Adhesive circuit for the often damaged flexible PCB in the battery bay (and I/O ports) of the HP-41c range of calculators. Shipping included in price.

Designed to be folded in shape, it has an adhesive back with a 3M Very High Bond surface.

Please looked to the attached manual.

Shipping included in price.

This piece is for repair of HP41c full nut calculator, substituting the damaged or corroded zebra connector that links the main board with the processor circuit. New design in a single piece, with 50 times more conductivity than elastomeric contacts.

Installation instructions

Open the calculator

First remove the rubber feet. To do so, use a small, flat screwdriver and introduce it beneath the rubber, taking care of pulling also the adhesive sheet that glues the rubber to the calculator. Try to make here a good job because usually you will have to reuse the rubber feet when you're done. Put the rubber feet aside so that they don't hinder the work (they have an uncanny ability to adhere to shirts, tools, calculators and eventually get lost.

Unscrew carefully and slowly the four screws beneath the rubber feet. Turn gently the screws: they operate on plastic that, after 30 years, is very brittle and fragile. It is very easy to break a screw post or "overdo" the thread.

Remove the back side of the calculator and put it aside. For comfort, remove also the intermediate piece between backside and main body. Later we will take care of putting it the right way, since it has two ways.

Remove the processor.

There are two possible cases:

• In the old calculators the processor circuit is pressed down to the main circuit by a couple of nuts screwed on the plastic posts. The post do not have any thread themselves - the thread is carved by the nut itself. These have to be removed with care because you risk to break the post, by unscrewing counterclockwise. More than a couple of in and outs and the post will break for sure. Once done, lif the circuit. 
• In the newer ones, this pressure is exerted by two cylinders around the screw holes in the back side of the case. These are not present in the old models. You just need to lift the circuit.

Once the circuit is lifted, you will see the zebra connectors. These may be of three different types (thanks to monte Dalrymple for the image)

• The left one can be reused if treated with care and not corroded - clean it with paper embedded in Caig Deoxit. However, it doesn't work in some circumstances and there is no clear reason why.
• The second is completely useless and will need to be replaced. Every time.
• The thirs type is the best of the lot and can be reused with care if not corroded. It is also sensitive to friction and can be easily damaged. Discard if corroded.

Reassembling - closing the calculator

You can now reassemble the calculator. Put the keyboard face down on the table. Then, put the intermediate piece with the open side upwards, taking care that the wider part is up, like "opening up"  (this piece has a slight angle when looked from the front of the open side). This is important for the calculator to close properly. See picture.

Then put the back side on top. You can start by any screw - I usually start by the upper ones. If everything is ok, you will be able to screw through the piece and into the calculator. Screw carefully not to break the threads. If that happens, repair will be substantially more complex and will need the upper post repair piece and some drilling.

And that's it!!

Two long screw for the lower screw posts of HP41c calculators. As they are slightly longer, they can still hold when the original screws have destroyed the thread.

The screws are too "self-threading". Made in 304 inox steel.

Two long screw for the lower screw posts of HP41c calculators. As they are slightly longer, they can still hold when the original screws have destroyed the thread.

+ two screws for the upper posts - standard size

The screws are too "self-threading". Made in 304 inox steel.

Two long screw for the lower screw posts of HP41c calculators. As they are slightly longer, they can still hold when the original screws have destroyed the thread.

The screws are too "self-threading". Black 314 Stainless steel (A4)

Set of 8 pairs of spacers for nut-screwed full-nut HP41c calculators. Substitutes the nuts that are screwed on the lower posts to secure the circuit. These nuts can only be used a couple of times before the posts are destroyed. The diameter is just right for the original posts, and the height can be chosen to adjust perfectly so that the case can be closed completely and the pressure to the zebras is transmitted efficiently.

The heights provided are: 1,2mm, 1,4mm, 1,6mm, 1,8mm, 2,0mm, 2,2mm, 2,4mm and 2,6mm

Lower Post repair

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 the pressure between both halves 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:

1. Try to avoid the torque from the screw forcing on the glue with the substrate
2. Try to ensure that the front part of the calculator is pulled up when screwing the back side on.
3. 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:

• Be sure of covering the zebra circuit part with tape -we don't want the glue to cover it.
• Cover also all nearby pin holes of the keyboard. If glue comes into one such hole, the key will not register. In severe cases, the key will be down all the time, glued in place!
• When applying pressure, use clothes' pegs to hold the piece in place, preferably on the outer side.
• 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)
• Use epoxy glue, type Araldit. 
• Let it cure overnight.
• Remove the tape over the zebra circuit paths.
• Place the zebra connectors
• Assemble the calculator and test.

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.

HP41c back port cover - 3D, new - set of 4.

Repaired back side of HP41c. It can be used in case the original backside has the screw supports broken.