HP41CV Turbo Version- Modified Calcs



While I rarely use it, I did in fact do the double speed modification to my college issue HP41CV. I did not, in point of fact insert an actual turbocharger or intercooler to achieve this.

Anyone else do this?

Anyne else perform non-stock modifications to ANY hp machines?

All stories welcome!

John Kercheval
Georgetown University


I have a HP 48GX with the 'Digitalis' speedup modification. It supposedly increases the speed of the GX from 4Mhz to 6Mhz.

You can turn the 'turbo' on by keeping the ON key pressed in for 2 seconds when turning it on. Otherwise it acts as a normal GX.

Unfortunately, the speedup also messes up the clock settings because 1 second isn't 1 second anymore!!



Really? Is that true? How can I test that?

After you turn it off, does it slow down again? In other words, do you need to hold down the ON key for 2 seconds EVERY time you start it up?



At least on my 48G+ with speedup,

you have to hold the ON key for about a second

to signal the calc to switch to turbo mode.

For normal usage, just press the ON key as usual.

When turning off the calc, it will revert to

normal mode automatically.

There are easy ways to determine if turbo mode is active or not:

Just press an empty menu key, or press SWAP (right arrow)

while there's only one or no object on the stack.

The resulting beep will have a much higher frequency

than normal if the calc is in turbo mode.

Most of the time normal mode will be sufficient,

at least for me, since I made my own software speedup,

which also accelerates input forms and choose boxes;-)




I tried this with my 48GX (lux version) and I get no speed improvement. Does it not work on this model?

JK in DC


I modified one of my 41Cs. I somehow managed it to use the old capacitor, and using a switch to enable another capacitor. The switch sits where the charger plugs in.
Unfortunately the switch gets sometimes set when pulling the calc out of its pouch. You can switch back when the calc is on (but idle), but when I switch back during a calculation, I got MEMORY LOST!

The switch selects normal speed or 1.4*normal

Overclocking comes in handy when keying in alpha data and runing programs, and I would recommend overclocking moderately without the possibility of switching to normal speed - as the CPU is mostly sleeping during calculations, the battery life will not be greatly shortened!

Long live the 41C!


Instead of switching another capacitor in or out during operation, it would be better to switch another resistor in or out of the "R" part of the RC so you don't get the capacitor-charging glitches in the clock circuit. I did this in a product we marketed in the late 1980's. The clock normally ran at 200kHz to save battery power; but if there was a long set of calculations to do, the processor turned on a transistor that was in series with the resistor so the frequency would be bumped up to 1MHz to keep the customer from waiting. When the calculations were done, the transistor was turned off again, so the capacitor took longer to charge and discharge, resulting in a lower frequency. I don't know what's accessible in the 41, but I expect something like this could be done.


You are right, a resistor is definitely a better solution. The logic board of the Fullnut 41Cs offers enough room for a resistor. But since opening and closing the Case breaks out the screwholes, I will leave it as it is.
By the way, I have read some old issues of the PPC journal, and at that time, most people were complaining about the build-quality of the 41C and the software bugs. Somehow reminds me of the 33S today.......


Can you provide any info on those products? Can anyone provide any more info on the different versions of the HP-41 machines along with their speed?
Thanks a lot,

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