Thread: Zx81
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Old 13th Mar 2020, 10:56 pm   #49
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 9,291
Default Re: Zx81

I think the best way to answer your question is to suggest you Google around and see how many different ways people have done the composite mod on the ZX81. If you see enough articles independently suggesting that it be done the same way as you did it on the Spectrum then it probably works OK.

If the general consensus is that you need to use a buffer transistor, then I would go with that option. As I suggested before, using a buffer provides a degree of protection to the exotic and difficult to obtain ULA IC by sitting between the ULA and the outside world - if something untoward happens, it hopefully takes out a twelve pence transistor instead of the ULA IC.

I would favour the buffer method, and in particular this version of the buffer circuit which comes already made on a little PCB which is small enough to fit inside the modulator without removing any of the original modulator parts. All you have to do is add four wires, Power, 0V, Video in, Video out.

https://www.thefuturewas8bit.com/sho...video-mod.html

Although the headline describes as being for Atari it also works with the ZX81.

However, I would not discourage you from trying either the 'simple' video pass through mod like on your Spectrum or the homebrew transistor buffer version - that's how you learn.
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