Spectrum polish off

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Project Spectrum polish off
Spectrum with real keys and joystick interface.JPG
repairing a Sinclair Spectrum and providing it with a new (real) keyboard, a Joystick port, composite video input, and a much more sensitive microphone input.
Status Completed
Contact mahjongg
Last Update 2016-01-16

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 ZX81+34 (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 sensitive enough and needs a lot of signal to work properly.

Spectrum keyboard with real keys and joystick interface

Note that this spectrum, (and the CPC646 mentioned here) and about eighty other systems in my possession, were donated to awesome space on good Friday (April 3) 2015 to awesome space http://awesomespace.nl.