I have another problem : Teh key are weel mark but the "faceplate" where the sub functions are written is completely erase : where is the square root, the SIN, LOG etc... Was there a removable faceplate on these calculator or how can all these writing completely erase out - the rest of the calculator seem to be in nice cosmetic condition...

Michel

                  
Ok for the picture :-)
Message #4 Posted by Michel Beaulieu on 19 Mar 2003, 6:04 p.m.,
in response to message #3 by Michel Beaulieu

I found a picture on the site... So just answer my keyboard dissambley questions :-)

Thanks again!

                  
Re: WOW, but...
Message #5 Posted by Jeremy on 19 Mar 2003, 7:58 p.m.,
in response to message #3 by Michel Beaulieu

"But now how can i "open" the keyboard to clean the switch? it seems to be "plastic soldering" in place like my HP-41. I never try to open my 41 keyboard but i need to open this 25 : how to do it safely? "

Be warned, this information is completely from memory and is assuming that the HP25 is similar to the HP45 that I have. So work at your own risk ;)

There are 6 or 8 screws that hold the whole circuit board assembly to the front half of the calculator molding. If you take these screws out, it should easily just lift out. When *I* did that, the springy brass contact that completed the power circuit just fell right out. At the time, I didn't know what it was, I had to look around for a bit to figure it out. At that point, you will see that the formerly shiny contact points on the springy brass contact and the contact points on the circuit board are completely dirty instead of nice and shiny and clean. So then, you can just clean it off with rubbing alcohol and a clean, lint-free cloth. (I used an old clean sock) I just put my finger in the sock, dip it in rubbing alcohol and rub it until it's pretty clean. Then, put it all back together and away you go!

While I had mine apart, I also replaced the plastic sheet that keeps the keypad contacts relatively sealed, as it was cashed. I just used a plastic wrapper that was used to package some Radio Shack battery connectors I bought and trimmed it to size.

Using an old toothbrush, (dry) I also cleaned out the inside of the enclosure and the keyboard. It is amazing how much junk accumulates in the corners in 30 years...

It was great fun to work on that. It was designed with technicians in mind and it shows. If the cicuitry was fried, I don't know that I would be able to track it down, but many times it is something much simpler. Those simple things can intimidate lots of people and they shouldn't.

Good luck.

-Jeremy

                  
Re: WOW, but...
Message #6 Posted by David Smith on 20 Mar 2003, 11:32 a.m.,
in response to message #3 by Michel Beaulieu

The HP25 keyboard cannot be disassembled without causing a lot of damage. It is held down by a zillion heat staked melted plastic posts. If you look near the top of the keyboard while you flip the switch, you can see a couple of holes that are under the switch contacts. You may be able to fix the problem by spraying some contact cleaner there and working the switch a bit. If the cleaner leaks out, it may stain the keyboard (depending upon type). You might also try working a few bristles from a wire brush in the holes.


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