17 Feb
2003
17 Feb
'03
12:16 p.m.
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
This was a very common design of keyboard at one time. DEC used it
(VT100, LA120, etc). So did HP (HP85, etc). And Radio Shack (early Model
1s).
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
If it's the standard design, one contact is sold and fairly stiff, the
other has the end cut into about 4 fingers, and is much more flexible.
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.
[...]
(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.
OK, remove the keycap (but you've done that). The plunger (inner housing
to you) will also pull out if you grip it with pliers. HP claim you
should always use a new one when reassmbling, but it's not really
necessary. When you've got that out, remove the spring too.
Then try cleaning the contacts in place. Take a piece of card (a bit of
Hollerith card is ideal if you have some). Soak it in propan-2-ol and
carefully work it between the contacts. After doing that check that the
contacts test as shorted on the track side of the PCB (work on one
switch at a time so as not to have sneak paths through other dismantled
switches -- a dismanteld switch is, of course 'closed').
If it still doesnt work, remove the contacts. Desolder them from the PCB
and pull out the stiffer contact with pliers, then _carefully_ remove the
other one. Clean them, bend the 'figered' contact slightly to increase
the pressure between them. Then put them back. There was a special tool
for this, but it's not essential.
-tony