HP 21



#22

I have a HP 21 (the old one from the 70's)
It's out of battery and I want it recharged.
I don't have the original charger and was going to use another power source.
It seems (from HPmuseum) that 10 Volt AC / 1.8 VA is what I need.

Questions:
Is that right, should I really use AC not DC?
What are + and - in the little conector on the back?

Any help is apprisiated


#23

Hi!

Is the rechargeable battery pack from the 70's still OK?? I replaced mine with new accus and charge them outside the calculator.

Ciao.....Mike


#24

The battery pack is most likely original. but I cant get the the little door open. Do you think there ar any risk of damages to the calculator (or my health) ?

Do you know how to open the door - it seems to be stuck.

I'm aware that the battery pack can, with some work, be replased, but I would like to try to recharge them first...

Best regards
Martin


#25

Well, I have a charger for the HP-21. I don´t have a HP-21, so I don´t use it. It is switchable 220/110 Volt. I live in Germany. If you have a chance to pick it up....

Klaus


#26

Thanks Klaus for your most kind offer.

Considering the advice from others I fear that the accu.pack is not in good shape. I will try to open it to check that.

If the pack, against all ods, seems okay. I would like to get back to you on your offer.

Martin

#27

This Message was deleted. This empty message preserves the threading when a post with followup(s) is deleted.


#28

Hi Jeff

Thanks for the good instructions. I have, with little success, tried to open the back and remove the accu's.

Thanks to your hint about removing the rubber feet, and unscrewing and disasembeling the calculater I now got the accu.pack out, without use of force...

The accu.s are, as expected, corroded and have leaked. One of them has some small white crystals on the surface, especially arround the +pole.

I have no hope of reviving them though I have read about a electro chock treatment on HPmuseum.

Question:
Dose anyone know about a good way to clean up the cellpack, some way to dissolve the corrotion?
Would vinager be usefull here?

Best Regards
Martin

#29

If you can't take out the battery pack it may be a hint that the accus leaked and the springs are corroded. I would check first that the accus are OK _before_ applying any tension. It could cause very bad effects to your health if you damage the calculator. An HP-21 is a gem.

Ciao.....Mike

#30

The 21 does use an AC power supply. If you cannot get the battery pack out it is a very good indication that the pack has leaked. You may have to pry it out with a thin bladed screwdriver. Or try opening up the machine with the pack in it (two screws under the top two rubber feet).

If the pack has leaked, most likely you will have to clean up a lot of corrosion, etc. Vinegar and an ultrasonic cleaner help. Rinse very well with water. Shake dry. Let dry at least 24 hours (or overnight in an oven on LOW (like 140F).


#31

Hi David

Thanks for the good instructions. I have, with little success, tried to open the back and remove the accu's.

Thanks to your hint about removing the rubber feet, and unscrewing and disasembeling the calculater I now got the accu.pack out, without use of force...

The accu.s are, as expected, corroded and have leaked. One of them has some small white crystals on the surface, especially arround the +pole.

I have no hope of reviving them though I have read about a electro chock treatment on HPmuseum.

Question:
Dose anyone know about a good way to clean up the cellpack, some way to dissolve the corrotion?
Would vinager be usefull here?
What is ultrasonic cleaner - Is that a brand?

Best Regards Martin


#32

It is very easy to slice open the battery pack and replace the battery cells with new ones (use modern high capacity Nickle metal-hydride cells). Any good battery store will be able to do this for you for a very affordable price.

#33

I used to have an HP-25, same basic construction. If you cut the narrow plastic rib between the batteries you can get the batteries out. It is possible to replace them with rechargable cells, but if you don't plan on heavy use consider using Akaline. I ran mine on alkaline cells for several years. Also eleminates the charger problem. I've heard that reversing the polarity WILL cause immediate damage, so be carefull! I marked my battery holder so I wouldn't forget which way to install them.


#34

If you separate the pack at the seam as David suggests so that it can be re-celled properly, rather than cutting out the center rib (uuggh), you'll find HP permanently molded the cell polarity into the case for all to see. You can still install alkaline cells and put a small piece of tape on either side to hold the pack together.

If you recell it with rechargables, a few drops of plastic welding solvent will hold it together until the next time it needs new cells. It's then a simple matter to pry the case apart and repeat the process.

I cringe every time I see a mutilated pack missing the center rib :-(

PS: Paul is correct, reverse polarity cells kills a diode in the power supply instantly. Be careful!


Edited: 17 Nov 2004, 9:06 p.m.


#35

I have reversed polarity on these beaties more times than I can 1 + without any damage... just lucky?


#36

Lucky? Dunno - I'm 2 for 2 in killing a diode in the PS. Never have drawn out the circuit to see what was going on, I just put in a new one and try to remember positive is on the right side.

#37

I would like to make the calculator look as original as possibly, but I'm a bit in doubt here.

What, precisely, is the difference between "separate the pack at the seam" and "cutting out the center rib".

Would anybody have a photo of how this is done the good and/or the bad way.

:-) Martin

Sorry if this is clear to native english speakers, unfortunately I'm only a guest in your language...


#38

If you look closely at the battery pack you will notice a heat-sealed seam around the top edge of the pack. You can slice this seam open with a razor blade, replace the cells, and glue the seam back together again. If done carefully it will be hard to notice the repair.

The other fix is to hack out the center rib that lies between the two battery cells (on the bottom of the pack)and remove the batteries. This method is hard to disguise the repair.


#39

Sorry frinds - opening the seam seemed to provide more visible damage to the casing thean cutting out the center rib.

I have now replased the orgiginal NiCd cells with a pare of AA non-chargebel Alcaline cells.

The unit seams to work perfectly...!

Since I have no manual I might have a load of new questions now, but see you in another thread for that - A milion thanks to all of you.

:-) Martin Hvidberg

#40

Re: Your 23 Nov 2004 posting


Appreciate if you can indicate specs for the replacement batteries.

(Says NI-CD on side of the pack)

Voltage of the installed (and presumably) the repalcement bateries?


#41

Hi, Perry;

excuse me answering (or posting an aswer to) a question direct to another contributor, but I guess I can tell you about my own experience.

I bought many commercial National NiCad´s, 850mAh, and used them mostly to replace regular NiCad battery in standard HP battery packs: Classics (HP45, HP55, HP65...), Woodstocks (vintage HP2x, HP67), Spices (not so vintage HP3xE/C) and I also rebuilt a pack for an HP75D, but in this case I used a commercial, slightly modified walkie-talkie battery (pack of three regular NiCads; I had to remove a diode used for charging purposes, but the pack fit almost exactly in place). I rebuilt two pack for the HP97 (Topcat) with a couple of four NiCad cells from two mobile (cell phone) faulty battery packs. Most of the earlier cell phones used a five-cells battery pack, and in most cases, the faulty packs have only one deffective cell.

I also used regular NiCads with higher capacity, about 1100mAh, but they need extended charging time to fully charge. Because of the awkward "memory effect" observed in most NiCad batteries when not being fully charged and/or discharged, I also successfuly tested some NiMH cells. These batteries are more "well behaved" related to the memory effect. I currently use NiMH AAA cells (650 mAh) in my HP48/49 units mostly because of the power drain when communicating. In this case they are externally charged, so I always keep some ready to use spare units. The HP49G+ is the one that needs fresh cells more often.

About voltages: the NiCad units operate in the range 1.25Vcc (fully charged) to about 1.17Vcc (low-power indication) or less, depends on the voltage sensing circuit. The NiMH are close to that, and I remember reading that they actually go a bit higher. I found out later that my personal measurement instruments have not enough accuracy to indicate such difference, so I take them as close enough to work about the same.

Hope this helps a bit.

Luiz (Brazil)

#42

Just about any AA size NiCad or NiMH cells will work.I use 1800 maH NiMH cells. These are now a bit old technology and you can get up to 2400 maH cells in this size (but those are marketing department specs, not what you will get out of them in real life). To fully charge the NiMH cells you will need to keep the on the charger for at least 24-36 hours.

I have my local battery store spot weld a connecting strap between the two cells (you do not want to trust the spring alone to connect the cells electrically). You do not want the cells themselves glued together. They must be able to move up and down independently so that they can make reliable connection to the battery conatcts in the calculator.


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