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Repair of HP41CX - Printable Version

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Repair of HP41CX - Nenad (Croatia) - 08-29-2001

During the repair of my HP41CX I have found that the lower left screw post has probably defect thread, as the screw rotates but it does not tighten the back cover any more. This causes operating problems (intermittent display, with nonsense in it). The thread was probably damaged during the previous repair (not performed by me), which had obviously failed. The screw post is NOT broken.

Can anybody help me with an information how to repair this.
TIA


Re: Repair of HP41CX - Vieira, Luiz C. (Brazil) - 08-29-2001

READ IT CAREFULLY, PLEASE!

THE PROCEDURE BELOW SHOULD NOT BE PERFORMED IF YOU HAVE ANY DOUBTS ABOUT HOW TO DO IT. PLEASE, ASSUME YOUR OWN RISK AND TRY THE EXPERIMENT IN SOME OTHER DISPOSABLE MATERIAL BEFORE TRYING IN YOUR CALCULATOR. I ALSO SUGGEST WAITING FOR SOME OTHER SOLUTIONS.

I have once tried, with success, an epoxy cement to fill-in the defective thread (here in Brazil we have a so-called Durepoxy; I'm not sure this is the same you will find in Croatia). Try this:

- Open the calculator, remove the motherboard, I/O battery contacts assy, battery holder and screws and keep them in a safe place.

- Mix a bit of both parts of the epoxy cement and fill in the defect thread.

- Before the epoxy cement dries out (try sensing with your finger), you should insert the screw with the screw driver carefully, so it will rebuilt the thread (I have 'moistened' the screw with liquid soap and dried it out with a hair drier before insertion, so it would have a thin soap layer to avoid the epoxy cement to mess around the screw itself).

- Wait for one-two minutes and unscrew it with the same care you had to screw it in.

Then its just wait till the epoxy is dried.

This procedure worked fine for me. If there is another, better solution, please let me also know about it.

You will need special care with the epoxy cement so it will not smear over the keyboard's back side.

Good luck.


Re: Repair of HP41CX - David Smith - 08-30-2001

The epoxy method mentioned above can work but most epoxies do not like to bond well to the plastic used in the HP cases.

The technique that I use is to shave a few slivers of plastic from the bottom of the I/O contact assembly (the gold foil connector to the module ports). Just snap the assembly out of the bottom of the case and use a razor blade to shave the slivers off the bottom edge corners of the assembly. The slivers do not need to be any longer than the depth of the screw post holes. You will need around 4 slivers.

Place the slivers in the stripped hole. Then place a drop of plastic welding solvent in the hole (it is just methylene chloride (dichlormethane) and is available in many hobby shops in small bottles). It works by disolving/melting most plastics. Using a small stiff wire/drill bit/pin swirl the solvent around in the hole for a few seconds. Remove the wire (be careful to not extract the slivers at the same time). Let the solvent set up... I prefer wo wait 48 hours... overnight if you must be in a hurry.

This will rebuild the inner walls of the hole. You should then be able to reassmble the calculator. The HP screws are self-tapping and will cut new threads. Be careful not to strip your newly finished hole.


Re: Repair of HP41CX - Nenad (Croatia) - 08-31-2001

Thank you both (Luiz & David) for your information. As the method with dichlormethane and slivers looks a bit les complicated, I will try this, but I am not sure if I have understood everything properly:

1) May I cut the slivers from inside of a battery pack cover, or module slot covers. How thick should they be?

2) Shall the slivers fill the complete bolt hole? Is some kind of a void space in the middle needed?

3) Will at the end (before screwing the bolts) the hole be completely filled with plastic (arising from slivers and dichlormethane)?

TIA for your explanation.


Re: Repair of HP41CX - Tony Duell - 08-31-2001

Firstly, if you can find one, try a slightly longer screw. There is enough depth in the post for this without it breaking through the keyboard panel, and normally this will hold the case together. It's what I did to my 41CV when I
had this problem, and I believe it's what HP service centres did.
If you can't get a longer screw, then another repair is to take some scrapings of the case plastic (e.g from inside the bottom case -- you don't need much), put them into the hole, and put a few drops of 'plastic weld' (dichloromethane, available from _good_ model shops) on top to disolve the
plastic and fuse int together. Then just put the screw in as normal.
Incidentally, to avoid stripping the threads in HP calculator (and other) plastic posts, when you put the screws in, turn them counterclockwise ('backwards') until you feel the threads engage.
Then drive them in. Doing that avoids cutting a new thread with the risk of stripping the plastic.


Re: Repair of HP41CX - Tony Duell - 08-31-2001

I've done this repair, and my answers would be :
1) Any similar (chemically) plastic will work. Module covers or battery packs should be fine. Youi want thin (about 1mm) slivers of plastic
2) No, leave a speace in the middle for the screw. If you do fill the entire hole, then drill a pilot hole down the middle (after the glue has set) with a twist drill bit held in a pin chuck.
3) See (2). If you do fill the complete hole, drill a pilot hole for the screw.


Re: Repair of HP41CX - David Smith - 08-31-2001

Tony's explanation about covers it. I trim the slivers off of the bottom edges of the I/O contact assembly becuase it does not show and this is a fairly thick and strong piece of plastic. I do not attempt to plug the entire hole. You need just enough plastic to build up the inside hole diameter so that the screw threads may form. Ususally four slivers around a millimeter wide and a centimenter long will do it.

If you want to use the epoxy technique, I would try filling the hole completely with epoxy and letting it harden. Then drill a pilot hole into the epoxy (best to use a drill press). If I remember right the pilot hole should be 1.9 to 2.0 mm (measure the diameter of the screw inside of the threads with a dial caliper). If the hole is too small you risk splitting the post (bad). If it is two big the screw will strip and you will have to refill it and try again (not so bad).