With the help of Ingancio Sánchez, we have begun analyzing the failure modes of the HP41c, and we have been developing repair techniques for most of them. In the next articles, we will present the failure modes and the possible techniques in each case. 

• Corroded battery terminals
• Corroded main circuit
• overdone lower screw posts
• overdone upper screw posts
• Broken lower screw posts
• Broken upper screw posts
• Corroded or broken zebra connector
• Broken screw supports on the back side or the case (usually the upper ones)
• Missing back port
• Missing side port
• Corroded spring for battery holder
• Broken or missing battery holder
• In full nuts, overdone nuts to keep circuit pressure - an alternative to nuts.
• Broken plastic window

In next instances, we will go one by one with each of the chapters. Who knows, it may end up as a book!

Lower Post repair

We have gone through many interations, both to the pieces used and the process. We think we have found a process that is durable, easy to apply and works all the time. You don't need to have steady hands to succeed!

Material needed:

• Lower post repair piece
• Clothes' peg
• Keyboard support for clothes' pegs

Let's assume that both posts are broken or cracked. The effect of this cracking is that the screws do not do their job. Usually, using longer screws may help, but this typically is short lived since the cracks will continue downwards. Once this pressure is softened, the calculator doesn't work anymore. The solution is to repair the broken posts. 

Old methods to repair broken post is to glue them (but this doesn't hold together too long) and to tie them down with very small gauge copper cable, applying pressure so that it holds together. This may solve some cases, but not when the screw path has been destroyed - maybe due to strong tightening of the screws.

Other repair methods have been proposed, including drilling down the screw and placing in its place a cylinder with screw path, glued in place. We tried this method but there was a problem: the torque used to screw resulted in cracked glue and the cylinder separating from the substrate - every time.

Our method was devised with three goals in mind:

• Try to avoid the torque from the screw forcing on the glue with the substrate
• Try to ensure that the front part of the calculator is pulled up when screwing the back side on.
• Try to be compatible with all known zebra connectors - including our own.

The piece that we have designed requires drilling the old posts to the keyboard ciruit level, taking care of leaving the hole clear. The design on the piece is such that the pin will be glued to the inner part of the rest of the screw post; and the surface of the piece can be glued to the keyboard circuit, so that the torque on the substrate is avoided (it also helps that torque is also resisted by the other post, when using the full piece)

When gluing the piece, there are several precautions that need to be taken:

1. Be sure of covering the zebra circuit part with tape -we don't want the glue to cover it.
2. Cover also all nearby pin holes of the keuboard. If glue comes into one such hole, the key will not register. In severe cases, the key will be down all the time
3. When applying pressure, use clothes' pegs to hold the piece in place, preferably on the outer side.
4. To avoid pushing the keyboard down, and only push down the front of the calculator, use the keyboard cover we have designed to support the pegs. Pushing the keyboard instead will result on separation between both halves of the calculator, since the attachment of the substrate to the pin will be at a lower level (the keyboard is linked to the keyboard circuit, not to the substrate)
5. Use epoxy glue, type XXX
6. Let it cure overnight.
7. Remove the tape over the zebra circuit paths.
8. Assemble the calculator

We recommend to repair both posts at the same time. However, if you are confident on the resistence of the other post, you can use our 1-side piece. As it is symmetrical, it can be used for both posts, just twisting it 180º. Using the full piece, the other post side does not need to be glued - the torque resistence is done by the post itself over a much longer distance.

Alternatively, you can cut the other side of the 2 post piece with a tweezer and glue in place the rest of the piece.

Upper post repair

Same procedure. You need to use our upper post repair piece. Same precautions apply - be careful of covering with tape the zebra part; and also the pin holes of the keys, just in case.

We have already found solutions for some of the main problems in our beloved HP41c:

• The upper screw supports in the back side of the case, that are usually broken.
• The battery contacts that are usually corroded
• The zebra connectors for the full-nut models

But there was an area that we have not been able to find a solution for: the broken posts, both upper and lower. Until now.

So far we have tried withe metal or nylon posts, but the repairs never lasted too long: the torque applied by the screw on the limited adhesive area was too much when tightening the calculator case. But stay tuned for a slution that may work finally!

Most of the HP41c that we receive for conversion are missing the side cover. You can find in ebay side covers but as they are from overseas, they are quite expensive for us Europeans. And you can find some in 3D printing sites, but neither the material quality nor the design makes for a good replacement. So based in our experience with the battery-port assembly 3D printing and with a couple of new materials, we think we may get it right!

Here you can see a rendering of the peice we have designed. We have tried slightly thicker arms, since this is the breaking point, but not too much. We have ordered the first set of samples and we are eager to show you how it looks like in reality!

Some of you guys have several calculators to repair and have asked for a better price if you buy more units. Also we wanted a more convenient way of selling and producing these parts. So we decided to join them and sell them together. You can then cut them by the crosses and use them as individual pieces. Exact length of the cross is not critical.

Just wanted to tell you guys that I have fixed the article Battery-port assembly. Apparently it was configured with options, but no option was activated and it couldn't be bought!

You can find it here :Módulo conector de baterías y puertos de expansión de las HP-41c/cv/cx

Sorry for the inconveniences of those who wanted to buy it!!

Instructions for use of back side repair piece.

1. Be aware, before everything else, that the piece has a right side and a left side. This is due to the space for the tabs in the calculator that keep the battery-port assembly in place. You need to be aware of this at all times - more so if you want to cut it as explained below.

2. Decide whether you will replace both sides or just one. The decision is easy when both are broken, but if one is not, you'll have to check whether the old standing support will withstand the pressure for long. Our recommendation is to replace both sides. If later one of the sides breaks down, the repair will be much more difficult and probably less solid. And you have already paid for both sides!!

3. If you are going to replace just one side, cut with a hobbyist scissors just the side you don't need, according to picture enclosed, again, be careful of cutting the side you dont need, since the piece has a single correct orientation!. The rest of the piece will serve to increment the glued area, better align the piece in place and also to better support the battery-port assembly.

2. With a Dremel machine, file and level the area that had the support. You don't need to remove the tabs in between the two screw post, but you need to remove them if they are twisted or half-broken.

5. Put the piece in place. There is only one "right" way. The cross part is tight because it is needed to properly locate both holes at the extremes. Do not force it. Please make sure it is not bent downwards. Make sure it makes good contact. Remove the piece.

6. Put some glue in the bottom of the piece. Do not put too much or it will go out when pressed. We have used E6000 black glue with good results. Do not use cyanoacrylate glues. Put the piece back in place. Press and remove with a stick or a blade the excess glue, if any. If the screw holes are partially obstructed by glue, pass a screw in the opposite direction (to avoid separating the piece from the back side).

7. Secure the piece in place with a pair of clamps - lighter pressure like clothes' pegs will not be enough. Be careful that there is no excess glue from the other side of the case and the clamps are adhered to the piece! Leave for 24h for the glue to cure. And you're done! You have a repaired back that will withstand a lot of abuse again.

8. The piece is designed to accept both the original and the new battery-ports assembly - so you need to press it down to have it in place. The screw holes should fit a little bit tighter than in the original calculator, but should be perfectly aligned with the assemblies and the screw posts below.

Also, if you're buying from the US, it is maybe better to buy it directly from Shapeways there (just click on the link)

1. The battery-ports assembly includes the base piece and a flex circuit expertly adhered by our expert. According to him, the adherence over the nylon material used in thos piece seems to be better than on the original's material!

2. You can order the nylon piece alone at the price of 26 € each plus postage (while the finished piece includes shipment too within Europe), but then you'll have to paste yourself. My experience tells me that it is worth to leave it to the experts!

3. The battery-ports assembly fits tighter than the original piece. This is made on purpose: we have seen some instability on original pieces, so much so that it is predicated to assemble the calculator with the port covers in to make sure that the assembly doesn't move. This is not the case with the new piece.

4. The small piece used to repair the upper screw supports fits perfectly under the repair piece - it is DESIGNED to be like that. The gaps and the slots it has fit perfectly into both the original and the new assemblies. In fact, it was designed to improve it support in case the original calculator small stands are damaged

More questions? please ask to jose@thecalculatorstore.com

If you have bought a HP41C, chances are that you have found a machine that has a broken back half. Typically, this happens when the top screw supports break under the tension required to bring together the two halves, putting presure on the foam under the flex circuit. A strong pressure is required for the calculator to work properly, as all of us that have assembled calculators remember.

The problem when repairing the broken supports is that it is very difficult to paste again the broken pieces - and when you do it, they tend to break easily. When you see a completely broken post, it is easy to see that the adherence area is very small. So we decided to find a solution. We designed a piece that can be adhered over a wider surface, that can be used for both sides or one side, and that can integrate woth the original battery-port assembly or with our new 3D-printed model.

This is how a partial repair looks like: first we cut the part we need:

Then, we need to remove all rests of the original screw support with a Dremel machine or similar. Then, we put the part in place. The way it is designed, it can only be put on way and it also serves to keep it in place. Then, we paste it and keep together with a clothes peg or similar overnight. And this how it looks with the repaired piece:

The part will fit under the battery connector module. Also, the way it is designed, it helps to keep a strong connection between the module and the circuit, even if for some reason the glue doesn't hold.

Repairing the back side of your HP41c

One of the common problems that plague old HP41c is the breaking of the upper screw supports in the back side of the calculator. The lack of pressure thus causes the battery circuit to not make full contact - and the calculator doesn't work. The repair is quite difficult, since the contact area for adhering the broken plastic is very limited. We have created a piece that fits below the battery contact assemble, has enough surface to adhere to the back side, and can even substitute for the broken stands that fix the battery contact assembly:



You can remove the rests of the two screw supports of the case and glue this piece in place. If one support remains in place, you can cut the piece in half and just glue one part of the piece. Then you can put the battery assembly (original or new) on top of it, guided by the elevated parts, since it it designed to fit as below:



We will now work on the process: find the right glue, how to remove the rest of the screw stands, etc., so it is not ready for ordering yet. Please email to jose@thecalculatorstore.com if you're interested on repairing your back-side with this piece. In any case, it is a good item for your repair box!