External power for 41C


I notice that there are 2 little round gold contacts behind a small knockout in the side of the battery compartment.

Did HP make an external power supply that would contect to those contacts?

I have a 41C - with card reader, math pac and 2 memory modules.



Hi, Ivan;

I´ve been using these external connections for many years, and for me, they are a very good way to power the HP41 with external batteries. At the time I decided to use it, I cut a small piece of unused PCB and simply removed a small track of copper from the middle, to separate the poles. Then I checked the polarity and built (soldered) a small bar with standard terminals, the same used in computer mainboards. To connect the external battery pack I use a regular connector that is normally used as POWER LED (three connecting points, only two used) in standard IBM_PC compatible computers.

I'll add a picture later with polarity marks.

In time: halfnuts have the small holes, but their keyboard PCB has no matching connection points. To use these connectors in a halfnut it would be necessary to add thin, isolated connecting pads aligned with the holes.


Luiz (Brasil)

Edited: 26 Oct 2005, 9:13 p.m.


Vieira, L.C. (Brazil) wrote:
> I´ve been using these external connections for many years, and for
> me, they are a very good way to power the HP41 with external
> batteries. [...]

If you look at the HP-41C circuit diagram in the service manual (hp41srvc.pdf), pages 4-17/4-18 you will see that the J4 connector (one of the gold ball connectors) is Vbat (the other is GND).

This is not the same as the Vin connection to the +ve contact in the battery assembly.

Vin is connected to Vbat via the CR5 diode, while the J4 connector is sent directly to the Vbat bus line.

What this means is that if you use Vin and get the battery polarity wrong, you will simply lose the contents of the memory, if you use the J4 connector, you can kiss your 41 good bye.

Many years ago I asked a friend at the local HP repair shop about using the J4 - J5 connectors and he told me to stay away from them (giving me the above explanation).
He also gave me a photocopy of the service manual so that I could check the schematics. Really nice guy.



I read that HP intended to market an adapter to plug into that port but never did. That's why the gold contacts (but not the holes) disappeared on the later models. We will all regret that decision the day that N cells become unavailable. It's a good thing that Luiz has found a fix. That should hold us till "dilithium crystals" come online.


Hi all.

This may not interest anyone but...(at least it's on topic...)

Vassilis is indeed correct in that the ext. power connector goes straight to the V+ regulator input. Wrong polarity will kill the calc.

I got a really messed up 41c recently...
For experimental purposes I soldered wires direct to the pads on the keyboard pcb after removing the gold balls and springs in my 1980 vintage 41c from Ebay. The wires exit the holes where the springs and balls were. I run the darn thing from a battery pack of four AA cells. I am going to fix the hopelessly destroyed battery connector with conductive silver from a pen applicator when I feel the need. THERE'S a HOT TIP for d-i-y repairers for free...

A switching regulator using a reference zener diode does step-up regulation and one could tweak this diode down a little to ease interfacing to 5 volt logic if one desired. The factory part is a 6.2 volt zener and I should imagine (given the incredibly generous margins in the hardware) that a 5.1 volt diode would be fine.

The BAT annunciator kicks in at about 3.6 volts on the battery buss.

When picking up from ebay/wherever you may occasionally encounter really messed up machines. Suggestion: If you get them, just use a current limited power supply to check them first as that way you won't cook the switching regulator. Some of these machines come with all sorts of highly conductive contamination in addition to the usual corrosion problems.

If running the machine from a low current supply with say a clonix module or very certainly a card reader but even a timer module (remember I have very old c/cv stuff, guys) you will find the ssytem won't boot due to voltage dips on the power buss due to transient current demands in the tens of milliamp range or hundreds for the card reader. You may get very strange displays from it though...

Anyone interested in more info please feel free to write me.


Edited: 27 Oct 2005, 3:53 a.m. after one or more responses were posted


please note there's a schematic on the DVD which has a number of serious erros in it. he schematic to rely on is in the service manual. That one is correct. For more info write to me if you wish.



I don't think the N cells will disappear because they are use for a lot more things than just calculators.



I am using my hp-41c with 4 aa batteries. I just made an adaptor (using a block of wood), to replace the case of the batteries.
So, i took the original case of the batteries, put my adaptor on the back hole , and now I am powering my Hp-41c using the battery contacts, so I am still keeping the protection against incorrect polarity. I create some contacts in my wood adaptor using some small springs and some small pieces of metal. The cables between the adaptor and the 4 aa batteries are going trough the lateral hole of the calculator.

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