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HP Forum Archive 14

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HP 41CV -> HP 41CX halfnut
Message #1 Posted by Artur on 3 Mar 2005, 8:12 p.m.


I have one HP 41CV in working conditions and on 41CX not working. Both are halnut models. Will it work to transfer the small PCB that is present in 41CX model (I believe the time circuit) over the CPU to the 41CV?

Has anybody here ever tryed it? Best regards Artur

Re: HP 41CV -> HP 41CX halfnut
Message #2 Posted by Peter on 3 Mar 2005, 10:01 p.m.,
in response to message #1 by Artur


while there are many more knowledgable people out there, I rembered that I had read about this somewhere and a quick search brought back the threads below. It seems that the Giants here would suggest that it does not work, due to different "ROM codes in the display driver" between CV and CX" (though it does work for Fullnuts...)

Here are a few thread that might help you:



Re: HP 41CV -> HP 41CX halfnut
Message #3 Posted by Randy on 3 Mar 2005, 10:25 p.m.,
in response to message #2 by Peter

The daughter board in a halfnut CX contains the clock and rom. If it is working and you solder it onto a working CV, you'll have the equivalent of a CV with a time module installed, you will not have extended functions and extended memory since this is contained in the LCD driver module in a CX.

It is very hard to kill a halfnut 41. Most that have had battery leakage simply have lots of oxidation in the switches which includes the ON switch. If thats the case with yours, it is simply not able to turn on. Cleanliness in a 41 is not an option. It has to be absolutely clean inside. Any corrosion or dirt must be cleaned up.

To test, first, briefly short together the two leads of the largest capacitor to completely reset the unit. Connect the unit to a 5VDC power supply and check the current. Connect the (+) to the anode of the two diodes soldered to the top left side of the board, the (-) to the (-) of the 470uf cap (the largest one). Current should be less than one milliamp. Anything more and it might have a problem. You can try turning it on by using a small pin inserted into the ON switch from the backside, just be sure to touch the side of the plate-thru hole and switch dome at the same time. Many times a halfnut will come back to life with this treatment.

Re: HP 41CV -> HP 41CX halfnut
Message #4 Posted by Miki Mihajlovic on 3 Mar 2005, 11:49 p.m.,
in response to message #3 by Randy

If I can borrow this tread and ask a similar question (well sort of). I have a CX that I tried to fix for a friend. It has square LCD so I would say it is a fullnut, if this is a sign of a fullnut. It was working when I got it but the display was fading and there were some loose plastic parts in it (broken posts). I fixed the posts and cleaned it but when put together (I just keep the two halves thight in my hand) the LCD immediately lights up the upper half of all the segments and nothing works. This happens without pressing the ON key and the calc doesn't respond to any keypress.

Has anybody had similar problem. Can anybody give me a hint if maybe replacing the PCB or something else would revive the calc.


Re: HP 41CV -> HP 41CX halfnut
Message #5 Posted by Tony Duell on 4 Mar 2005, 6:14 a.m.,
in response to message #3 by Randy

You're right about the result (adding the daughterboard to a CV will result in a CV with time module but no extended functions), but not quite right about the hardware details.

That daughterboard contains the timer chip, time functions ROM, extended functions ROM and extended memory. The problem is that the system ROM (in the display driver hybrid) is different between the CV and CX models, and the CV version doesn't look for the extended functions ROM in page 3.

However, if you have a CX with a bad extended functions ROM, then replacing the daughterboard should fix it.

Thanks, I'll try and let you know the results!
Message #6 Posted by Artur on 4 Mar 2005, 10:34 p.m.,
in response to message #5 by Tony Duell


Re: HP 41CV -> HP 41CX halfnut finished!
Message #7 Posted by Artur on 6 Mar 2005, 12:37 a.m.,
in response to message #1 by Artur

Thanks to everybody in this thread! I removed the Time PCB from CX and reimplanted it in a CV. Now, time functions appear to work fine. I just have to try some special time funcitions from CX (I'll see manual for them),but I'm satisfied with results by now!

Thank you! Artur

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