Internal User Modifications

Any of you guys know of or tried to modify the hardware?

I did the double speed mod to the 41CV.

CalcPro is advertising an internal memory upgrade for the 48GX.

We tried to hardwire ROM chips directly into the INSIDE of the 41CX (failed)

Any other mods tried?

John Kercheval


I have doubled HP-41 modules within the module cases. These are described by David (Ebras-) White in the PPC Journal under a series of articles titled "Butcher's Block".

I have also done the internal speedup on the HP-41. I did these circa 1984, and have not taken the time to do another one. I could find the articles if needed ... they are easy to follow.


The speedups only involve the replacement of one capacitor.

Sadly, they must be done on older machines as they newer ones had a blob of epoxy on the to prevent tinkering.

John Kercheval


I have opened up well over 100 HP41 machines of all types and ages. I have never seen one with the timing capacitor epoxied over.


I never did the speedup on my CX, but I'm actually
thinking that perhaps I should do it now...

However, I have the ZenROM and two X-mem modules
built into my 41CX.

Have worked like a charm for 10+ years now.


I've only looked inside one Halfnut, and it
was probably 15 years ago, but if I remember
correctly, the Halfnut don't have the same
design for the clock. I remember not finding
those capacitors at all. Either with or
without epoxy on them.

The fullnut have them visibly enough. No problems

However, I no longer remember which one to change,
and to what.
Anyone care to remind me?

Actually, if I remember correctly, the inside of the
halfnut turned out to be very different from the
fullnut (I was comparing two 41CX).


I have not done that mod in about 18 years.

But I'm sure someone on here has the instructions.

We could modify CV's easily enough but all of the CX's we tried had a blob of epoxy over the circuitry. We looked into modifying about 5 or 6 of them and all had it. However there may be those that do not.

Good luck & let me know how it works out.



The halfnut has an SMD capacitor that you can swap using the same rules as the NUT. If you don't use the card reader or the IL, you can remove the cap altogether and your speed will be 3x. Most units will work well with 80% increase (33pF).



I did not know you could do that. Without the capacitor at ALL did you experience crashes or lockups?


Apparently the halfnut IC and the PCB had gone through enough techno iterations to not be bothered and - of course - the location of PCB tracks close enough and long anough to act as a capacitor.

As far as I remeber the original cap was 150pF at 1x the 33pF at 1.8x and the wires must be but a couple of pF.

No no crash, no lockups, no nothing - except a lot of battery consumption (probably part of the original design tradeoffs)



I thought internal user modifications were best
left to an MD... I certainly wouldn't want to put
my hands into a user that lightly.



Okay, I have my CX open now.
No epoxy, no nothing. Now, which capacitor should I
change (and to what)?

I see three:

1 470 uF (probably to maintain power when battery is changed).

1 grey with onknown value. Looks like normal electrolyt.

1 blue drop capacitor. Marked "33+ 10F" if I manage
to read it right.

If I remember correctly, it's one of the three in the
upper left corner of the small PCB with the CPU
and memory stuff.


Hi Johnny,

After all this thread and as interested in speed up my 41CX, could you tell me how can I do this? I've tried to find out any instruction here but unsuccessful. My 41CX is of 1987 and have reduced electronics.

Thanks All!



Unfortunately I don't know much about the newer
halfnut hardware.
Until recently I was under the impression that you
could not speed it up at all, unlike the older hardware.

Someone here claimed otherwise though. I hope he
can provide you with more details on how to do it.

For the older hardware, the speedup is achieved by
just changing a capacitor. I'm just trying to figure
out which one. Most of my friends did the speedup
(I can't even remember why I didn't), but this was
in the mid '80s, and memory have faded some, so
now I don't remember which capacitor... :-/


OK, it seems like everyone has some level of amnesia, after all this modification has not been in fashion for about a generation.

Older NUTs

One capacitor, usually green (very old red), resembling something like 150pF. That's the one (LC curcuit)

Half NUTs (what an expression)

the Cap is a TINY cheramic SMD component (20 mil by 30) located (as far as I recall) on the lower left of the CPU chip (could be epoxy covered) around 7 o'clock. Again some sort of indication should be present that this is 150pF.


You are on you own, this could Zap the MOS circuits (Might require the earlier indicated MD). I personally changed speed on about 500 units some 10-12 years ago (commercial approach) and NEVER zapped one unit. But I used all the Bells and Whistles in order to protect against ESD.



Let me know if I should take this by email instead.

I don't see either a green or red capacitor. :-/

Holding the CPU card with the large 470uF capacitor
at the top left corner I have two other capacitors
right below it. One grey and one blue. We're talking
around 10 to 11 o'clock here. I was under the impression
that it was (probably) the blue that I should change,
but the markings on it have me confused.

It was a long time ago, but I seem to remember that
my CX looks pretty much exactly like a CV in this area.

The only green component I have is a small thing under
the piezo speaker. Looks slightly like a diode, and I
cannot read the markings on it.

Is *that* the capacitor to change?


The capacitor can be in different places depending upon the board revision. It is in parallel with an inductor that has a very low (less than 50 ohm) resistance. If you have an ohm meter you can try buzzin around for the cap with the inductor across it. On all the full nut boards that I have seen it is a light blue-green color with a value of 150 pF (usually inicated by the numeric code 151, but sometimes 150)


Ah. Silly me, the grey unidentified thing was the
inductor. That was pretty obvious (and approx 4 ohm).

But I cannot find any capacitor in paralell with it.

I found one capacitor the the marking "151K" but
that one is definitely not in paralell with the
inductor. :-/

This is slightly silly. Unfortunately my digital
camera is in for repairs, so I cannot take a shot
at what I have right now either...

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