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How to create your own HP41CL

How to create your own HP41CL

I have finished the current batch of 41CL from Systemyde. Time to reorder. Please show your interest if you would like to buy one. If not, it’ll take longer to receive the board. I am only purchasing as many boards as interest shown from customers. I will order another one this time since the damned loss of my calculator during the latest business trip. Every time I think about it, it drives me mad.

It seems that many people are attracted to this calculator, but they do not dare to assemble theirs. On one side, they may have an old HP41c laying around, but for personal reasons, they want to keep it as is; on the other hand, to spend over 100€ in a bet in Ebay (where you can end up with a non functioning calculator, or one with deficient aesthetics) is a risky proposition. And they do not feel comfortable opening their calculator to do it.

While I am very satisfied by doing this work, I would not like European calculator amateurs to be put off to buy from us to get this wonderful calculator. I encourage them to try by themselves. If they fail, they always have professional repairers that can finish off their job properly.

Just a couple of hints to a successful HP41CL conversion.

  • It is fundamental to start off a good calculator. It does not need to work - in most cases it will work; but you need to have good battery contacts. Rust in them will spread though the calculator and ruin your investment.
  • There are alternatives to repair the battery/port connection circuit. You can buy from us a self-adhesive printed circuit that will substitute your original rusting unit.
  • Be careful to clean all contacts with a good contact cleaner. I use Craig’s Deoxit D5L for a start, then rub the contacts to remove all grease and rust, and then Craig’s Deoxit Gold for further protection. (the latter enters the metal interfaces protecting them. It is widely used in the Hi-fi specialist area, when there are noise in the connections.) The contacts to clean are
    • The contacts of the new board
    • The contacts that will connect with the board

- The contacts between the battery module and the circuit itself (on both sides)

- The contacts in the ports

  • The more tricky part comes on the assembly. You need to make sure that the board presses on the connection pad below it. This is the point where most full nut calculators fail. This makes indeed buying a non-functioning full nut such an easy bet, and the reason why non-working, good condition full nuts reach 60-70 € in Ebay - it is soo easy to repair them. Coming my way, there have been a number of repaired units, where you can see different methods used by different repairers. The most common is to put some springy matter around the holes, so that the box makes pressure on them and thereon to the pad below. Other methods with good results are putting oversized thin nuts on the calculator back, around the screw posts, so that when closed they make pressure on the board (be sure to cover all metal on them so that no electrical connection can happen) Be aware: you need to put pressure on the board, not on the screw posts. These do not help in making a good connection!
  • Do not be put off if it does not work on the first time. Discard the thought of the CL board not functioning - it has never happened to me. When I had problems, it always came down to bad connection between module and pad below. Add more springy matter, check the screw posts and repair if needed (Loctite’s Superglue 3 is good if you allow it to cure properly. Good practice is to secure it with a very fine transformer copper cable around it), and reassemble it again. It will work!
  • As a result of the CL project, I have several original C and CV working circuits, that I can install in your calculator if you ever fail. But I insist: no CL board has ever failed to me.
  • Regarding the serial connector: So far, in my previous and lost CL, I had it installed but to be honest, I have never used it. It occupies a full physical port permanently and gets little use. For my next CL, I will not put it initially and will consider instead other interface options, given that I already have a HP-IL. (look forward to a HP-IL special. I am preparing a “vintage laboratory”, HP-IL based, for home, complementing the HP-IB laboratory that I already have in the cabin.)
  • You can find in Ebay lots of additional parts. You can buy from us battery holders, rubber feet sets, etc. Beware that the rubber feet, while practical and cheap, can only be used once, compared with the original rubber feet that could be used several times.

Once you have assembled your own CL, you will enjoy a sense of achievement (even though it was so easy), and admiration on how can a 30 years old machine be so well designed. As well, you will feel grateful to Monte Dalrymple and his outstanding achievement.

But if you still do not dare to do it, even after the above, please contact us at for quotes and pictures of the donor calculators. We’re more than happy to help!!! As well, we’re willing to buy your malfunctioning full nut HP41c or cv.

Now I am looking for a way to produce overlays for all the software we now have in a convenient way.

Last but not least: software. I do not know what you will fill your CL with; but I draw the Library #4 software suite from Angel Martín, including the PowerCL, Sandmath 44, advanced matrix #4, OS additions and others. Programmed mostly in Mcode, these change your calculator to a very different animal.

PS: Gardermoen airport has informed me that they have found the HP15c LE that I have lost the past week. Not all is lost! However, I had already opened another one, so I will give it to one of my children.

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Más sobre: HP calculator repair, HP41cl

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