What Glue is best to repair HP 41-CX case plastic ?



#2

I've just repaired my 19 year old 41-CX which has been dead for about 4 years, by using the Fantastic Instructions on this Museum Site. Thank you very much. My problems were twofold:

1) due to broken case screw seats (not screw posts) on the back cover at the top of the calculator which of course didn't allow the internal electrical "pressure" contacts to be made; and,

2) several disconnected (but originally soldered) "spring-clip like" wire connections between the main circuit board and the circuit board right behind the LCD screen which were visibly separated by about 1 to 2 mm and easy to spot. Under a 10X magnifying glass I saw atleast another 5 solder contacts on the LCD board side of the contacts that had cracks in them (common problem with "cold" solder joints) so I resoldered them to avoid a similar problem in the future.

I don't know how many pieces of electronics that I've repaired over the years just by resoldering cracked solder joints, usually due to "cold solders joints" made with just a fraction too little heat when originally soldered, which leaves them brittle and easily cracked with time and regular equipment use and vibration.

I used a 5 minute epoxy glue and let it set for 15 hours before reattaching the two halves of the calculator, however within a half hour of using the calculator, the glued case screw seats (that little recessed piece of plastic under the foot pads that allow the screw heads to pull the back case together) came apart. Also the epoxy glue peeled right off of the both pieces of plastic. It appears that epoxy glue doesn't work that great on HP-41CX case plastic, however I may have to wait 24 hours for it to really work. I have never had much success using cyanoacrylate glues (aka "crazy glue") to work on much more than my own fingers, so I didn't use it as was mentioned in the section of this web site tittled "Repairing an HP-41 CX" as listed in the "Main Hall"

My questions are
1) Does anyone know what exact type of plastic is used to build those robust HP-41CX cases; and more importantly ?

2) Does anyone know what glue works best to fix cracks in the plastic used in HP-41 CX cases ?

Thanks


#3

I fix the broken screw bosses on the case back as follows:

1) Grind off the old remains on the screw bosses using a dremel type tool. Grind it flush to the rest of the case back. If you have old epoxy traces, make sure to remove them.

2) Cut some 0.25 x 0.375 pieces of 0.10 inch thick styrene rod (available at hobby shops as packs of five 0.10 thick rods of 0.25 inch thick strips about 12 inches long).

The thickness is important... (3/32" / 0.096 will also work). You want the blocks to be thick enough to just fit flush to the bottoms of the recesses in the battery contact assembly plus just a little bit (a few thousanths). Sand around 0.02 inches off the sides of the blocks so that they fit in the contact assembly recesses with a little room around the edges.

One problem with the original screw bosses in the case is that they did not seat to the bottom of the contact assembly. When you tighten the screws, the bosses bow inwards and place great strain on themselves... hence the cracks.

3) Using styrene welding cement (dichloromethane / methylene chloride) also available at hobby shops, wet the back of the case where the old screw boss was and one side of the styrene block that you just cut out. I like to make sure the plastic is very thouroughly wet. The solvent works by melting the plastic and drys very fast so work quickly.

4) Press the block onto the case back and try to align it so that it will fit properly into the battery contact assembly recesses. Hold it for a few seconds then quickly check the alignment by snapping in the contact assembly.

5) Remove the contact assembly and the clamp the block in place at let it dry at least 48 hours. You want all traces of the solvent under the block to be gone before you put any strain on it.

6) Remove the clamp and drill a hole though the block for the mounting screw.

7) Voila... if all went well your repair will be much stonger and more reliable than the old screw bosses.

The styrene welding fluid also works very well for fixing cracks in the screw posts.


#4

Very clear instructions. You should get this added to this site under HP 41-C repairs.


#5

Perhaps Dave can copy it to its proper place? I can't get it to cut and paste properly.


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