TRS-80 Model 1 Keyboard Repair

If you have been involved with retrocomputers for any time, the chances are very high that a machine that worked perfectly when last used has now developed a new fault (or two or more.....).
This happened to me recently with a TRS-80 Model 1.  In the time I have had the machine it has been very reliable.  I pulled it out of storage after 6 months to do some testing with the Model 1 FreHD Interface and found that the Shift Keys had stopped working.  
Failure of individual keys is a pretty normal occurance on machines this age.  Failure of a single Shift key wouldn't be a big problem because there are two Shift keys.... but in this case neither worked.
The TRS-80 Model 1 keyboard is memory mapped.  The ROM polls a set of memory addresses in the block 0x3801 - 0x3880.  When keys are pressed this pulls up the matching bit on the data bus.  Most addresses link to 8 different keys except for 0x3880 which on the Model 1 is used only for the two parellel wired Shift keys.
The only component that would affect 0x3880 and the Shift keys but no other keys on the keyboard is Z2 (74LS05).

After Z2 was replaced the keyboard was fully operational.


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