TLC Ideas for LA120 Keyboard?

From: Ian Primus <ian_primus_at_yahoo.com>
Date: Mon Feb 17 08:00:00 2003

I've had pretty good luck fixing keyboards like that. First, take the
keytop off. Then pull out the inner plastic housing. To do this, you'll
need to use some small pliers and be very careful not to damage it.
Once it is out, remove the spring. Now you have better access to the
contacts. Take a scrap of paper and get it wet with isopropyl alcohol,
then put it between the contacts while pressing them together a little
bit with a screwdriver or something, and pull the paper out. This does
a pretty good job of cleaning the contacts, and I have also had luck
with dry paper. Rougher paper helps, and if they are really dirty, I
would imagine you could try using some very fine grit sandpaper, being
careful not to damage the contacts. Once you've cleaned the contacts,
put the spring back in, and the inner plastic housing (be sure to get
the plastic bar going the right direction!). I have used this technique
to fix my DEC VT100 keyboard, as well as some other terminal keyboards
that have a similar mechanism. Good luck!

Ian Primus
ian_primus_at_yahoo.com

On Monday, February 17, 2003, at 01:54 AM, Jeffrey Sharp wrote:

> Well, my DEC LA120 is almost perfectly operational. Its only problem
> is a
> set of keys that do not work when pressed. I'd like to fix that.
>
> What I see when I remove the keycap is a square plastic housing that
> slides
> vertically within a larger square plastic housing. The smaller housing
> slides down when one presses a key. A spring below the smaller square
> housing pushes the housing back up when the key is no longer pressed.
> Up
> through the smaller housing shoot two electrical contacts. The
> contacts are
> fixed and do not slide with the housing. When the key is up, a plastic
> bar
> across the middle of the smaller housing holds the two contacts apart.
> When
> the key moves down, the bar moves down and no longer holds the contacts
> apart. The contacts touch and complete a circuit, and the LA120 senses
> a
> keystroke.
>
> Top view (key not pressed):
>
> +-A------------------+
> | +-B-----+-+------+ |
> | | | | | |
> | | | | | | | |
> | | D |C| E | |
> | | | | | | | |
> | | | | | |
> | +-------+-+------+ |
> +--------------------+
>
> Side view (key pressed):
>
> | | /\ | |
> A B / \ | |
> | | D E | |
> | | / +--+ \ | |
> | | / |C | \ | |
> | | / +--+ \ | |
> | +----------------+ |
> | /\/\/\/\F/\/\/\/\/ |
> +--------------------+
>
> A = Outer (fixed) housing
> B = Inner (sliding) housing
> C = Bar that holds contacts apart when not pressed
> D = Electrical contact
> E = Electrical contact
> F = Spring
>
> The problem is that the circuit isn't being closed when the key is
> pressed.
> If I stick a screwdriver in there, bridge the gap between the contact,
> and
> thus close it manually, a keypress is sensed. Actually, it senses
> several
> keypresses very quickly, probably because of the noise caused by the
> conductive screwdriver scraping across the contacts. When I press the
> housing down, the contacts *appear* to touch, but no keystroke is
> sensed. So
> I figure there are either or both of two possible things going on:
>
> (1) Tiny space between the contacts
> (2) Nonconductive material (corrosion? oxidation?) on the contacts
>
> Two keys were fixed by using the screwdriver to bend the contacts
> toward
> each other in the hopes of creating more force pushing them towards
> each
> other. The ENTER key does not seem to be responding well to that
> treatment.
> One thing is for sure. All this stuff is so tiny and hard to get to
> that it
> is difficult to work on.
>
> Any suggestions?
>
> --
> Jeffrey Sharp
Received on Mon Feb 17 2003 - 08:00:00 GMT