Thread: Zx81
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Old 14th Mar 2020, 5:04 pm   #55
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 8,875
Default Re: Zx81

In any case, your membrane keyboard seems to be working. If it isn't broken, don't fix it.

In the case mentioned above it would have been possible to use a meter to ascertain which keyboard connections join together whenever a particular key is pressed, without (potentially) destroying the original membrane keypad.

John: Tell us which transistor you have ordered and we will tell you which of its leads are the base, emitter and collector.

If done correctly, the video output from the ULA goes to the 'base' of the transistor, then the buffered video-out signal is taken from the 'emitter' of the transistor. The transistor therefore sits between the ULA IC output and the video-out connection on the socket and is first in line if any damaging signal or voltage is inadvertently connected to the video-out socket.

The transistor also assumes the role of video output driver, relieving the ULA of that load. The ULA now just has to generate the video signal: The transistor toughens the video signal up a bit before passing it on to the monitor / display.

To be slightly more technical about it, most composite video inputs on TVs and monitors have quite a low input impedance, 75 Ohms. The ULA IC, originally intended to drive the input of the RF modulator, may not be intended to, and may not like, being asked to drive such a low resistance load directly.
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