Hi, I bought some of your pads. I thought you did a very nice job making them and I am grateful for your efforts. However, they do not work well for the keytronics keyboard in the Sol-20 (they probably do in other types of keyboard) I found out the reason why. Here is some text from an article I wrote about fixing the keyboard:
4) The keyboard required the usual replacements of the foam pads with the dielectric
coated discs.
The original arrangement uses a 0.07mm thick dielectric (possibly mylar or polyester)
on a foam disc with the conductive layer sandwiched in the middle of the dielectric, so
about 0.035mm from the surface.
Experiments showed that the capacitance has to rise to about 10 to 15pF to produce a
reliable negative going output pulse from the transistorized differentiator and threshold
circuit, of about 800nS width. This sets the S-R flip flop U14 on a key-press.
Prior to getting the Sun 4 keyboard replacements pads, I had experimented with
conductive rubber discs, which also work and running in a DC coupled mode the
output pulses are 5uS long.
I bought some other capacitance pads to try which had a similar thickness dielectric, but
the conductive coat was on one surface and therefore the capacitance didnt rise high
enough for reliable key detection as the dielectric layer was too thick.
......So if you can find a material where the conductive layer is only about 0.035 from the surface, they would also be fine for the Keytronics/Sol-20 keyboard.
