HP 21, key problems

[teenix]

(02-04-2018 05:17 PM)Matthias D. Wrote:  Hi everyone,

Have problems with my Woodstock 21. Sometimes keys are not registered, sometimes double. Which keys are affected seems to change a little. After cleaning internal with alcohol the shift key and cos got better. Others, 4 and 1, and today especially the zero are registered often two times.
Most keys work reliable...
Does somebody know, which part is defective? Will the panamatik full repair chip be helpfull?

Yours

Apart from dirt inside the keyboard, it could also be that the underlying contacts have lost some of their spring tension or are cracked which would mean those keys feel spongier to press than the others. It is also possible that the plastic under the actual key may have worn down and wont allow the contact to mate properly with the PCB.

When keys are pressed or released, the contacts can bounce around for 10's of milliseconds before making proper contact or releasing. The calculator software works so fast that it can see these makes and breaks. To try and ignore them, the software usually only tests the keys periodically. The HP21 has a small delay after processing a key press and just before it looks to see if the key is still down, released or if another key is pressed. This small delay only uses about 22 instructions so is quite fast.

If any of the above problems exist, they may be allowing the contacts to make and break for longer than normal and the calculator software may not be able to deal with this amount of contact bounce causing it to "see" multiple keypresses. Pressing keys slowly or softly can have the same effect.

If contact bounce is the problem, I don't know if Bernhards LP Woodstock would fix it unless his software can deal with larger delays. A programmable delay might be a good thing if not already there :-)

In my own emulator software I had to extend the delays for the same reasons mentioned as it could register multiple key presses from old worn keys on the Classics.

Having a good look for damage, broken or cracked contacts, worn keys etc, and a thorough clean under the spring contacts will hopefully fix the problem.

cheers

Tony