HP41 Brain Transplant (CV vs CX)


I recently "inherited" a used 41CX with a sticky
keyboard and corroded battery contacts. After some
inside cleaning and soldering the battery
contacts, it seems to be almost working. It does
not recognize the Math module in any slot, though, but
it works well with the card reader and the

Well, there is still the sticky keyboard.
So I wonder if I can risk upgrading my good old 41CV
to a CX by transferring the CX's logic board into
the CV? Both models are fullnut. Any suggestions?

Sincerely, Michael


Hello, Michael;

if both 41's are fullnut, the LCD assy is exactly the same for both of them. The CX's mainboard should work fine in any C/CV keyboard/LCD, provided they are fullnuts, of course.

You should take special care with both:

1- the zebra connectors between the mainboard and the keyboard
2 - the pressure over them;

in the second case, using plastic washers will help if you build the CX's mainboard in the C/CV case and have problems switching the "franken"calc on. I do not know the thichness, but others in here will surely post the measure. The washers are required only if after rebuilding it, the calcualtor does not switch to on or freezes/shoot off by its own.

Oh, yes; if you can, keep the original CX's back plane. The CX's back label shows active ED's keys instead of regular ALPHA keyboard, shown in the C/CV's back label.


Luiz C. Vieira - Brazil


If the keyboard zebra is a single piece with round connectors, you shouldn't need the washers. If they are two separate flat pieces, the washers used by HP were 0.015" thick. These are impossible to find in the standard places. I believe they were HP custom. You can find 0.032" nylon washers of approximate ID/OD in most hardware stores. The extra thickness will result in less than perfect case tightness, but the CPU board will certainly be in positive contact. You will absolutely need longer screws (3/8") when using the washers, or you'll strip the posts out.

I have found #2 x 3/8" sheet metal screws in the lower posts without the washers will do the job. They have enough engagement to prevent striping and provide more clamping pressure.

You can find #2 hardware in hobby shops in the R/C model section in small quantities.


The 41C/CV/CX fullnut CPU boards are interchangeable. Some very early 41C CPU's for use with the LCD modules that have the square black epoxy blobs over the chips are an exception.

A little work with some UnDu adhesive "remover" and a strong thread will let you remove the front CX label and transfer it to the new machine.


Thanks all for your kind replies and suggestions!
The surgery was successful, the patient is alive
(and via card reader even got back part of its
previous personality :-) ).

Sincerely, Michael

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