Re: restoring electrical conductivity of PET keyboard pad tips

I have done this many times.  When I was maintaining PETs this was a 
constant problem.

First, clean everything with 91% isopropyl alcohol.
Next, clean the circuit board contacts with a non-abrasive eraser.

If this doesn't work, then the rubber contacts have shrunk with age.
I then put a small piece of flexible eraser inside the rubber contact so 
that it pushes out the rubber contact enough to make a good contact.

There's no point in trying to measure the conductivity of the rubber 
contact.  This is a high impedance circuit and takes special equipment to 
measure.

If you have the version of keyboard with the shift lock key, it's a good 
idea to replace the two wires going to the shift lock key with longer 
ones. You will need to clean these keyboards periodically.


On Fri, 3 Nov 2017, william degnan wrote:

> Hi...I am working on a PET 2001-32B with a bad keyboard.  The circuit board
> is OK, the keyboard logic is OK, the computer itself is fine.  I have
> isolated the problem to the black carbon tips that touch the circuit board
> upon a tap of a key from the keyboard.
>
> I have cleaned the tips with 91% isopropyl alcohol and that has had little
> effect.  I speculate a glaze has formed on the tips and the conductivity
> has been lost.
>
> Should I clean "harder"?
>
> I have heard one should paint/brush a metallic paint onto these tips, but I
> am thinking there must be a way to restore the conductivity without adding
> paint, something that will lead to issues down the road.  I have also seen
> a page stating the solution is to scribble on the tips with a pencil.  Also
> not to me a good long-term solution.
>
> Would it make sense to simply use a pencil *eraser* on the tips instead, or
> a very smooth sandpaper/file to buff them in order to clear off a thin
> layer/film that may have formed?
>
> I figure someone here has been through this.  I searched the web, surprised
> to have not have found more written about this.  Up to this point I have
> had spare keyboards and this issue never came up.
>
> Thanks
>
> Bill
>

wlevak@sdf.lonestar.org
SDF Public Access UNIX System - http://sdf.lonestar.org

       Message was sent through the cbm-hackers mailing list