|Re: HP-71B sunk key|
Message #2 Posted by Vladan Dugaric on 8 Feb 2009, 10:36 a.m.,
in response to message #1 by Marcus von Cube, Germany
My HP-71B over time developed three sunk keys - gold f, blue g, and right arrow, and space and down arrow were getting softer. In addition, there was a bit of dirt under many keys and some of them were getting stuck or did not register unless pressed firmly.
After many years I decided to disassemble the keyboard by cutting the heat stakes, and found out that all the snap discs were fine, without any cracks or dents, but the rubber film that is between the key caps and the snap discs got punctured by plastic posts that project from the middle of each key and press the snap disc. My solution was to turn the rubber film over around the horizontal axis, so that bottom was now top, and front was now back. The holes in the film that correspond to the heat stakes are almost symmetric with respect to film, but slightly shifted up, so you either have to cut off a millimetre or so from the other end of the film, or just tuck it in (what I did). Then reassemble (after cleaning all snap discs and key caps and contacts as needed).
Beware: you only have one go at re-melting the heat stakes; check carefully that you did not misplace some key caps, since if you re-melt heat stakes and then realise you need to break them again, you will be left with too short heat stakes.
After that, the keyboard is nice again, without any sunk keys. Gold-f is still somewhat soft, but that is probably because it was used a lot and the hole in the rubber hole under that key is a little larger than other few holes that were punched through by the keys.
Edited: 8 Feb 2009, 10:43 a.m.