On Fri, 8 Mar 2002, Al Feng wrote:
> Speculation about the material used in the QL's membranes by Sinclair or
> otherwise seems futile.
I disagree. Here's why.
They fail. We have to produce something that does the same function. We
don't want it to fail the same way.
> It is worth noting, IMO, that the reason that the membranes have failed
> in the past, and will probably fail in the future in a standard QL
> configuration IS singularly BECAUSE of the proximity to the heatsink.
The heatsink is at the opposite end of the QL, and if you put smoke into a
QL you'll see it is drawn in at the expansion end, drifts slowly to the
heatsink end, then accelerates rapidly and comes out of the grille at the
back of the heatsink.
> Think about it.
I have, for a couple of weeks now, full time. :o)
> As long as the standard PS configuration is used, you are baking the
> tails. The rest of the keyboard membrane does not appear to suffer from
> the problem of becoming brittle.
The baking is what causes the pthalates to evaporate. The heat comes from
components on the board.
> If you want to make a BETTER *membrane*, then get hold of a TS-2068 and
> see how Timex made their keyboard -- printed circuit, carbon pads to
> close the circuit, ribbon wire tails attached to a connector.
These are approx 3mm deeper than the current design. I am experimenting
with them. They work very well and are durable. I'm just waiting for a
better selection of expansion cards to check clearances, etc...
> Worrying about the pressure needed to actuate a "key" is a silly exercise
> if you are simply trying to create a replica of the original membrane
> because your keyboard membrane's tails will inevitably suffer the same
> fate which the original membranes experienced ...
Worrying about the key pressure is very important. Part of the total key
pressure required comes from the inertia of the keycap, to accelerate it
down (about 5%), part comes from the rubber bubble mat (about 60%), and
part comes from the membrane itself (about 35%). If it requires 1 unit of
pressure to press the key, and I produce a membrane that requires, say,
double the pressure to operate, it requires a pressure of 1.35 units. This
different will be enough to throw off most fast typists, and if you've
been using a keyboard for 18 years, the last thing you want is for its
characteristics to change. It's a small bit of effort to make a permanent
quality product that will last forever and not have any negative side
effects.
After experimenting with the QL I have here, and checking specifications
for a type of kbd that uses the exact same construction as the QL and OPD,
I can report that the force specified is 2-3 Newtons. The force actually
required increases as the keyboard ages, because the bubblemat and
membrane top layer become slightly stiffer.
If I am going to invest several thousand dollars in a project, I am going
to make sure I understand 100% all the issues involved with my designs,
and with the design decisions of the original membrane. Only by
understanding the specifications and requirements for performance will I
make something that is *perfect* :o)
If I can see a way to make this work, every membrane will be stamped with
a "Born on" date, so people can tell how old it is.
Dave
ql.spodmail.com
