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 - 00:56:00 GMT