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I bought a small LCD TV today in part to use with my retro computers. My Sinclair ZX Spectrum 48k works perfectly with it - I can manually fine tune Channel 36 as shown in this video.

With the ZX81 I get frustratingly close. I can see the K icon, and the results of anything I type, but the image will not stay still, and half the time the TV claims there is no input (usually while still showing the flickering image).

I have read the discussion on this forum thread, though it is 10 years old, and not everything is there or linked any more*. Also as I understand it, most solutions involve some replacing/soldering of components onto/into the ZX81 and I absolutely don't want to do that for fear of breaking anything.

Having said that, I have opened up my ZX81 and ascertained that the root of the problem is the ULA 2C184E which, unlike later versions of the ULA, does not control video signal strength.

So my question is - is there an external solution to my ZX81's weak video output?. I'm open to doing some soldering and creating some kind of device, as long as it is external to the ZX81. Or is it impossible to improve the situation without internal modification? Has anybody had success using a generic TV signal boosters with the ZX81 video output to solve this problem?

(As an aside, even in the complete absence of a TV for a time, I was able to use the ZX81, using the printer as a kind of Teletype :-D).

* But the thread does prove that this is not just a problem with my TV, and is a general problem with ZX80/81.

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    Googling "tv signal amplifier" gets lots of hits on ebay, amazon etc. Wouldn't that work?
    – Tomas By
    Dec 26, 2019 at 18:24
  • Sounds promising, but I'd like to hear from someone who's tried it successfully with the ZX81 (and ideally an LED TV).
    – harlandski
    Dec 26, 2019 at 18:31
  • Well, it might be either due a general weak signal or, more likely a jittering modulator, moving the signal more than the TV's logic likes. While the first can be solved by adding some antenna amplifier, the second needs a new modulator - or at least some components changed - it might in fact resolve both issues. Then again, why not just adding a composite out? It doesn't have to be a high end ZX8-CCB - a simple transistor and a resistor hooked up to ULA video out, +5V and ground will do it. [edited by moderator]
    – Raffzahn
    Dec 26, 2019 at 18:39
  • Like here: classic-computers.org.nz/blog/…
    – Raffzahn
    Dec 26, 2019 at 18:39
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    There seem to be at least two revisions of the ZX81 — one which produces a back porch (which TVs use to judge the amplitude of the incoming signal) and one which doesn't. Does your ZX81 contain a 2C158E/184 ULA, or a 2C210E? It should be printed on top of the ULA in big letters, e.g. this is a 2C210E: stevenjohnson.com/pics/timex1000-board.jpg
    – Tommy
    Dec 26, 2019 at 19:18

1 Answer 1

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I would skip the existing output entirely.

Buy a Chroma 81, which plugs into the expansion slot — no internal modifications necessary — and then monitors the internal bus to reproduce the video stream of its own volition. It has a SCART socket that you can then connect to your TV, modern or retro.

The Chroma's primary purpose is to add colour graphics modes, but they're entirely optional.

EDIT: re that final assertion, see this archived version of the usage instructions:

Chroma 81 is a multi-function SCART interface for the ZX81, providing a clear, sharp picture on most modern TV sets.

...

Switch 6: ON=Enable colour facilities / additional 16K RAM [$C000-$FFFF].

Though you shouldn't usually need to switch colour off at a hardware level — it's off by default when you start the machine and software must write to an $xxEF port to enable one of the two available colour modes, which most software won't do by accident.

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