Fix for sticky buttons on Tek 1240/41 logic analyzers

[Noel Chiappa]

So one of my Tek 1240s had incredibly sticky buttons on the front panel
(basically, _all_ of them would stick, and have to be pried out manually). I
had _very_ good results with the following very simple procedure (which is
_not_ in the service manual :-) to totally cure the problem.

Your results may differ, since the problem may have different causes in other
cases; in mine, there was lot of fine dust in the keyboard area, and it
apparently had gotten into the button mechanism, which has a lot of sliding
parts with tight tolerances.

i) Remove the keyboard PCB: the Service Manual, Volume I (available online)
explains how to get the keyboard out - roll up the logic cage, and then you
can undo 4 small bolts, after which the keyboard just comes off. You will
also need to remove two flat cables. Next, remove the scroll button (usual
hex set screw holds it on the shaft), and then separate the cover from the
actual keyboard PCB by removing the four hex stand-off posts.

ii) Take the button assembly out of the PCB; it's just a press-fit, not glued
- just push on it from the solder side of the PCB (may need a fair amount of
force). The buttons consist of precisely 5 parts: the keytop, which comes
off/on easily (I usually remove them, but you could probably leave them on),
the housing, the shaft/actuator, a spring, and a gold-plated copper contact
bar (which connects two gold-plated contacts on the PCB, to form the switch).

iii) Immerse the button assembly in water, and _while immersed_, push the
shaft/actuator back and forth a number of times. If it's totally binding, and
won't come out at all, use a small object of some kind down the cylindrical
shaft to get it to go back and forth. If there is an air bubble trapped in the
cylindrical shaft, make sure to clear it - that shaft is one of the places
dirt can get and bind things up. Pretty soon it should operate completely
freely.

iv) Needless to say, dry thoroughly before replacing... :-)

In theory one could take the button assembly apart to clean it, but I'd be
_really_ worried I'd damage the contact, or something. I just left it all
together and dunked it in water, and that worked wonderfully.

For the 5 LED buttons, it's basically the same. The LEDs are _not_ glued into
the button assemblies, just inserted into the cylindrical shaft. _Carefully_
remove the LED from the shaft, and bend it back out of the way. Then proceed
as above.

Hope someone finds this useful! It's tedious, doing all 30 or so keys, but it
works well.

	Noel