Opening (and repairing) a halfnut

Hi, HP41 maniacs.

I'm opening an HP41CV halfnut because it has a problem I cannot repair without spreading its guts around: after a MEMORY LOST and SIZE 000, it gives me .END. REG 63 instead of .END. REG 319. I did not test regular memory modules (82106A) so far, but I'll do it later. I read in some posts (some time ago) that RAM chips are in the LCD assy, and I'm gonna open it. I'm scanning all intermediate images so far, and I found out two somewhat important facts:

1 - tactile sense in halfnuts is solely due to a thicker rubber sheet between the keyboard and the keys; parts used in both half- and fullnut keyboards, i.e., the metal caps, are essentially the same;

2 - based on the ones I have in hands, calculator's plastic case has the same geometry for both types, meaning you can remove the halfnut guts and build a complete fullnut in the same halfnut case. I removed the halfnut guts and tested a fullnut keyboard. Fits perfectly. (in fact, after testing, I could not identify which case was the original one except for a missing post in the fullnut's one)

I am now with a strange thinking: I have an HP41C's case and a fullunt's guts that acts like an HP41C. I'm tempted to keep the halfnut's guts as is and build it as an HP41C. And I'll have a genuine... O.K., O.K., a cheated HP41C fullnut. In any case, I'll open the LCD assy in order to check for the cause of the missing 256 registers. As the plastic rivets were untouched, it's definitely a problem to be solved instead of an original HP41C halfnut in an HP41CV's case.

As I have all images scanned and saved, I'm posting a new message.

Suggestions? Comments?

Luiz C. Vieira - Brazil



just to add an information.

Although the halfnut guts fits in a fullnut's halfcase, there are 7 (seven) plastic rivets that must be removed from the fullnut's case because there are no equivalent holes in the halfnut's mainboard for them. Tha mainboard fits in, but cannot be settle unless they are removed.

When I post the images, I'll indicate which rivets are the ones to be rmoved in this case.

Best regards.

Luiz C. Vieira - Brazil


It is very interesting to hear about the difference in keyboards; I have often wondered if the difference in feel of the case, and the creaking of a halfnut, was due to a poorer grade plastic. Or is it the construction?

Anyway I am tempted to send you my halfnut 'as new' CX and ask you to upgrade it to to old fullnut keyboard and display for me!


Hi, HP41 maniacs;

I have concluded my incursion into the inner guts of the HP41 halfnut. I scanned some interesting sites, mostly the hybrid RAM/ROM Display Driver, from what seems to come the Voyager R2D2 + uP. Yes, believe me, I removed the display driver IC in search for a failing jumper, connection or the like, but I was not successful.

Even after being completely disassembled and reassembled, the halfnut insists on seeing only 64 registers instead of the expected 320. I also compared the hybrid code to another existing 41CV halfnut and the codes match. What to do?

I already built the mainboard in a former HP41C fullnut case. Yes, it fits fine. I had to remove eight (not seven as I thought) of the small plastic posts (rivets) used to settle the mainboard in the upper half case. Now I have a custom HP41C halfnut.

I'll post images as soon as the webmaster explains to me why can't I access the page where my previous images were loaded. I cannot even open the page, not necessary to say I cannot upload the images.

I'll take a little time to treat the images and prepare the one showing which plastic rivets must be removed in a fullnut's case in order to accept a halfnut's internals. The opposite operation - using a halfnut case to build a fullnut internal - needs no further operations; simply stuff it in.

I hope the images to be posted and the information herein help others to handle the halfnuts internals.

Best regards.

Luiz C. Vieira - Brazil


Hi, Gordon;

thank you for your always valuable and supporting comments.

But I have to warn you that the inner guts cannot be transplanted. The fullnut's LCD cannot be used in a halfnut mainboard (at least I did not try to...). What I did was using a fullnut's upper halfcase to hold a halfnut internal. But it must be a complete halfnut internal. What can be done is replacing the thicker halfnut rubber sheet, under the keys, for the fullnut's. I tested and the key feeling of the fullnut is kept: it feels as if you are using a fullnut.

I promise I'll go deeper into the LCD's compatibility. I did not disassemble any fullnut LCD so far, but it seems to me there is one malfunctioning LCD comming closer to my hands, and if I need to, I'll open it and compare it to the halfnut. I'll let you know...

Best regards.

Luiz C. Vieira - Brazil

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