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HP 41 Battery posts, another repair

Posted by Geoff Quickfall on 20 Apr 2009, 2:58 p.m.

I saw Nigel Bamber's posting earlier and coincidentally was in the process of repairing a CV myself.

Mine came with the x functions, x memory and a time chip with a really worn keyboard. I mean REALLY worn. Well thanks to Wlodek in England I secured a new looking keyboard with two cracked posts.

Heres how I went about the repair:

Firstly, I preassembled the new keyboard to the rest of the CV and had to use some finger pressure to ensure contact as two of the posts were cracked. I determined the 'on' rocker key was not working as well as the E+ and 1/X keys.

I removed and soaked the keyboard in a solution of liquid detergent and warm water. The keyboard was placed vertically with the LCD above the level of the water. I rinsed the keyboard with water, dilute vinegar, water and then 99% alcohol. The keyboard was placed in a drying box with the dryer set for ambient air temperature.

The calculator was again assembled and the keyboard tested perfectly for all key punches and rocker arm switch function.

Now to the posts:

Tools required:

1.  liquid plastic cement and brush.
2.  old piece of soft plastic (ex. memory module housing).
3.  exacto knife.
4.  silicon tubing or small o-rings (inner diameter smaller then
    outer diameter of post).

silicon tubing: ID 2.55 mm OD 5.60 mm Fuel line hose for model aircraft. Specify military specs as the hole is dead centre and therefore usable for pinch roller o-rings ID 2.55 mm OD 6.50 mm Hardware store.

In figure 1, you can see the reverse side of the keyboard. The two posts on the right are cracked in two places, almost opposite of each other.


Figure 2 is a close-up of a typical cracked post, usually in two places nearly opposite to each other. The correct procedure for screwing an HP screw back into it's socket is to place the screw in the hole. Then reverse the screw motion, that is unscrew the loose screw until you feel it click. Then screw gently in the correct direction for tightening. By doing this you set the threads of the screw in synch with the threads of the post thereby not cross threading.


Once the cracked posts have been identified (or stripped posts as this routine works for these) a clamping system must be formulated. I used silicon tubing, the spare tube I have for fixing pinch rollers in HP card readers. Conversly the o-rings for the same purpose, suffice to say that the tube must have an inner diameter smaller then the outer diameter of the post (Figure 3).


Since the cracks are to be repaired we can assume that the internal threads of the posts are either stripped or damaged. Every time the screw enters and is removed it grates away at the internal threads of the post removing material and leading to a stripped post. I remedied this by securing a soft plastic material in the form of memory module housing. Shave off some of the plastic using an exacto knife. When the shavings flake off some of them form a perfect circle. I inserted these into the posts along with some straight flakes. This is done before the glue is applied. (figure 4)


Here you can see a series of photos where the cracks have been clamped together by the silicon tubing or the o-rings. Once these posts are clamped, add the shavings into the post.




Using the liquid plastic cement, add a good dollop (a drop) to the inside of the clamped post with shavings added. The glue will seep into the cracks and soften the shavings. It will also bond the shavings to the posts, especially the cylindrical shavings added before hand. The clamps will insure the posts cracks have the requisite pressure exerted for glueing purposes.


Now let the post dry. I left it for a day; you certainly don't want to rush things.

As a last step I screwed the screws into the posts gently starting them as I outlined earlier. I also did this with the silicon clamps in place and the back of the case not attached. Do not screw all the way down but just start them until they bite. Since the back of the case is not in position the screws will be too long for the posts and you can damage the keyboard!

Now remove the clamps (silicon or o-rings) and replace the back. Using the screw technique mentioned above, tighten up the case.

It's that easy ;-)


Edited: 21 Apr 2009, 1:51 a.m.


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