HP25C repair, help needed

[ijabbott]

(02-19-2019 01:48 PM)albertofenini Wrote:  Here you go ..
Amazon Vinavil
It's a very common glue, also used in schools, and it's quite cheap.

It seems to be polyvinyl acetate (PVAc) glue, or "polivinilacetato" in Italian.

[burkhard]

I think in the US, the equivalent would be Elmer's, specifically "Elmer's Glue-All"

Elmer's Glue-All

Normally I think of this and other PVA glues as more of a craft glue for porous material like paper, felt, cardboard, wood, &c.
But maybe this is a good application (as long as it doesn't eventually release on its own or from the calculator suffering a bump)

Keep us posted how it all works!

[albertofenini]

As described in the post, we re-opened the keyboard.
The initial work with the copper tape was not working for all the keys.
Probably the glue was not so conductive as stated.
We removed all the copper tape, we cleaned the plastic film and following the advice
we have cut the new keys with a shape that allowed us to reverse one side.
Once done we tested with a multimeter and apparently all the keys were working.
However, when assembled we tested the combination of rows and columns and again, not
all the keys were doing contact properly.
Then we tried to leave a thin layer of soldering material, but nevertheless it didn't worked out.
We abandoned the idea, also because by soldering the heat had deformed the plastic domes.
We recovered then an old keyboard from another HP25 which was missing some keys and by using
the same donor we used for the HP67 we completed the work.
The domes are a little bigger so that we have cut and soldered the domes one by one and
in the end it looks a little like a frankenstein, but it works.
Soon we will glue it back and we will post pictures of the vinavil work.