HP 41 screw post repair

[Karel]

I finally finished repair of my HP41C calculator.

Thought someone may find helpful how I managed to repair the screw posts that had stripped threads. It may also help repairing cracked posts.

I took a piece od brass tube with id 1.7 mm and od 2.4 mm, cut four pieces under 10 mm long, cut M2 threads in them, glued them inside the posts. I had to clear the holes a bit with 2.5 mm drill bit before the tubes fit in.

I found the correct diameter brass tube on Aliexpress. I could buy SS M2 screws locally.

All the best,

Karel

[John Garza (3665)]

Excellent work!

I've come across old electronics that use the same principle; brass liner in a plastic post. An old Simpsons multimeter comes to mind. Never thought of a DIY version of the technique. This looks like a definitive answer to the long-standing problem with 41Cs. Makes them maintainable/repairable without fear of too many disassembly/reassembly cycles.

-J

[rprosperi]

(03-06-2024 02:44 PM)Karel Wrote:  I finally finished repair of my HP41C calculator.

Thought someone may find helpful how I managed to repair the screw posts that had stripped threads. It may also help repairing cracked posts.

I took a piece od brass tube with id 1.7 mm and od 2.4 mm, cut four pieces under 10 mm long, cut M2 threads in them, glued them inside the posts. I had to clear the holes a bit with 2.5 mm drill bit before the tubes fit in.

I found the correct diameter brass tube on Aliexpress. I could buy SS M2 screws locally.

All the best,

Karel

Nice idea and good photos to help explain.

When sharing something like this, it's always helpful to also share a link to the exact thing to buy. You did supply precise specs. however an exact link, it removes all doubt that it is the exact same item and that it will work.

Thanks for sharing this!

[Karel]

Thank you for your kind words.

I posted just a gist of the repair to see if it could be of some meaning for anybody.

I can write a paper about it with more photos and tricks and links if it would help. My quota of pictures is rather low. So a how-to pdf writing would be better for me.

For instance - it is rather tricky to cut threads in thin small tube without lathe or anything. I used a plastic block with 2,5 mm hole and superglued the tube in before cutting the thread. Used some heat to release the tube after. And so on.

Thanks again!

Karel

[rprosperi]

Yes, I'm sure there is interest, new techniques for restoring HP 41 machines is always of interest here.

A step-by-step PDF makes sense, its the most universal format and likely the most efficient as well.

We look forward to it, but take your time, it's better nice and later than thrown together and quick.

[aurelio]

Compliments Karel and Thank-you for sharing.
Good job and very good project!!!
All the best

[Martin Hepperle]

In the past I have used ready-made brass inserts with M2 threads and knurled outer surfaces e.g. to replace posts in HP 75 calculators.

These parts can be found im many, many variants, e.g. on Aliexpress. They are usually for being heat pressed into injection molded parts or added directly during the molding process.
There are also small boxes with an assortment of lengths which are very handy. The smallest thread size seems to be M2, the outer diameter is about 3 mm but could be reduced slightly on a lathe.

In these repairs I have removed to old, completely cracked and brittle posts completely and drilled a small hole to about 2 mm depth to screw the brass insert into the plastic base plate.
Then I applied steel filled gray epoxy to the connection. Depending on the counter-part one has to add a small M2 washer unter the screw head to provide enough surface area to distribute the pressure exerted by the screw head.

Martin

   

[Karel]

Yep. I thought about the M2 heat inserts too. But the outer diameter 3 mm put the idea off. There would be too little meat left in the posts.
You are right it can be slimmed with lathe. I do not have one.
But, maybe if I put a screw in the brass insert to have something to grab a drill chuck on and then use a file to slim the brass down.....

Thanks everyone!

Karel