HP41CX halfnut circuit diagram


Dear HP41 enthusiasts,

I am looking for the circuit diagram for the HP41CX halfnut version.

Searching an electrolythic condensor which is wired in parallel to the +UBatt terminals. I want to supply the upper housing shell of the halfnut on the workbench - without using the original battery holder

I use this setup for replacing the internal silicium battery cell from a HP41CY (HP41CX halfnut with internal RAM-Box)....

Regards - Christoph Klug


I certainly have it (I have the full 41 service manual which seems to cover all versions, fullnut and halfnut). I thought it was on TOS.

However, there are no PCB layouts in said manual (component-level repair was not supported by HP), so it's not going to be a lot of help in finding the battery connections. I have powered up a halfnut by connecting wires to the logic PCB, I seem to remember tracing the connections from the contacts for the I/O flexiprint.

It's been a long time since I did this, so check this carefully, but I seem to remember that the -ve battery contact is the -ve side of the largest capacitor on the board, and the +ve side goes to one of the connections to the 2 diodes soldered together near that capacitor.


Dear Tony,

thanks for your answer - I hoped that I get an input from your side :-)

On which page of the HP41C/CV/CX service manual I will find the wiring diagram for the 41CX halfnut ? Please give me this number - than I am able to download this page from TOS.

Regards - Christoph


I should learn to check before posting...

While the service manual is on TOS, for some reason the halfnut diagrams are missing. I am sure they are in my paper copy...

Let me check and get back to you...


Hi, Christoff;

I once dismantled an HP41CV halfnutt and powered it with an external battery arrangement. I was able to produce some good images with my scanner (had no digital camera t that time), as you can see:

If the big capacitor is the main capacitor, it is identified as a 470uF by 6 dot something volts. The DC wiring is also visible in the upper part of the bigger image (red - positive; black - negative).

I have no other picture for now, just let me know if you need some extra images.

Best regards.

Luiz (Brazil)

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