how to accelerate an HP 41C???


HI Everybody,

I am looking for one "Expert" who can accelerate my HP 41C.
Does someone have still this precious knowledge.
i am ready to pay for it.


Edited: 24 Apr 2005, 4:06 p.m.


In Educalc's glory days I bought the kit called something like "Speed-It-Up" which was supposed to nearly double the speed, and read throught the papers before doing anything with the hardware. It said you shouldn't use it if you had extended memory, combined modules, or used the HPIL. I had all three, so I sent it back.

I believe it was after that that I got into some synthetic programming which made a dramatic difference in certain small portions of my programs, but I can't say the overall performance was affected a whole lot. I never did get into M-code, but that could make a huge difference in speed, depending on what you're doing. Some of the modules had functions written in assembly that were tons faster than doing the same functions in user language.


Though compared to the real Giants here I'm an amateur, yet let me try to offer some info, until they come in and fix the wrong part.

If you have a Fullnut (no rounded corners in the display) the necessary change is relativly easy to do. It involves adding a second capacitor and a switch to the timing capacitor (150pf, in parallel to coil, on various places depending on the board layout).

I;ve taken pictures of about 5 board layouts so far and marked the 150pf, yet have not found a place to publsish them (I checked out Dave's comments yet he asks, rightfully so, for pictures not too large and I had not time to reduce the size of my pictures yet. lazy bug I am and then this other thing, might be called "work" gets in the way every so often. Very annoying...) But I can email them to you if you are interested.

One needs the switch to be able to move back to normal speed to use the card reader or cassette drive. The wand on the other hand works just fine.

I have a set up at home that uses a variable capacitor and a testprogram (I think it is from a PPC or HPCC journal) to find the maximum speed a certain calc is still working perfectly. Older boards sometimes only work well until about 160% speed, while newer boards (especially Halvenuts, more to them later) work up to 270% or even more.

There is a service manual available at the DVD of TOS (please search this site for this expression to understand it). It has most/all layouts to locate the 150 pf for the halvenuts. Unfortunately it does not have the shematics for the Halvenuts.

As for halvenuts, they generally can tolerate higher speedus, yet it is a bit more tricky to perform, as the cap is a SMD and (I believe) even for the C one would have to desolder the "bridge" (I've actually never seen the pcb of a 41 C halvenut, only CV and CX), which is a delicate business in and by itself. The cap in question can be found right under the board (thanks to a description by Tony Duell...)

That's about all that I know. As for doing it - where r you located?





Many thanks for your kind answers.

Peter, if you are an amateur compared to the giants, what should I say for me?????
I have actually some halvenut and fullnut calculators (C, CV, CX).So I think we can try to modify some of them at my own risk!!!!
I am located in France and my Email is
I really thank you for your proposal and I am sure that some other fellows of our community could be interested as well.....

Again many thanks to take some time for answering to my mail despite our desesperate hard working lives.....


Have a look at this picture:

It shows pretty clearly the location (the two blue wires) of the original cap in my fullnut. The taped 'thing' contains a reed relais and two caps, but I do not recall the values. Just experiment and verify at which speeds the HP41 works reliable.

There are reports that the 41CX will not speedup as good as the original 41C. I also have a CV and CX, and I have decided NOT to do any speedup, they are too precious for me. If you want a really fast machine then use a simulator like V41.



You are too kind. I'd love to help you however I'm located in the US which would drive up the shipping costs I presume. There should be quite a few people in Europe and even in France (Emmanuel, you listening?) that could do the trick for you.
Alternativly, I might be selling a couple speed up CV's and maybe even a CX in the near future.





All HP41s -- in fact all machines based on the NUT processor -- use an LC parallel 'tank' circuit for the clock oscillator. Changing the resonant frequency of that will change the speed of the machine.

Connecting a capacitor in parallel with the existing one will slow the machine down (obviously). The normal speedup mod consists of replacing the capacitor with one of a lower value, then having a switched one in parallel with that to bring the machine back to the original speed for card operations, etc.

The HP41 service manual on TOS contains halfnut schematics (or at least it should do), but not PCB layouts. It's fairly easy to trace the PCB tracks from the pins on the NUT (the only PQFP chip on the board) to the LC circuit. That's the capacitor you want.

Halfnut C's and CV's do not have a bridgeboard. The capacitor is easy to get to on those machines. Halfnut CX's have the bridgeboard, the capacitor is under it. You will have to desolder the bridgeboard, then fiddle with the capacitor, then solder the bridgeboard back in place.


Hi Everybody,

Thank you for your help.
Peter, I know very well Emmanuel and he is very busy at this time.
Could you please send me some pictures you told me.
Again many thanks for your time dedicated to this question.

kind regards


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