Spectrum polish off

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Project Spectrum polish off
Status Completed
Contact mahjongg
Last Update 2014-05-29

Sinclair Spectrum polish off is a project to restore a donated defective Sinclair spectrum to working order, and to add some additional features:

All Fixes are done, including:

  • Replaced blown up 7805 voltage regulator.
  • Repaired the +12V & -5V generator (replaced two transistors).
  • Completely replaced foil keyboard by 40 6x6mm buttons, hot glued to the inside and hard wired, the rubber keypad was modified to make place for the buttons.
  • Built in a 1 transistor preamp to make the cassette input sensitive enough so it accepts signals from a tablet or laptop.
  • replaced a missing rubber feet
  • added a composite video output cable
  • Add a built in joystick port, that works by activating certain keys, by using FET's connected between the keyboard matrix points for the keys, 5,6,7 & 8, and 0 for fire.

The latter was also a test case for an identical solution in my ZX14 (ZX81 rebuild). It works, but only when using the right kind of FET's, the first FET's I tried (BS170's) did not work! Probably because of it's largish (200pF) drain-source and gate source capacitances, but using 2N7000 FET's did work. In the ZX14 I will be using the 2N7002, which is a 2N7000 in a SOT-23 SMT package instead of the 2N7000's TO-92 PTH package.

A less positive note is that even with the transistor pre-amp the cassette input is still quite finicky and still not very sensitive and needs a lot of signal to work properly.

Spectrum keyboard with real keys and joystick interface