HP-41c back case screws holes repair

[tribidouille]

Hello Forum Members,

I was searching how to repair my HP-41c back case screw holes.
As the guides between holes were also broken, none of the solutions I found suited my case.
As you know, the repair pads have to be placed very precisely, otherwise the plastic flex circuit support will not fit in anymore.
So I had to find something with another approach, that's what i am sharing, if one has to do it too, one day.


   
Here are the broken screw holes. I bet it often happens on the hp-41c.
Also 2 of the 4 the middle positioning plastic pieces are broken.
Before to proceed to repair, scratch or file the bottom to remove remaining parts of the screws housings. Obtain a flat (at least not convex) surface.


   
I tried this system by making one myself, but with missing middle positioning pieces, repair pads placing is not precise.


   
So I built a guide (blue) and two simple pads.
This way, repair pads positioning is very precise.


   
First, try the whole process with thin double sided tape:
-Place the pads along the blue guide.
-Delicately pull the guide out.


   
Verify that the plastic flex circuit support comes in place easily, going well down to the bottom.

Max attachments reached. Dare I continue in a next post or are there too many pictures already?
EDIT: pics and text errors

[tribidouille]

   
Remove everything, degrease. Clean the repair pads too.


   
En-glue with a slow glue. I am not qualified enough into gluing plastic to tell exactly what type is the best. I personally use slow bi-component epoxy glue.
Put some glue under the pads too.


   
Hold the pads and clean the un-useful glue as much as you can. You can also scratch it later but glue should not stay, as it will disturb the plastic flex circuit support positioning.


   
Hold it with... well maybe you have something better. Eventually screws from under the case would provide the best pressure. But don't en-glue the screws!


Last step:
Very delicately remove the guide by lifting it up, with both hands, without to move the pads.
Clean the guide from glue.
Clean the remaining glue under the guide.

Be patient, let harden long enough, rebuild.
I'd be curious to read your comments and how some of you did, in a similar case.

EDIT: Had forgotten to describe the last step!

[tribidouille]

Other sine now.
On the other side, holes were looking fragile.

   
I filled the holes with epoxy glue and carefully drilled new holes.


   
I surrounded them with tubes I made.
These tubes are not glued, to allow future disassembling if necessary.

[tribidouille]

   
A certain amount of tril/error and missed pieces...


   
Lot of tries until I found the right dimensions (talking about some 10th of mm).
My thoughts were these:
If the pads are too thin, the pressure that has to be applied on the flex cable plastic support will be done by the rear case inner face, leaving the pads press on nothing (only pull on glue), hence not good.
If the pads are too thick, the flex cable plastic support will press too much, risking of damaging itself and the board and not allowing the case to fit correctly.
I decided to let the pads pass over 1 or 2 tenth of mm the flex cable plastic support. This way, the glue will never have excess traction.

[Raymond Del Tondo]

Looks great:-)

Maybe this repair guide should be added (or moved) to the Articles section.
In any case, I think it may be a good idea to add a link to this guide on the "Repairs" page of the MoHPC;-)

[tribidouille]

I forgot the very important last step of the first repair, in post #2!
Edited + corrected.
@Raymond Del Tondo
Yes, why not . It was said "advanced ... repair instructions". Didn't know at what point...

[tribidouille]

   
Here is the result on the out side.
Not bad. The screw fits, just, but well.


   
As at this point it is not a big effort, while I had the good settings, material and before I forget the details, I made some more units.
If someone need to repair his HP-41C just let me know!

Have a nice day,