HP-41 CV Charger?


I've got an HP-41 CV, an i want to let it run from the power outlewt... i know i have toput a connector in de hole on the left side (with cap over it), but what foes that connector look like? Anybody got a pic? I already know ill heve to put 8V~ on it.


There are one only type of connector for the whole 41 system (82240A/B printer apart), anyhow, and regarding the 41C, earliest models had a gold plated small balls with springs that allowed the calc to be powered (directly) thru a suitable connector. Those components were removed somewhere along 1980, the corresponding PCB golden contacts remains during the Fullnut production run so, if this is your case, you can still open your calc and place a pair of apropriated sized balls and springs (I did). Halfnut design changed the keyboard PCB layout and the required pads were no longer present.

AFAIK HP did never release such a connector to the market, tough lots of users made their owns (I also did... ;-). The power thru this kind of connector MUST be 6v DC.

HP 82120A is a rechargeable battery pack that fits into the 41's N-cell battery compartment, and it's thru this pack how the calc obtains its power. Inside the 82120A are the corresponding rectifier and charging circutry that provides the 41 with the apropriate 6v DC power supply. Thus, 82120 carries the male connector where the female plug fits.

If you need further info, please post back with some details about your calc.

Kind regards from Spain.

P.S. I'll send you a picture of the HP-82066B (it's 220v, 8v AC, 3VA according to Europe's standard), you can see the connector there.


thx, i rcvd your pic, but how can you put it in the calculator? I reckon there isnt enogh space for it... :-?

Anyways, about the balls, you need to clear that up for me, i dont really understand i guess.... maybe another pic would help :)

Anyways, yuor pic for avry1 else:
<img src="http://home.hccnet.nl/orion.boot/junk/HP41_Power.jpg"> <-- if HTML-tags work ;)

Cheers, ValHallA


If you have the balls, or can find the balls, or can even use the balls (what's happened to this discussion group?!!), I can send you an article from the PPC Journal detailing how to use an even earlier HP calculator AC adapter to power the 41C via the connection Diego is talking about. You just need a stable 6VDC but this article show how you can make the connector to mate with the... balls!


Well... er... about the balls, I can swear I've got no pics of'em... Ok, just kidding, let's say first I used a pair of seteel balls out from a bearing, they are 3'15mm diameter, the springs are 8mm length x 3,25mm diameter, I know for sure as I still keep a pair of each as spares.

The main question here, prior to attepmt any task, is to determine if your 41 is a Halfnut or a Fullnut, Half's have a rounded corner bezel in the display... (Luiz, some of your great pics may help a lot in here!) while Fullnuts have a squared corner display.

If yours is a Halfnut theres nothing you can do with the "balls/spring" solution, unless you *really* want re-engineering your 41 guts (Well, not that serious but somewhat messy). If it's a Fullnut then the PPC article Ellis mentioned should be of great help.

The standard HP connector shown in the picture I sent you comes into the 82120A Battery Pack installed in the calc thru the rigth side small port. This is obviously useless if you dont have the said 82120A. Anyhow if you remove this small port cover you'll see two small holes from wich the said balls shows a bit and make their contact with the (so called) "DIY" unnofficial connector.

I'm sorry but I've got no pictures of my calc (neither... ;-) nor my camera handy.

Keep the contact and kind regards.


Why not just use the 82120 battery and charger? If you don't want the battery you can remove the battery and install in the pack regulation cicuitry to regulate the power. That way you don't have to mess with the inard of the calculator.


Hi folks;

I simply did not pay enough attention to the thread's main theme, only now I woke up for it.

I think all important issues have already been discussed here: the stable 6Vcc power supply, the (originally golden plated) connecting balls and springs, the difference between full- and half-nuts, and so.

There is only one thing I'd like to mention about Chan Tran's suggestion of using the regulator only. I thought about using the 82120A without batteries and simply plugging it in so it would rectify and stabilize the inward voltage. The only problem is that the rechargeable cells are "part" of the regulating circuit. You can remove them and the only active component that will keep voltage under the 6.2 Vcc is a single zener diode. It goes in parallel with the batteries. I'm affraid it's too much responsability for a single component. If it fails, 12Vcc through a low-value resistor (about 300 ohms) will reach the circuits...

I think I understand that Chan Tran suggests not to open the calcualtor to add balls and springs (is it correct?), and I agree with the fact that it's necessary a bit of care and skills to accomplish the related tasks. Hey, ValHallA (from Switzerland, sounds "godish"), do you have the skills to open it and try? I have some pictures that show an HP41 being disassembled. I'll find them and post them soon.

Good luck!

Luiz C. Vieira - Brazil


Yes Luiz. I suggest not to open the calc to intall balls and contacts. I think if we install a 3 pin voltage regulator and a capacitor we can be quite sure that we will get a stable supply.


Also, the internal charging circuit in the 82120A will only provide about 16 mA, not enough to run the card reader.


I've found this pic, from a later post of Luiz, that surely helps you to determine your type of calc.

This image is an original MoHPC.



Got both a halfnut (my new HP-41) and a fullnut (my old HP-41) :)

Uhm, i'm from the Netherlands by the way, ValHallA|SW is jus a bit stolen from RedHat: ValHallA is 7.2 and SW = SeaWolf=7.1, im not sure 'bout the versions ;)

Where can i find a battery with charger? ATM there arent any @ ebay I guess...

and 'bout the balls, if i'm right, you put those springs IN those holdes with a ball on top of it? :)


Right: one end of the springs relys on the keyboard PCB golden pads, the othed end pushes the small balls against the case hole, making them to rise about 0,3mm wich is enough for the appropriated sized connector to have a good electrical contact.

I'm sorry not having pictures of the "real thing", anyhow you seem to got the main idea.

Remember this is only suitable for the Fullnut!

One more thing, if you decide to face the task, remember to check the screw-post and make sure they are in good shape, as springs will require a little more strength to hold both case's halves tighten.

I agree with Luiz regarding the battery-less 82120. By the other hand regulated 78xx circuit re-design, though simple in concept, may fall far beyond the common user skills. Please ValHaLa, let's know if you feel comfortable with soldering irons and pliers in your hands, if this is the case, we may have a lot more to talk about.

Kind regards from the Canaries.



I'm comfortable with soldering irons, but i'm gonna try something else firt; i'll take onde of the battery-cases, drill a hole whare the hole in my calculator is, make some sort of connector there & connect it to a transformat. Voilá! (i hope :))

Anyhow, i'm at least /interested/ in your HP41-wirering, so please post it.



Hello, ValHallA;

just to tell you that exactly in the same direction of the square hole there is a "male-type" connector so you can directly connect an original AC adapter; this AC adapter is the one reated 8VAC, 3W, as you mentioned in your first post. I'm not sure you'll be able to drill it, otherwise you can remove the 82120A male connector, instead. I'd suggest, if you don't mind, keeping it as it is and try to build a connector to be used with the existing 82120A male connector. Just another possibility, you're the one that knows better what to be done. As you mentioned, what matters is your HP41 working.

A bit off-topic: I have many Philips tape-decks form the 70's, some of them disassembled and others ready to play/rec. They are still good equipments, and looking at their schematics, you could teach how to design basic amplifiers, pre-amplifiers, oscillators and (in some cases) power-supplies. I still have an operational N-2510, a stereo-cassette recorder that uses a motor with a mechanically-coupled generator. Bothe are built in one single piece and share the same axes. It's a piece of engineereing design. I am shure Norm would like having a look at it.

I hope you have success, my friend.

Best regards from Brazil.

Luiz C. Vieira


I have emailed the PPC Journal articles to you. They are gif files and I don't have anywhere to host them.


<getting-tired@check-one-of-my-earlyer-posts.com> :-)

I rcvd it, dont worry 'bout the size, i got DSL :) (btw, it was only 700K)

Uhm, i dont really understand it i guess.... if i understand it right, the idea (part 1) is to make a connector for those golden balls (???)
part 2 is for the charger adaptor (if i'm right)--

I guess a pic woukld help :(



you're right: these are different subjects.

The "golden balls and springs" connectros allow you to directly connect a pure, glitchless, noiseless 6VDC directly to the batteries terminals. SPECIAL care is needed here. I use a spare set of four D-size cells so when I have to read cards or use the wand I simply plug it in, regular batteries inside or not. Works great.

The 82120A rechargeable battery pack use an 8VAC, 3W AC adapter. The first thing you have is a four-diode complete-wave rectifier and an electrolytic capacitor. At its terminals you have about 18VCC. After this you have a 12VCC regulator, a 1W, 390 ohms (about that) resistor, a 6.2V zener diode and the four rechargeable batteries.

The rechargeable pack uses AC current, and you MUST use DC in the balls connectors.

Hope this clarifies a bit more.


luiz C. Vieira - Brazil


ppcv7n2p25-27 is an early article describing how to modify an AC adapter for an HP Classic calculator so it can be used to run an HP41C via the "balls connector". The pictures aren't very good but the description of how to build the plug to mate with the balls is very detailed.

pssch is just the schematic of the original Classic recharger from this article.

ppcv13n9p11-12 is a later article discussing the reason why the card reader causes alkaline N cells to have a very short life in the 41C. At the end of the article is a discussion about a possible AC adapter to run the 41C and another description of how to make a connector to mate with the balls.

82120sch2 is my schematic of my 82120A rechargeable HP41C battery pack charging circuit (I've heard there are different versions). Chan had suggested you could just use this circuit, without the NiCads, to run the calculator. It biases a 6.2V, 400 mW Zener at about 100 mW, so I think it would be reliable, but it will only provide about 16 mA which is enough to run the calculator but not the card reader (6mA, 250 mA per ppcv13n9p11a). This would only be an option for you if you had an 82120A anyway.


I must have missed something. How do the gold balls get the power from the charger plug into the calculator circuits? Do they perform the same sort of function as the gold (brass) plate does on the HP45, HP67 etc.? If so how?



the golden-platted balls are connected to the keyboard by two golden-platted springs that also keep them in place. Only the fullnuts are "balls-ready", what means they already have the (-) and (+) pads under each "ball+spring" set. You can use an external 6Vdc power supply to "feed" the circuits.

Many fullnuts and all halfnuts do hot have the balls, and no halfnut has the pads. One can use extra self-adherent copper trails and create these pads, taking care not to shorten existing trails, and connect these extra pads to the batteries connectors with extra wiring. I have not done it so far, but I have fed halfnuts under repair through external batteries set. Not hard to add.

Hope this answers your questions. If not, let us know.

Best regards.

Luiz C. Vieira - Brazil

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