HP-41CX Speedup


Is there any information out there that describes hoe this is done?
Please let me know.


Hi Jeff,

There are several ways to accomplish the speed-up (upgrade?) mods to an HP-41. Being it C/CV/CX won't make any difference, however, being it a full-nut or halfnut certainly does.

Basically, you have to reduce the time constant of the LC resonant circuit that conforms the Nut processsor clock.

Most of the common mods switched the capacitor for a smaller one. The switching method varies from a simple DIP switch (often located in the charger port hole; to a reed bulb activated by a small magnet placed "strategically". (I built the latter on my first HP-41C -quite a while ago- :-))

However, and despite the interest you can find in the speed-up mods, I wouldn't make it.

Opening a 41 is a risk in itself, as screw post tends to crack too easily... you will only get 70% faster... which,in fact, is still way too slow when compared with newest models... HP-41, is slow, ok, we know, so... let it be... ;-)

Anyhow, if you've positively decided to face the speed-up mod... feel free to ask for any detail you need... but first, you need to tell if your CX is half or full "Nut".

Best wishes.



Opening a 41 is a risk in itself, as screw post tends to crack too easily... you will only get 70% faster...
And, weren't there a lot of limitations? I ordered a speed-up kit from Educalc back in the day, and sent it back after reading the "manual" (probably just a sheet) which said you shouldn't use it if you have extended memory, HPIL, or combined modules. I had all three and used them regularly.


I have two HP-41's that have been sped-up. One I modified myself in early 1988 with a speed-up kit from Synergistic Design that I got from Edu-calc. I did it to an HP-41CX halfnut. The manual stated that the half-nuts sped up to a higher speed than the full-nuts. They advertised up to a 200% INCREASE in speed, but all I got was exactly 100% increase (twice as fast). It is switchable with a switch in the charger port.

The other speed up I have was a surprise from an ebay purchase. It is an HP-41CX fullnut, and it is sped up at about 75-80% faster than standard. I have not figured out if it is switchable, I suspect not, that it is permanently sped up. My speed-ups work with just about everything, except: My triple module conversion (HP-41 Advantage, 2 Extended Memory Modules). And Diego's No-VRAM(32). My single modules, ADV HEPAX module, single Extended Memory modules, card reader, HP-IL interface, and Infrared Printer Module all work fine at the double speed. I have not tested my Wand or 82143A printer. The documentation says to beware of module doubles and triples, and HP-IL interfaces. Port extenders are probably a problem as well. I assume that the more capacitance you add to the HP-41 bus, the more likely things won't take the higher speed.

Switchable is definitely the way to go. And double speed is certainly very nice. The thing I really like is that alpha executed functions, which can take a while for the HP-41 to locate, are found much more quickly. It is my primary HP-41, but unfortunately the keyboard has developed problems unrelated to the speed-up, and I am looking for a replacement keyboard (see Classifieds).

Side effects are that the tones and beep are all half frequency (higher tones) and duration, and catalogs can go by so fast you can't read them. And there is the smoke that starts coming out of the seams if you run it for too long (just kidding).



My speed-ups work with just about everything, except: My triple module conversion (HP-41 Advantage, 2 Extended Memory Modules)
This is kind of O.T., but is anyone combining modules anymore? I have a 41cx with a double extended memory in one module, extended I/O built into the HPIL for another module, then Advantage and ZENROM in separate modules. If I want to add anything else, can the
Advantage and ZENROM be combined?

Edited: 29 Sept 2008, 4:20 a.m.


When combining application modules, these must be forced to be addressed in a specific HP41 port. This would prevent to plug another application pac in the port that is (virtually) taken by the doubled module. So there is no real advantage to do this.

Doubling modules is only useful when the additional module is not port addressed, like extended memory.

I would especially keep away from touching the more rare modules like ZENROM. Todat there are better and more flexible ways of combining functionality, like Clonix or MLDL2000.



Hi Garth,

Though it is physically possible to combine Adv + Zeprom, this will turn up into a very restrictive module.

The reason behind this is that Advantage already uses both pages of the port it is plugged in, thus Zenrom (or whatever othe module you may want to combine with it for that matter) must be hardwired mapped to a different physical port. Therefore, you must keep the port, "occupied" by Zenrom, free or use it with a System addressed module (the double X-MEM for example).

According to the list of modules you've supplied, and assuming you need all of them plugged into your calc saving as much ports as possible; the best module combination to my mind should be:

Zenrom (usses upper page) + Ext I/O into one module. -> Port 1

Advantage + HP-IL into HP-IL housing. -> Port 3

Double X-MEM is ok as it is. -> Port 2

This will leave port 4 free, both phisical and address, to use any other module or peripheral you may need.

However, I'd rather prefer to get the original modules as they are and even remove Ext I/O from the HP-IL housing and place it back into its housing; and use a Clonix instead... and no, I'm not saying this because I build Clonix, just because, these original modules like Ext -I/O or Zenrom are rare and valuable, and I wouldn't put'em into risk by sending them to someone to open and re-solder. While you can have Adv + Zenrom + Ext-I/O into a single Clonix and still have one extra page to load any other 4k .ROM module image. Further more, you'll still have port 4 empty (only lower page) to plug yet another 4k ROM pack, or the Card Reader, or Wand...

Hope this helps.

Cheers from The Canary Islands.


Edit: Meindert, You've said it faster, clearer, and into fewer words... kudos!! ;-))

Edited: 29 Sept 2008, 5:15 a.m.


Thanks for the responses. I really have all I need for my daily uses of my 41cx, although I have had a mild interest in one of the bigger memories which could hold not only many modules' images but also have the MLDL RAM for user-written Mcode, and, I expect, more memory for the user's data. I have half-heartedly been watching the unmentionable auction site for a dead module or card reader to use the shell and connector for the Clonix or MLDL2000 or even building my own I/O that would bypass HPIL. I know I would have to get more serious about learning the 41's insides-- Mcode, memory maps, etc..

Diego, as I was reading your page at http://www.clonix41.org/Projects/Clonix-41/Cloni41_00.htm , I see it ends in the middle of a sentence. What happened to the rest of it?

You mention the high value of the ZENROM which I bought on sale for $25 US when it was being discontinued. I have been in awe of the prices my 41 and 71 and their modules and peripherals have been going for on the market today-- thousands of dollars if I total it all up. BTW, I bought the HPIL and XIO combined module that way from EduCalc. I never saw them as separate modules.


Hi Garth,

It's a nice while since I wrote this *unended* page. I'll try my best to have it completed as well as "corrected" (I know my English is not *very good*... but certainly not *that* bad.. :-))

About the Zenrom, I consider "replacement value", as the one you have to pay to recover a missing good... whatever it was.

Zenrom "replacement value" may well be above $200, that's why I said it is valuable.

Many of us will also agree that Ext-I/O should have been included into HP-IL enclosure at HP factory... alas, they're separated.

Best wishes from Spain.



I have a triple module conversion, an Advantage Module combined with 2 Extended Memory modules. Educalc sold the conversion service (SOS, I believe did it) around 1987-88. That little unit cost me a total of about $225 back at that time (around $450 in today's dollars)! I have used it for many, many years. The downsides are:

1. I guess it was a tight fit inside the module, as the case came out a little too big, and is difficult to get out of the HP-41.
2. The extended memory does not work in my speeded up HP-41. Fortunately I have a switch that lets it run at normal speed if I need to access Xmem (and the unit knows there are two Xmem modules when it is running at 2X, so it does not try to clear the extra memory when running at high speed)
3. The 'door handle' was rendered inoperable; the handle is there, but it does not slide out.
4. The little sticker that they used to cover up 'HP-41 Advantage' with, that read something like '2XMEM Advan' fell off a long, long time ago.

I also recently obtained a double module (2 Extended Memory) in an ebay sale. I have tested it in my sped-up HP-41 and it works well. It also fits real well in the HP-41; the module itself looks as good as it came from the factory. The module is not labeled a double in anyway (maybe a sticky label fell off), and the door has also been rendered inop, but since it slides in and out like it is supposed too, it's no big deal.

I think an HP-IL and Extended I/O combination would be a good one to have.



I must confess I haven't really fiddle too much with the speed-up setting.

By the time I made the mods I only have a barebones HP-41C + Quad Memory, I also iserted that Quad memory into 41's guts in the same "surgery", and the whole thing works as espected. But have never tried other modules except Clonix-41 (configured with Clonix6P) in order to make sure it works with speeded-up 41's.

However, I do remember reading some reports form the "old-good-days" revealing several drawbacks, limitations and uncompatibilities of turbo mods with different modules and/or peripherals.

Kind regrds.



Thanks for the responses. I have a 41 Full and Half nut both. I guess that I was curious what the pros and cons would be. It looks like this would end up being a negative. I use the MLDL2000 and occaisionally HP-IL to PC. I suppose this would not work that well.


I agree with Diego. I have one 2*& speedup 41C (I did that myself back in the old days), but I will not open my other machines unless I must.

The switching between speeds was only needed for the cardreader (and then only when writing cards I think). The MLDL2000 will run just fine. The tools I use say that I could run the MLDL2000 up to about 8 MHz, while the HP41 clock is around 500 kHz (at about 2* speedup).




Whats the difference between fullnut and halvenut other than the location of the capacitor in question?


(I think I have copied most of the speed-up articles from PPC, CHHU, etc into a little stack and could probably try to locate that stack and make a pdf if that is helpful to some)



According to the information that I have, a full nut conversion could be installed much simpler, without significant de-soldering/soldering. If I remember correctly, my half-nut had a 'bridge' board soldered over the main board that had to be removed to get to the capacitor, I think I had to de-solder about 20 pins to get it off, and then solder them all back on afterwards. Not a task for someone who is not comfortable soldering.

Once that board was off, the existing capacitor was cut off (I still have my original!) and replaced with a circuit that was also connected to battery power, that would switch between two capacitances to change the processor speed at will, even while a program is running.


I once had a 41C with switchable speedup. It used a switch to activate a second capacitor, like described in the posts above. The drawback is that switching while running a program results in "memory lost". I suspect that switching in a different coil in the LC-circuit might do better, so that it won't produce a memory reset.


If the speed is governed by an RC, it's always better to swith the "R" in and out, not the "C", since the latter produces glitches depending on the charge of the second capacitor at the time it is switched in, and those glitches can crash the thing if it's running at the time. If there's bounce in the switch, you'll get a whole string of glitches.


Hi Peter,

As pointed out above in this thread, the main difference is that half-nut allows higher clock rates (suppossedly up to 4x) and there is plenty of space to place the required switch allowing even several "shifts". Half-Nut CX required bridge board to be removed, thus the operation is somehow more complicated.

I also remember reading of a "soft-driven" speed-up (but I cannot trust my memory too much on that... :-))

All in all, the speed-up mod is sort of a try/error process as not every nut processor can handle the same top clock speed.



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