HP-41CX halfnut speedup

I have an HP-41CX HALFNUT that I would like to modify to run at 2X (or faster) normal speed. I have seen posts on speeding up hp-41 fullnuts but not half nuts. I have successfully increased the speed of a 41CX fullnut many years ago and would now like to do the same on a spare half nut that I have.

Any help would be greatly appreciated.



I got the kit from Educalc in the 80's when they were selling it, but when I read in the instructions that you couldn't use it with HPIL or combined modules (both of which I had and used regularly), I returned it. I believe it was for both Halfnut and Coconut. (There never was a "Fullnut.") Mine is a halfnut.

Edited: 10 July 2010, 12:48 a.m.


I remember reading about the EduCalc kits long after production of the 41 had ceased - I think there were two kits available, fast and Turbo. However, I was a PPC member many years ago and read where members had increased the speed of their 41's by replacing a 140pF cap with anywhere from a 33pF to 22pF cap.
I tried this - using a 22pF cap on my first 41CX (which has the same 41C LCD display and 41C/CV/CX common logic PC board - hence the 'fullnut' reference) which I still have and all modules including x-memory, HPIL AND card reader worked (and still do, although the card reader now has a gummy wheel) OK.
I thought I would speed up a spare 41CX halfnut that I have, however, opening the machine reveals a totally different setup - there is a small PCB mounted on to the main PCB. I am assuming that any mod to increase the speed would have to be done to this small PCB....?????


Hi Theo,

I am assuming that any mod to increase the speed would have to be done to this small PCB....?????

Nope! The LC resonant circuit in on the keyboard PCB.

The main question you'll have to answer is if you want a "permanent" speed-up or a "switchable" one.

In the firt case just replace the SMD capacitor between pins 9 and 10 of the CPU (just under key [5]) with one in the range 22pF to 47pF (depending on how much "thrust" you want to make your 41 run at)

The second case is somehow more complicated since you'll need a suitable means to turn the "turbo" mod ON and OFF; and involves some type of switch and cabling for the task and, eventually, some case mods (filing, drilling, etc.) to gain acces to the switch.

Wish you the best of lucks with your project.

Cheers from Spain.


PD. Not intended to refrain your enthusiasm, but IMHO 41's were not "born to run" (even at 4x they're still pretty slow). As I said, just my opinion.


Hi Diego,
thanks for you help and advice. I think that I will leave it as it is and continue to use the one that is already modified. It must be a fluke that my 2X 41CX works with all modules and peripherals - maybe modding another might not be so lucky.

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