Keyboard Restoration work
Keyboard Restoration work
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Having taken the keyboard out of the case applied some power I fond the following:-
some of its output data lines have outputs of +5v and -12v, some of the data output lines go to -12v when you press some keys and others go to 0V. The strobe line is sitting at some strange level of +2.1v. The encoder chip is an SW20457K (40 pin) by the logo made by AMI there is no information the device that I can find.
Testing the keys
The Keyboaed uses to types of hall switches one which is Sink pulse is similar to sink level, but current is only passed momentarily (in the range of 10–100 µs). Switch actuation is signalled, but release is not. The Hall sensor has dual isolated outputs and the other is Sink level switches draw current when active, and remain active while the key is held. The Hall sensor has dual isolated outputs suitable for two-of-N encoding these are used for CTL, RPT,Break, Shift and Shift lock. A number of these keys switches are dead either they only produce a single output or no output, with one holding it output low all the time regardless of switch position.
In the progress of finding parts to repair this keyboard I found DSM Components they have the Tesla MH3SD2, proximity sensor with Hall probe and MH3ST2, Schmitt trigger
The data sheet is here MH3SD2 MH3SS2
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